Autocross nerd thread: The Ultimate E9X A-Stock SCCA Autocross Build?

[kitw]

Originally Posted by CSBM5

Rich,

How is your front bar working out? I tried to run some numbers on the stock versus various aftermarket options, but the stock bar is hollow, and without cutting one in half, I don't know the inside diameter. So I ran across a FAQ on Turner's web site on the H&R front bar where someone supposedly did the calculation for a 27mm solid H&R versus stock, and they claim that one is 1.425 times the stiffness of the stock one. I assume the moment arm is equivalent to the stock bar.

I'm looking at all of this since I ended up buying a Dinan 28mm solid front bar on sale for 40% off, and I'm wondering if this will prove to be too much front bar on street tires. Interestingly, Dinan adds this bar to their spring kit (i.e. moving from their stage 1 to stage 2) without changing the rear bar, so clearly a car with just their springs versus springs + front bar is going to be a good bit different, but they must feel it isn't too much bar. All that is an aside...

So if I assume Turner's H&R vs stock calcs are correct, then the Dinan 28mm bar (same moment arm as stock) should be: 1.4252 x (28^4 / 27^4) = 1.65. Hence it is 65% stiffer than the stock bar.

The stock E93 bar is 28mm arms with a 26mm middle section. Just about impossible to accurately model without FEA I suspect. The stock E90/E92 bar is 26mm or thereabouts, looks like constant diameter.

the Dinan 28.5mm bar is a whole hell of a lot stiffer because it's a solid bar. I was originally just going to toss the E93 bar on but went with the stiffer Dinan bar... and found that it indeed makes the car understeer a decent amount, even with -2 degrees of camber. I may try the E93 bar on the new car.

[Richbot]

The RDSport bar is quite a bit stiffer than either, I have it set at full stiff on my car

[CSBM5]

Thanks kitw. I definitely can see some steady state understeer, but this thing is a beast in a slalom (granted I'm under-tired at the moment with 245 Rivals all around, lol) with great balance. 95% of the course I ran it felt great with only mid-corner push on a large turn-around spanning the width of the runway.

I'm going to run this season as it is I think and wait to see where the cards fall for 2013 as I might just then need to figure out what adjustable rear bar to install if the rules go through similar to proposed.

Anyone have any thoughts on camber plates that work with stock springs that won't raise ride height? I noted Vorshlag's notes on their plate here: http://www.tcklineracing.com/webdocs...Details848.cfm

[kitw]

For anyone looking for a beefy front bar, Dinan has their's on sale:

http://www.dinancars.com/shop/D120-0...ar.aspx#page=1

$257! Not too bad.

[kitw]

Originally Posted by CSBM5

Anyone have any thoughts on camber plates that work with stock springs that won't raise ride height? I noted Vorshlag's notes on their plate here: http://www.tcklineracing.com/webdocs...Details848.cfm

The Ground Control street plates are supposed to accomplish this. Not sure if they do or not, haven't run them personally or measured them.

[CSBM5]

Originally Posted by kitw

The Ground Control street plates are supposed to accomplish this. Not sure if they do or not, haven't run them personally or measured them.

I have the Ground Control street camber plates on my E39 M5. Many years ago I bought them to use with the stock springs after GC assured me that they wouldn't increase ride height. I installed them and sure enough the ride height went up a bit over 1/4". It looked like crap. I went to the Dinan springs after that solving the ride height issue. Hence I don't trust what GC says in this regard. Hopefully someone will have measured the stack height of the stock hat versus the GC setup?

[J!M]

I think Vorshlag released an OEM spring/strut camber plate recently:

http://www.vorshlag.com/product_info...roducts_id=535

From that page:

This camber plate can only be used with OEM diameter springs (OEM springs, lowering springs, some coilovers) and can be used with virtually any strut type.
...
Designed specifically to match the factory strut height, these do not alter ride height!

I'm a slow, and not a very committed autocrosser, but I like the new proposed stock rules. Mostly because I personally prefer to keep my cars close to stock, and when I do make changes, the first things I have changed in the past are camber, sway bars and sticky-ish street tires (Z1, RE11, etc).

[CSBM5]

Originally Posted by J!M

I think Vorshlag released an OEM spring/strut camber plate recently:

http://www.vorshlag.com/product_info...roducts_id=535

From that page:


I'm a slow, and not a very committed autocrosser, but I like the new proposed stock rules. Mostly because I personally prefer to keep my cars close to stock, and when I do make changes, the first things I have changed in the past are camber, sway bars and sticky-ish street tires (Z1, RE11, etc).

That's the one I was trying to find again. Thanks for the link.

[kitw]

Maybe it's just a bad picture, but they appear to increase the stack height substantially.



well, or maybe not:

[Be Stock]

Having the same total height as stated in the picture doesn't mean anything if the springs are compressed different amounts. The strut on top appears to have more spring compression as would be expected with a taller stack. The springs on both struts will reach the same compressed length when loaded. The strut with the taller stack would result in a higher ride height.

Originally Posted by kitw

Maybe it's just a bad picture, but they appear to increase the stack height substantially.



well, or maybe not:

[CSBM5]

Yeah, I think the pictures are deceiving and hard to draw accurate conclusions from; however, I'd trust Vorshlag claim...at least enough to buy and install them (carefully measuring things along the way). Speaking of which, in the proposed SCCA rules, since they are allowing a change in suspension mounting (i.e. camber plates), I wonder if they will now have to enforce ride height measurements as I can imagine someone doing a custom fixed camber plate/spring hat with a very low stack height.

[m3kerry]

That is a grey area. You can convert coilovers with the stock spring perches and still have ride height adjustment. I have not read the rules in a while, but i think you get a 1/4 or 1/2 inch.

Rich can explain it better than me.

[Richbot]

It's not gray at all, it's black and white in the rules, see 13.5.4. The distance between the mounting points of the shock and the spring load bearing surface must be exactly as stock under the current ruleset. This means that you cannot change "ride height" by changing where the spring sits relative to the body or the suspension. However, there are plenty of ways ride height can be altered legally. Shorter tires, nitrogen pressure, extreme rebound damping, etc., so a "stock ride height" requirement is not a good idea and is not enforceable. The allowance for ride height to change in 13.5.2 is in anticipation of the other factors which influence ride height that I mentioned

The ONLY play in the Stock-class rules dimensionally is for EXTENDED length of the damper itself, you get +-1" to allow for different damper designs, which has ZIP to do with ride height. This is a very commonly misconstrued rule. It does not allow you to run 1" lower than stock, or +-1" where the spring perch is, it only has to do with how long the damper is when fully extended, which is usually irrelevant once the car is on course and therefore is allowed to accommodate aftermarket dampers without allowing extreme shortening or lengthening

And remember, if it doesn't say you can, then you can't.

Which all has absolutely nothing to do with the proposed rules for camber plates because camber plates aren't currently allowed. The location of the upper "spring load bearing surface" as contemplated in the current rules already changes depending on some of the same factors I mentioned already, but up until now we haven't had to worry about the dimension changing due to some modification because we required people to use the stock upper strut mount.

But doubt the rules will go forward as proposed. Stay tuned for the next revision next month.

[lacartus]

Anyone know what length adjustable end links to get for the front RDSport bar to take preload out? Assuming a bit longer than stock, anyone know what they bought in millimeters? Rich?

Also, anyone use the Turner seat adapters? http://www.turnermotorsport.com/p-174116-e9x-m3-front-sway-bar-link-seat-adapter.aspx

[Richbot]

I'm using the Turner 310-355mm links, which are close to stock but not as long. The seat adapters are a nice feature, I think the GC endlinks have those too.

[Richbot]

Woops. Car is wider than I remember

Can't wait to get on street tires though. I'm really starting to envy the people who are showing up on tires they changed in their garage the night before or whatever rather than doing it in the paddock after chasing cones all day

I think I'll be trying the 275/295 Rival combo once I kill these A6's. I took a close look at various 255 and 275 street tires mounted on 9" wheels at the event and I don't think the 275 Rival will be pinched much on an 8.5, but we'll see. If I get really bored I'll try them against a 245/275 setup and see which is faster

[CSBM5]

Same here. I'm still running on 245/40 Rivals all around since the others won't be available for a while yet still. I love these tires. I would really enjoy testing 245/275 vs 275/295 on the stock width wheels of course. I wish you were closer Rich as we could swap out wheels/tires and test this out if you get the 275/295s.

I've got a range of opinions from extremely experienced autocrossers to tire engineers on whether street tire stuffing on stock width wheels will be as beneficial as A6s -- clearly it won't at the extreme (like your 295/315s), but it's the middle ground where it seems to be undefined. I guess we'll find out this year. I would also guess that different street tire designs will react differently as you allow tread width to exceed wheel width by ever greater amount

Oh, and I LOVE showing up at events and not having to fool with changing tires...especially at the 100F heat index days. I've been running STX co-driving with my daughter in her E46 for a couple of years now, and it sure has been fun. Same now with the M3 -- I change tires/wheels in the garage the day before an event.

[Richbot]

That's what I'm thinking too. Sure going +50mm on a tire that already runs wide, as I've done with the A6's, is probably not going to be that great an idea since the A6's are much more cantilevered than any street tire I've seen. But maybe +30mm isn't so drastic, it's only 0.5" inside of the recommended rim width and especially using a 35 aspect ratio instead of a 30 it might not be so bad, and nothing has changed with respect to a bigger tire being able to handle more heat, wear better, have more ultimate grip, question is whether the slow response due to squish and undersupported carcass allow enough squirm to be a dealbreaker on ultimate grip

Running as seldom as I will this year I'll probably be able to nurse my A6's for a while

[lacartus]

Originally Posted by Richbot

I'm using the Turner 310-355mm links, which are close to stock but not as long. The seat adapters are a nice feature, I think the GC endlinks have those too.

Thanks Rich. I installed the RD Sport front bar and the Ground Control adjustable end links. I just missed being able to install them for a test and tune on Saturday, but had time to take them out into an empty parking lot today.

I extended the GC end links to match the length of the OEM end links and had to turn them out one turn on each side to install them with no pre-load using the holes furthest in on the bar. This leaves 8 threads of purchase within the aluminum rod. The links are mounted like the OEM, top nut is on the tire-side and bottom nut is on the engine side. I applied ample grease to the bushings on Rich's recommendation in this thread.

Observations: On the stiffest setting, the inside rear wheel does not appear to lose as much traction (e.g., lift on a decreasing radius turn) and I think I'm able to apply more throttle earlier coming out of a turn.

The bar slides a little from side to side within the bushing mounts, just enough that it will tap a frame piece on the passenger side -- barely, but it makes contact. On the driver's side it doesn't hit as I push on it on level ground, but there is only about 2mm of clearance. When I swerve I hear a tap, loud enough to hear with the windows open, but not with them up. The pictures of the bar below show the clearance if I push the bar to one side and take a picture of each side. I think if I shortened the end links there would be more clearance, but it would also be slightly pre-loaded.

Questions for anyone with the RDS front bar:
1) Does the RD Sport front bar make a noise on your car (e.g., tapping sound)?
2) I assume the holes at the end of the bar are the softest settings, right?

Thanks!
Ian

Passenger side bar clearance

Driver side bar clearance

Passenger side end link

[Richbot]

Yes, longer moment arm = less resistance to roll

I did notice any clearance issues on mine but maybe I did not look closely enough? I don't have any tapping noises

[lacartus]

Originally Posted by Richbot

Yes, longer moment arm = less resistance to roll

I did notice any clearance issues on mine but maybe I did not look closely enough? I don't have any tapping noises

Thanks again Rich, I'll shorten/tighten the end links gradually and see if that gets rid of the tapping noise.

[CSBM5]

Another outstanding experience with the M3 at an autocross at NCCAR where the pavement is perfect and extremely grippy. I love these Rivals, and they are wearing well. I managed to pull off 1st place PAX out of ~95 cars -- really loving the M3 in F-stock. I'm continually amazed at how well this car puts down power coming off elements. I had to execute the 1-2 shift in the middle of a two gate complex, and it stuck much better than I was giving it credit for (too much experience with the M5 wanting to bust the rears loose any chance it gets I guess -- M3 wins on a more useable torque curve).

I dropped almost a full second on my last run by super late apexing the sweeper at the far end of the course. It felt "slow" being wide down there, but I got way around the back side of the last pointer cone and was able to rotate well and put down power well coming off it.

Did I say I love the E90 M3 on an autox course?

https://www.youtube.com/watch?v=c5VpjPytJis

More videos of the next autocross coming soon.

Chuck