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      01-09-2010, 09:19 PM   #1
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Suspension Upgrade Options for the M3

It’s winter and thoughts turn to the coming of spring – it’s time to think about suspension options!

I have to thank RLDZHAO for starting the survey thread of suspension kits. It would have been helpful before I started, but I was well into my analysis before it first appeared. This is the first of a number of posts. I’ll try to hold each post to one element of the suspension “situation” and focus on background rather than the merits of the various kits.

In the end, I built a set of motion analysis spreadsheets for the suspension and took four pages of notes and diagrams of measurements of the suspension itself. There’s a lot of misinformation out there on suspensions, and a lot of opinions. My goal is to just lay out the facts.

The most obvious fact about the M3 suspension is that the BMW engineers did a brilliant job. It’s going to be difficult to improve on it. You can re-prioritize your goals (handling, looks, etc) and improve one at the expense of the other, but there will be tradeoffs.

Before I dig into the M3 itself, I came to understand that there are three broad types of suspension kit for the E92 M3 and some interesting non-kit options as well.

The first broad type (let’s call it “Type One”) is “lowering springs”. Lowering springs drop the ride height of the car. They function on EDC and non-EDC cars, and while you can swap the dampers out, you don’t have to. They are usually form-fit-function replacements for the stock springs. Some (like Dinan) come with other replacement components; most don’t. You have the option to use replacement dampers from Bilstein (non-EDC cars only) to replace the stock dampers when you install lowering springs. The effect of lowering springs on handling is hard to predict – more on this later in another post.

The second broad type (Type Two) is “street-sport suspension kits”. These kits are essentially lowering springs matched with adjustable dampers. Examples are the Bilstein PSS-10 and the Moton StreetSport. They usually provide both adjustable ride height and adjustable damping rates. You can tell a street-sport suspension because it uses the factory upper spring and strut mounts – it doesn’t use race springs and it doesn’t come with camber adjustment plates.

The third broad type (Type Three) is “club-sport suspension kits”. These kits replace the stock upper spring mounts so that standard-sized (60mm or 2.5” ID) race springs and camber adjustment plates can be fitted. With this type of suspension, you can not only adjust camber, ride height and damper settings, you can swap out the springs to change the rate. The dampers are usually double-adjustable, which will demand some expertise to set properly. Examples of this type are KW Clubsport kits, Moton ClubSport kits, and products from TCKline and Ground Control.

The Type Four suspension is really the do-it-yourself “non-kit” kit. Buy dampers, buy hardware, buy springs and put it all together. Any of the club-sport units can be reinvented as Type Four suspensions just by changing your mind about a component. You can also buy racing dampers and build them yourself. Information on these is usually scarce – for instance, Bilstein makes a “VLN Suspension” for the E92 M3 that was aimed at the VLN racing series in Germany. It has to fit a stock M3, but beyond the fact that it exists, there’s no data sheet for it. Oh, for 2010, VLN’s governing body excluded cars over 3.5 liters, so the E9x M3 is no longer eligible. Of course, speaking of racing kits, Ohlins and Moton both make damper sets (just add springs and stir) for the M3 GT4. Buy those, you’re good to go. Pricey, finicky, but rewarding when you get them dialed in.

Next post will be on the challenges that the factory design presents for designers of each type of suspension.
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      01-09-2010, 11:03 PM   #2
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The second post in this series looks at the challenges of the M3 front suspension.

The front suspension has some particular challenges: travel (always) and camber change. The BMW engineers solved both by putting a really well designed bump stop in the front suspension. At static (sitting there waiting to go) the both the front and the rear suspensions are compressed by 4 inches (100mm) at the wheel. Static is 355mm from hub to fender edge, full droop is 455mm. By limiting the travel in the front suspension design, they controlled the camber change. Your tires are very happy about this.

The front suspension moves the damper at virtually the same rate as the wheel (motion ratio is 96%, which is essentially “one” for all practical purposes). That means the damper compresses 100mm (4 inches) from full droop (lifted into the air on a jack or a lift) to sitting on the ground ready to go. The front damper has a total travel of 150mm, or 6 inches. That means that there is 2” travel left when the car is sitting on the ground. The question is, what do we do with the two inches?

The factory chose a 167 pound-per-inch spring, so weight transfer in a corner or a bump of 334 pounds that pushes the wheel up into the wheel well by two inches would run out of travel. The factory put a cleveryly designed bump-stop in place that augments and raises the spring rate smoothly and rapidly so that the suspension doesn’t run out of travel suddenly with a metal-on-metal collision. Travel just comes to a rubbery end at about 1.5 inches rather than a hard bang at 2”.

So, how does this play out in suspension mods? Well, if you just put “lowering springs” in place, the suspension has to sit lower than stock (duh) so the spring has to be SOFTER than the stock spring. You've also used up some of the two inches available travel. The four inch rise of the suspension has to become five inches to get a lowering of one inch, and the remaining travel is reduced to one inch. One inch of travel is "not much" in anyone's language.

Now, along the way, the BMW engineers gave the spring companies a “gimme” by pre-loading the 167 pound spring by about 133 pounds so that it's producing 800 pounds of force at 4" of compression (133# + 4"x167#/" = 800#). A spring with no pre-load that sits just-barely slack in the OEM spring seats can have a 200#/inch rating (four inches of compression with about 800# sprung load) so it CAN be stiffer than stock at the STOCK height. If the spring lowers the car by allowing five inches of drop instead of four, it has to be 160 pounds per inch (160#/inch times five inches drop = 800 pounds) so you get a lowering spring with the same rate as the stock spring.

What about handling, though? You’ve got the stock spring rate, but instead of two inches of travel (334 pounds of weight transfer) you only have one inch, or 160 pounds. Needless to say, your car’s handling will suffer – too little travel left to soak up bumps before you’re into the bumpstop. The car will be fine on smooth roads, choppy on rough ones.

I'm going to stay out of the "progressive spring" discussion until I get to Type Two suspensions. Most Type Ones look like linear-ish springs, let's just leave it at that. Progressive springs change the game, but they have their own problems to cope with.

The Dinan kit dodges this bullet with two important design choices: First it limits the drop to about ½ inch. Second, it increases the suspension travel (at the front) by the same amount, keeping the same travel as stock. Travel remains the same as stock, rates are slightly higher (I’m guessing) and overall the performance of the suspension doesn’t change that much.

My opinion (I promised to keep them out of this, but it's just too tempting) is that Dinan springs and suspension parts matched with Bilstein's Sport dampers would work really well together.

When you do the same analysis for Type Two, Type Three and Type Four suspensions, the situation improves somewhat. After a brief discussion of the rear suspension, I’ll post about Type Two’s next.
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      01-09-2010, 11:32 PM   #3
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The third post in this series looks at the challenges of the M3 rear suspension.

The rear suspension has different challenges than the front: As with the front, the static (sitting there waiting to go) compression is 4 inches (100mm) at the wheel.

The rear suspension moves the damper at 83% of the rate at the wheel (damper motion ratio is 83%). The spring motion ratio is 57% because the spring is mounted inboard of the damper, and not concentric with it (a coil-over design) as it is in other BMW vehicle series like the E39 5-series, for instance.

At this point, I haven’t done the full motion analysis of the rear suspension, but some problems with the factory design are evident. The full analysis will (probably) show that the rear suspension has more travel than the front, but that the spring design is a serious challenge for high-rate springs.

The reason I say the rear suspension is a challenge from a spring rate perspective is that the rear wheel-rate is about one-third of the actual spring rate. The stock M3 rear spring has a rate of about 600 pounds per inch, giving a wheel-rate of 200 pounds per inch. Combined with the front rate of 167, the rear rate provides a good frequency balance. The car rides very well, if a little stiff.

Now, the rear spring is mounted between the rear suspension “Camber link” and the body, and it sits just outboard of half-way between the two ends of the link. It’s a teeter-totter setup that tries to pry the rear subframe DOWN and AWAY from the body when the wheel rises and the spring compresses. What’s worse, the higher the rear spring rate, the more prying force is applied to the subframe when you go over a bump. No wonder E36 and E46 rear suspensions have had problems with body damage – every little ripple in the road is another attempt to pry the subframe off the body.

To prevent damage to the bodywork, it’s necessary to limit the rear spring rate. Later, in discussions of Type Three and Four suspensions, I’ll talk about how this is solved, but for the moment, take it as given that rear spring rates that are significantly higher than stock are a bad thing.
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      01-09-2010, 11:38 PM   #4
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The fourth post in this series is about Type One suspensions – coil-over springs – and the M3. It’s been talked about already in the second post, but it’s time to close the loop.

The M3 front suspension is travel-challenged, a problem for the spring designer, and a blessing for the tires. You’ve got two inches at static OEM ride height to use for compression travel. How do we use them wisely?

First, what choices are there? My survey turned up:
- Two H&R spring sets – sport and race
- Dinan springs – stage doesn’t matter, the springs are the same
- Eibach springs – sport setting
- RD Sport Suspension kit - Bilstein non-adjustable dampers, RD Sport fixed springs

That was about it. All types seem popular, the reviews of the users are varied. Dinan customers, with the smaller drop and increased travel, seem to be happy. Actually, everyone seems happy, but some do say the suspension is choppy on rough roads. Given the design constraints (see Post #2) it’s not a surprise.

My opinion (here’s another one) is that track handling will be WORSE with lowering springs. Good track handling is built on travel and spring rate. Lowering springs don’t improve either. That said, if it’s as good as stock, it’s still pretty good.

So, should you expect with lowering springs? A better look, for sure. Handling will vary. Ride will be roughly stock - spring rates won’t vary much and with the exception of Dinan, you’re mostly running on the stock bump stops anyway. If you don’t change the dampers, the car will handle OK. With new dampers, it might handle better or it might not. You have to try it to know.

Last edited by JAJ; 01-10-2010 at 12:41 AM.
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      01-09-2010, 11:38 PM   #5
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The fifth post in this series is about Type Two suspensions – Street-sport suspension kits – and the M3.

Street-sport kit designers have more tools in their toolbox to attack suspension travel problems. They can use radically wound springs (ie. Bilstein PSS10) to get a high rate in the “normal travel” range of motion, while keeping the ride height at rational levels. They can also change the damper body height and piston stroke to control static height and manage free travel. Either or both might be required to get the springs to deliver the correct ride height and rates.

The challenge that Type Two designers can’t overcome easily is the problem of managing the last two inches. Bilstein solves it by saying there’s a limit on lowering. Moton, in the first version of their kit, didn’t solve it, and their increase in travel (that improved grip) left the tires free to grind holes in the fender liners. Moton is very competent, they’ll figure it out. The key point is not that they messed up; it’s that when someone that good doesn’t get it right, then it’s a really hard problem.

If you look at pictures of the springs, you see one interesting difference between Bilstein and the others. Notably, the Bilsteins have a two-stage progressive wind. The rate is low at low compression, high at high compression and the transition will be quite sharp. It’s much like a racing spring/helper spring combination – two rates that become one high rate for all practical purposes when driving. Other springs tend to look more linear, with one pitch that is closer to stock.

So, what’s available?

- Moton Streetsport kit (double adjustable dampers, height adjustable springs)
- Bilstein PSS10 (single adjustable dampers, height-adjustable springs)
- KW Variant 3 (double adjustable, height adjustable springs)
- AC Schnitzer / Sachs (Single adjustable, height adjustable springs)
- (Future) Ohlins Street Sport (Single Adjustable, height adjustable springs)

One observation I’d like to make on the Type Two kits is that it’s a hot market. Everybody wants in!

Both Ohlins and Moton, damper-makers to the stars, have introduced kits. For Moton, it’s the only twin-tube shock they make, and the initial results have been challenging, shall we say. My opinion on the market (not the products) is that the damper companies who’ve made a good living for years selling high-end racing dampers saw an opportunity to design to a price-point and deliver kits to a waiting enthusiast market. Springs and mounts are new territory, as are low-priced dampers. Teething problems will be rampant. Bilstein, who’ve been doing this for years is in a good position, KW not far behind. The new guys will struggle to find their way.

Overall, the Type Two street-sport products will outperform lowering springs, in that the designers can get the best of the travel design variables and good streetability. For track work, they are limited because of the lack of camber adjustments. Go out a few times a year, they're going to be great. On a steady diet, they might show some limitations. Overall, they'll make a great-handling car handle a bit better without the downside of lowering springs.

They're good value.

Last edited by JAJ; 01-10-2010 at 12:52 AM.
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      01-09-2010, 11:39 PM   #6
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The sixth post in this series is about Type Three suspensions – club-sport suspension kits – and the M3.

Club-sport kit designers have no excuses for not producing a good product. They can select from a broad range of springs and dampers, and they don’t have to be daunted by the travel issues that plague other designs.

That said, it’s not a cake-walk. Essentially, you have to allow for at least 100% weight transfer (ie – when your inside front tire is in the air in a corner), preferably more, within the available travel of the suspension. While the stock suspension travel is limited by the damper design, it’s not because BMW couldn’t have made it longer. The tire will hit the inside of the wheel-well at some point, and that’s the true limit of suspension compression. Of course, the clientele that buys club-sports is going to run wider tires than stock, so just letting the tire ride up into the wheel well is just not going to work.

What the Type Three designer can do then is pick a spring rate that keeps the tire under control and allows for 100% weight transfer. That’s about a 500 pound per inch spring at the front. If you’ve got a droop problem, a helper spring will take up the space. Aren’t racing springs wonderful? It’s just that easy, at least at the front.

The rear’s not so easy. Remember the point about the rear spring rate limits because of the suspension design? You know, where a high-rate spring will try to rip the rear subframe right off the bottom of the car? Yeah, that one.

Your Type Three designer is on Easy Street with the front, but the rear is going to be a compromise. The stock rate is 600#/inch, yielding 200#/inch at the tire. There’s a safety margin, but it’s probably not much more than 50%. Let’s say that rear rates over 900# per inch are out of reach for mechanical reasons. That means you can raise the front rate by a factor of 3 from 167#/” to 500#/” but you can’t raise the rear rate from 600# to 1800#. The subframe would be in serious jeopardy.

So you have to go with what works. KW has decided, after a lot of testing, that’s 800#/”. In my survey of suspension kits, I never found one that was higher than that. So, you can triple the front wheel rate, but you can only increase the rear by 33%.

As for what’s available in Club Sport, there are only two: KW and Moton. The Moton kit is the more expensive of the two, and it’s not clear how they’ve dealt with the rear suspension issue. The limited documentation available says “coil over”, which is the only real solution. More on that in the Type Four post.

Last edited by JAJ; 01-10-2010 at 01:15 AM.
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      01-09-2010, 11:40 PM   #7
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The seventh post in this series is about Type Four suspensions – DIY Engineering Specials – and the M3.

The Type Three suspensions solve most of the front suspension travel issues. They might be a bit stiff, but you’re in linear mode with reasonable travel most of the time. It’s the rear suspension where the going gets tough. Type Four suspensions essentially pick up at this point and keep going up the development curve.

The bad news is that unless you can afford to buy a Type Three suspension kit and throw away the rear suspension, you have to start over at the front. You have to find dampers and strut bodies that will fit and work in the front of an M3. Moton, KW, Bilstein and Ohlins *appear* to have parts that work but you’re in territory where there are no guarantees. YMMV takes on a whole new dimension. You can also go with front kits from Ground Control and TCKline – I assume they’ll sell you axle sets one at a time. My own dealings with GC were very satisfactory on this point when I was upgrading my E39 M5.

The solution to the Type Three rear suspension compromise is to convert the rear to coil-over. By putting the coil spring around the damper, the motion ratio goes up from 57% to 83%, drastically reducing the leverage on the subframe. It increases the load on the body where the spring mounts, but the leverage is much less than on the subframe mounts. Also, because the action is more “direct”, the spring rate is much lower. If you want the rear rate to be a good match to a new, higher rate 500# front, then you can use a 600# coil-over spring, delivering a rear wheel rate of just over 400# that will work nicely. To do the same using the stock spring location would call for an unhealthy 1200# spring.

At first, the coil-over conversion sounded a little abstract to me, so I spent considerable time looking at photos of the BMW M3 GT4’s racing. I eventually found one with a shot of the rear suspension with the car in the air and the wheel off. Sure enough, it was a coil-over rear spring setup. It’s what the big boys use. Interestingly, the photos of that car also made it clear that the rear dampers and the front dampers were from different suppliers. The rear dampers had an “Ohlins” look to them; the fronts were probably Moton, if the orange color means anything.

So, what are your choices? Well, you can contact Sachs Racing, Bilstein Racing, Moton, Ohlins, KW and probably a few others to get real racing dampers that will support a rear coil-over setup. While nobody is talking about it, I suspect that a call to Turner Motorsports (who make a race damper spherical ball upper mount for the M3) would turn up other choices. Certainly, GC and TCKline have the capability, although perhaps not the motivation, to build something for you using Koni single or double-adjustable dampers.

I wouldn’t be surprised if sometime soon one of the Type Three club sport companies comes out with a coil-over rear setup for the M3. In fact, it’s possible that Moton has one already – my research showed that they refer to a coil-over rear setup on the Moton Clubsport, but I found no drawing or picture that confirmed it.

Last edited by JAJ; 01-10-2010 at 01:44 AM.
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      01-09-2010, 11:41 PM   #8
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The eighth post is where I tie it all together.

My conclusion after all the research, and a very helpful online discussion with ORB, was that the KW Clubsport would be the best choice to meet my needs. I've placed the order and I should have it in a few weeks.

My goals were:

- stockish ride height (maybe 1 cm lower)

- improved precision

- little or no increase in NVH

- more negative camber than stock - 2 degrees or so, switchable back to 1.1 (stock) for just driving around

- linear springs that don't bottom, that don't ride the bump stops and that are properly damped

- overall, I want a car that is predictable and consistent on the track with a decent "drive to work" ride

My intent is to install it (maybe even do a DIY) and run it exactly the way it comes out of the box. I will contact KW directly to get the "bad news" about a rear coil-over conversion, but I expect the cost will put it out of reach for this year.

One conclusion I came to during all this was that there are "damper companies" that make suspensions on the side and "suspension companies" where integration is their core competence. The only "damper company" that has a well-integrated kit at this time (January 2010) is Bilstein and it's Type Two. Moton isn't there yet, but they will get it right in due course and it's reasonable to expect that Ohlins and Sachs will too once their kits are out. Strangely, Koni is conspicuous by their absence. As for "suspension companies", KW is the only one that really jumps off the page with a high level of integration. I'm not discounting GC and TCK - they're good people doing good work - but they don't present as cleanly as KW. JIC Cross shouldn't be discounted, but frankly I just didn't look at them - they don't have an M3 "purpose built" kit, so I didn't see the point.

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      01-10-2010, 01:23 AM   #9
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Nice write-up so far (#4). Anxiously awaiting the rest.
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Let me get this straight... You are swapping out parts designed by some of the top engineers in the world because some guys sponsored by a company told you it's "better??" But when you ask the same guy about tracking, "oh no, I have a kid now" or "I just detailed my car." or "i just got new tires."
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      01-10-2010, 02:22 AM   #10
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Thank you for your efforts. Glad to see someone on the forum who shares my passion

A couple comments. (more to come)

**

1) Great explanation on why most of the lowering spring kits are bad for handling. When you drop the front by 1" and have very little of bump travel left, roll resistance will spike up even with slight compression.

When this occurs to the outer wheel during cornering, unpredictable high roll resistance means more weight transfer to the outer front wheel, which means more understeer...

**

2) I remember that Steve Dinan said that the OE suspension has 0.5" of travel before making bump stop contact. The bump stop is 2.375". Therefore, the total amount of compression travel (before considering the bump stop) is 2.875".

I also measured my OE front shock and there is precisely 5.75" of total travel (bump and droop). Thus, per my measurements, the total front suspension travel (bump and droop) is divided 50/50, e.g. 2.875" for bump and 2.875" for rebound. (give and take 0.25" due to variation in equipment, spring rate, etc.)

Further, let's assume that the stock bump stop will be compressed to about 1" at the limit (from 2.375" unloaded). This means that only 2.875"-1"=1.875" of effective bump travel is available for the M3. (consider a kit that drops the front by 1" and you will see why this is a bad idea)

(note: I use bump/compression, rebound/droop interchangeably)
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      01-10-2010, 02:53 AM   #11
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Quote:
Originally Posted by rldzhao View Post
Thank you for your efforts. Glad to see someone on the forum who shares my passion

A couple comments. (more to come)

**

1) Great explanation on why most of the lowering spring kits are bad for handling. When you drop the front by 1" and have very little of bump travel left, roll resistance will spike up even with slight compression.

When this occurs to the outer wheel during cornering, unpredictable high roll resistance means more weight transfer to the outer front wheel, which means more understeer...

**

2) I remember that Steve Dinan said that the OE suspension has 0.5" of travel before making bump stop contact. The bump stop is 2.375". Therefore, the total amount of compression travel (before considering the bump stop) is 2.875".

I also measured my OE front shock and there is precisely 5.75" of total travel (bump and droop). Thus, per my measurements, the total front suspension travel (bump and droop) is divided 50/50, e.g. 2.875" for bump and 2.875" for rebound. (give and take 0.25" due to variation in equipment, spring rate, etc.)

Further, let's assume that the stock bump stop will be compressed to about 1" at the limit (from 2.375" unloaded). This means that only 2.875"-1"=1.875" of effective bump travel is available for the M3. (consider a kit that drops the front by 1" and you will see why this is a bad idea)

(note: I use bump/compression, rebound/droop interchangeably)
Thanks for the feedback and thanks for firming up the front suspension measurements - I haven't taken mine apart yet, so they're not as precise as yours.

We might differ a bit in our details - my measurements have the front damper further than half-way through its travel at static OEM ride height, while yours have it half-way. Other than that, our measurements are remarkably consistent.

It doesn't matter which numbers are the more precise; either way the conclusions are the same. The issue in the front is the limited compression travel and the problems it causes for suspension mods.

Edit: By the way, when I measured mine, I concluded that my front bump stops touch (lightly) on the top of the damper when the suspension is static. Steve D and I disagree, and it' s more likely he's right than I am.

Last edited by JAJ; 01-10-2010 at 03:21 AM.
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      01-10-2010, 07:30 AM   #12
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Nice write up.

Just some info you may want to check out regarding type 2 and 3 kits with the possibility of rear coilover option. It´s a UK based operation called Nitron, they are known to be good in Europe. They have e46 and e92 M3 kits available.

Here a pic and link, maybe give them a call:

http://www.nitron.co.uk/nitron09/ind...cPath=2_60_327



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      01-10-2010, 09:34 AM   #13
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Quote:
Originally Posted by JAJ View Post
So, how does this play out in suspension mods? Well, if you just put “lowering springs” in place, the suspension has to sit lower than stock (duh) so the spring has to be SOFTER than the stock spring. You've also used up some of the two inches available travel. The four inch rise of the suspension has to become five inches to get a lowering of one inch, and the remaining travel is reduced to one inch. One inch of travel is "not much" in anyone's language.
Thanks for the write-up JAJ.

Not sure about your conclusion above though regarding the stiffness of the "lowering" springs. It is possible to use shorter but stiffer springs to achieve the drop (shorter natural length but higher k). That could offset some of the drawbacks of using up the "avaliable" travel built into the stock suspension since the stiffer spring would compress less under the same loading conditions. In that scenario, the main issue is changing the spring stiffness without modifying the damping ratio. In a properly tuned system, they need to be modified in conjunction if the modifications to the stock system are significant. Of course, one can debate what "significant" means.
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      01-10-2010, 10:48 AM   #14
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Could someone explain what the helper spring does? I thought it was to keep the main spring lightly compressed in case you got a wheel off the ground? But it sounds like it takes part in the total spring rate in JAJ's discussion.

I also know Orb had some issues with the KW kit, but he did some modification's to his kit.

.
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      01-10-2010, 11:37 AM   #15
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interesting, great write up
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      01-10-2010, 11:42 AM   #16
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Great writeup JAJ! Very informative.

Do you think the internal progressive bump circuit and adjustable pressure in the Moton racing shock will help eek out the most perf. from the available travel in the front?
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      01-10-2010, 12:47 PM   #17
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Originally Posted by lucid View Post
Thanks for the write-up JAJ.

Not sure about your conclusion above though regarding the stiffness of the "lowering" springs. It is possible to use shorter but stiffer springs to achieve the drop (shorter natural length but higher k). That could offset some of the drawbacks of using up the "avaliable" travel built into the stock suspension since the stiffer spring would compress less under the same loading conditions. In that scenario, the main issue is changing the spring stiffness without modifying the damping ratio. In a properly tuned system, they need to be modified in conjunction if the modifications to the stock system are significant. Of course, one can debate what "significant" means.
The perch-perch distance at full droop can't be changed on the stock setup. The shortest spring you can install must be long enough to get from one perch to the other with no gap. Otherwise the spring will come adrift when you jack the car up.

So, the length is what it must be. The only other parameter is the rate, and the spring rate is determined by how far you need it to compress at static position. The softer the spring, the lower the ride.

Because BMW set the suspension up with preload, they created design flexibility to shorten the spring and stiffen it to about 200# per inch while keeping the stock ride height.

At that point the damping will be out of tune. However, if you now reduce the spring rate to lower the car, the damping comes back into tune as the car gets lower. The reduction of travel is a whole other matter - the best damper tuning can't compensate for the rapid climb in spring rate created by the bump stop.

The only "get out of jail free" card in this equation is the prospect of doing a bimodal progressive wind on the spring. It starts with a softer section that allows the car to settle most of the way to static, then that section goes into bind and the stiffer section takes over and controls the motion in the narrow stroke range above and below the static height. The Bilstein PSS10 spring shows this exact winding design.

However, the progressive spring strategy requires new dampers because the stock dampers are tuned for much lower spring rates. The car would be pretty bouncy on stock dampers. Of course, now that you have to replace the dampers, you might as well make them height adjustable - oh, wait! We've just designed a Type Two (street sport) kit!
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      01-10-2010, 02:50 PM   #18
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Edit: By the way, when I measured mine, I concluded that my front bump stops touch (lightly) on the top of the damper when the suspension is static. Steve D and I disagree, and it' s more likely he's right than I am.
Here are a few pictures of my measurement. Mine came out to be around 3/4", probably because my wheels are fully turned during measurement.





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      01-10-2010, 02:54 PM   #19
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Originally Posted by ee8 View Post
Nice write up.

Just some info you may want to check out regarding type 2 and 3 kits with the possibility of rear coilover option. It´s a UK based operation called Nitron, they are known to be good in Europe. They have e46 and e92 M3 kits available.

Here a pic and link, maybe give them a call:
These look like kits for the E46. Any pictures for the E92?
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      01-10-2010, 04:09 PM   #20
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Could someone explain what the helper spring does? I thought it was to keep the main spring lightly compressed in case you got a wheel off the ground? But it sounds like it takes part in the total spring rate in JAJ's discussion.

I also know Orb had some issues with the KW kit, but he did some modification's to his kit.

.
A helper spring is a gap-filler. They're usually wound from ribbon instead of round stock. They're deliberately easy to collapse and have a low collapsed height. When extended, they keep the main spring in place so it doesn't slop around while the car is in the air on jacks or a lift. Once the car is on the ground, they're fully collapsed into short cylinders. They don't contribute to the "springiness" of the suspension.

From my read of ORB's report on his 335i, he modeled the suspension and computed the spring stacks that would give him the performance he wanted. He chose Swift springs with "longer-than-normal" travel and had some special mounts made to allow him to achieve both the desired suspension travel and resonant frequency that he wanted for his car. He very kindly gave me some spring recommendations that I may well implement after I've had a chance to shake down the out-of-the-box KW setup on the track.
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      01-10-2010, 04:47 PM   #21
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Great writeup JAJ! Very informative.

Do you think the internal progressive bump circuit and adjustable pressure in the Moton racing shock will help eek out the most perf. from the available travel in the front?
It's actually the spring choice that will have the largest influence on performance in the last two inches of travel. However, once you have the spring picked out, you need a damper that will control the motion of the sprung mass as you transfer weight around under braking, cornering and going over bumps.

Racing dampers have lots of adjustments, and if you get them right, they're awesome. However, I'm positive I don't have the tools to set them up properly, and so if I had them, my probability of disappointment would be pretty high.

The best resource I know of to answer your questions is Dennis Grant's blog:

http://farnorthracing.com/autocross_secrets.html

He's a bit harsh, taking the view that all dampers are junk until proven otherwise, but he's done a lot of damper dyno work and he's been kind enough to share his experience.
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      01-10-2010, 05:14 PM   #22
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Quote:
Originally Posted by rldzhao View Post
These look like kits for the E46. Any pictures for the E92?
I just used the pics they got on their website... don´t know if they are specific for the e92... but if not they shouldn´t be much different...

I´m seriously loooking into this kit depending if I need to do much modifying to acomodate the coil type in the rear.

Their topmounts also allow for camber and caster settings.

I just posted the link cause the op was saying their are not many kits on the market for the rear coil conversion... this is just another option already out.
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