My E90 M3 Dedicated Track build (with pix!)

[dogbone]

Originally Posted by admranger

Gabe, have you thought about trying out the agar in Catalyst (no affiliation)?

Agar in Catalyst? What is that? Searched Google didn't find anything.

[admranger]

Originally Posted by dogbone

Agar in Catalyst? What is that? Searched Google didn't find anything.

Fvck auto incorrect.

Garmin Catalyst.

[dogbone]

Originally Posted by admranger

Fvck auto incorrect.

Garmin Catalyst.

Ahhhh——yesssss, I just saw one for the first time at Buttonwillow a couple weeks ago. Looked very interesting. Don’t know anything about it though.

[derbo]

Originally Posted by dogbone

Ahhhh——yesssss, I just saw one for the first time at Buttonwillow a couple weeks ago. Looked very interesting. Don’t know anything about it though.

It's a self-closed system that is aimed to give you actionable items for improvement based on its capturing of GPS/camera system to help you be a better driver.

https://www.garmin.com/en-US/blog/au...nce-optimizer/


Happy to see how well the car turns. Amazing!

[dogbone]

Figured I should make a note of this here since I'm starting to get inquiries:

I sold the JRZs at Chuckwalla last weekend. Someone there had an E90 M3 and was mentioning they wanted to get a suspension. So, I was able to work the deal out at the track and hand them over.

[dogbone]

Originally Posted by tsk94

Interesting you note the stiffer front springs, but how'd you feel the rears were? 728 in a coilover in the rear is equivalent to ~1450 in a divorced, that's almost 50% more rear spring then you were using at BW last weekend. I'm planning to go stiffer in the rear but not sure how stiff - how'd the big increase in rear spring rate feel?

I'm glad you asked this question because it's a good way for me to introduce a topic that has surfaced during this suspension upgrade.

The topic is: How do you calculate spring rate comparisons when comparing a divorced rear setup to a coilover rear setup?

It seems that the math to calculate the comparison is not universal----meaning it looks like people measure the physical dimensions and then use different formulas and get different results.

I've gotten 4 different numbers from 4 people about what the 13k (728lbs) rear coilover spring converts to on a divorced setup.

-tsk94 says 728lbs coilover ~ 1450lbs divorced
-Another forum member did the math and said that 728 lbs coilover ~ 1550 lbs divorced
-Casey at Racewerkz said that 728 lbs coilover ~ 1280 lbs divorced
-And someone else said that to achieve the equivalent of 1000 lbs divorced, I would need to run an 800lbs coilover....in essence saying that the 728 lbs coilover that I had did not even equal 1000lbs divorced.

This wide range of answers has me curious. What the heck do my 728lbs springs ACTUALLY equate to on a divorced setup? I really would like to know. So, I would love it if tsk94 would share his math and his reasoning behind it. I am also asking everyone else to share their math. I would love to learn more on this topic.

To answer your question about how the rear felt, it's more active than it used to be---but not harsh. It's not bad at all----just different. I've been joking with people that the only thing that was different at Chuckwalla was-----everything! That's easy to adapt to, right? hehe Let's look at what I mean:

-First, the track has been repaved so the track surface had more grip. This messes with your previous experiences of what the track felt like. Plus, the old braking markers on the side of the track are gone---replaced with cones that are in different locations than the old markers. So all that had to be relearned.
-The suspension was new.
-Spring rates new.
-alignment new.
-never been to Chuckwalla with Yoko slicks before.

haha that's quite a bit to digest all at once. But I really enjoyed it. I spent the whole weekend adapting my driving style to work with the new dampers. And by the end of the weekend, my confidence in the setup was building very nicely. I was willing to toss the car around pretty hard, and it just kept holding. I sent the ass out several times in the first few laps, but never spun. The only time I went off track was on a cool down lap when I was being lazy and dropped two wheels turning onto the back straight.

My JRZ suspension was setup to tend to understeer, and I drove it like that for 8 years. The MCS is setup so that it's much easier for the rear to rotate. fsmtnbiker made some tweaking recommendations based on my comments. I liked how it felt after the tweaks. Ultimately, the new MCS setup required me to change my inputs. An understeering car with toe-in on the front requires a fair amount of steering input. When I used that older driving style with this setup, the rear kept stepping out. Now, I need to turn the wheel less and a bit later because turn in is nice and sharp. Many people have commented about my videos that it looks much more hectic in the cockpit than my previous videos----saying that my previous driving looked docile and mellow. My take away from that is that an understeering car is mellower at the limit, and that a more neutral car requires more attention as you approach the limit. I also think that as I get more used to it, I may not need to be quite as active. We'll see. I definitely like the new setup. One example of how it's different: In the Bowl at Chuckwalla----the old setup would understeer in the Bowl, meaning if I was in the middle of the Bowl and gave it some solid gas pedal, the front would just plow and walk away up the Bowl. Now, if I'm in the Bowl, and I gas it, the rear can step out. That's new.

So, I spent the weekend learning and adapting my driving style to work with how the MCS's felt. I think by the end of the weekend, I was getting comfortable with how the rear acts. Moving forward, I could definitely drive it like this. But some people I have been talking to said the car may look a bit loose. And I don't think I want it to be too loose, especially at a place like COTA. So, I have ordered 12k springs and I'll be curious to see how that feels. I also have some softer springs for the front if I want to experiment with that. 850 is on there right now. I have 800 and 750 on-hand if I want to try that. Swapping springs is not very difficult--even trackside. I believe I will go to High Plains for the first time with the car in its current state. After I hit it at least once, I may try going to 12k in the rear as the first change to try.

I would also appreciate anyone else's input on how the math is calculated to compare coilover spring rate to divorced spring rate.

[SYT_Shadow]

There is only one math that's correct for the divorced rate equivalency. It's literally math and does not depend on opinions or feelings.

Insideline/Edmunds used to have great writeups where they would measure lengths etc.
With a measuring tape you can see what the old position was and what the full coilover is now.

Found it:
https://www.edmunds.com/car-reviews/...alkaround.html

[dogbone]

Originally Posted by SYT_Shadow

There is only one math that's correct for the divorced rate equivalency. It's literally math and does not depend on opinions or feelings.

Insideline/Edmunds used to have great writeups where they would measure lengths etc.
With a measuring tape you can see what the old position was and what the full coilover is now.

Found it:
https://www.edmunds.com/car-reviews/...alkaround.html

I looked at the article but it seems to only be a general write up of what's on the car.

Do you have an equation?

[SYT_Shadow]

Originally Posted by dogbone

I looked at the article but it seems to only be a general write up of what's on the car.

Do you have an equation?

I don't, because as usual I pay no attention to these things. If Bimmerworld mistakenly sent a piece of granite instead of a spring I would mount it and not think twice about it.

The article makes mention of the efficiency or motion ratio IIRC. The shock is 1:1, the stock spot is 0.56:1 or something like that. So that's the division.
I can't verify it's 0.56:1, this is just a random number. You'd have to measure distances of the silver alu arm where the spring is mounted.
But for this discussion let's assume it's 0.56:1. That means the 1000lb rate divorced spring is 1000*0.56=560lb at the shock which is what we said earlier was 1:1.
If you have a 750 full coilover rear spring, the equivalent divorced one is 750/0.56=1340

Edits ignoring the angle of the spring and shock:
Distance from car to divorced spring is 9.25, distance from car to shock is 12.8125
Equivalent divorced rate = 12.8125/9.25=1.385. So multiply your full coilover by 1.385 to get the divorced equivalent

[tsk94]

I've measured the motion ratios myself to confirm numbers I've seen online. The divorced motion ratio is 0.576 and a rear coilover motion ratio is 0.819.

All you need to do is calculate the wheel rates for a given spring rate and compare. A 1450 divorced spring is roughly equivalent to a ~730lb spring in a coilover at the wheel - which is really all that matters when looking at spring rates.

I can tell you with absolute certainty that a 730lb rear coilover is NO WHERE near a 1000lb/in divorced spring, it's mathematically not possible.

I'm not sure why there's so much confusion on this subject as it's relatively simple to calculate:



Above are the calculation for the wheel rates of a 728lb/in rear coilover spring - resulting in a 488lb/in wheel rate and a 1450 lb/in rear divorced spring. As you can see the wheel rates are very close. The conversion between divorced and coilover spring rate is roughly 2:1* - again, easy to see if you do the math.

*Importantly these calculations do NOT take into account spring angle, however the spring angle doesn't make a huge difference. The angle of the rear coilover will slightly reduce the effective wheel rate, meaning the actual conversion ratio is slightly less then 2:1. The divorced spring angle is quite low so its impact will be fairly small.

[tsk94]

If someone can provide me with the spring angle of a divorced setup I can calculate a very precise conversion, but roughly 2:1 is pretty accurate.

[tsk94]

Originally Posted by SYT_Shadow

I don't, because as usual I pay no attention to these things. If Bimmerworld mistakenly sent a piece of granite instead of a spring I would mount it and not think twice about it.

The article makes mention of the efficiency or motion ratio IIRC. The shock is 1:1, the stock spot is 0.56:1 or something like that. So that's the division.
I can't verify it's 0.56:1, this is just a random number. You'd have to measure distances of the silver alu arm where the spring is mounted.
But for this discussion let's assume it's 0.56:1. That means the 1000lb rate divorced spring is 1000*0.56=560lb at the shock which is what we said earlier was 1:1.
If you have a 750 full coilover rear spring, the equivalent divorced one is 750/0.56=1340

Edits ignoring the angle of the spring and shock:
Distance from car to divorced spring is 9.25, distance from car to shock is 12.8125
Equivalent divorced rate = 12.8125/9.25=1.385. So multiply your full coilover by 1.385 to get the divorced equivalent

That conversion rate doesn't hold when you calculate wheel rates, which is how you should be comparing spring rates:

*Again ignoring spring angle*:
Coilover: 1000 * (0.819)^2 = ~670lb/in wheel rate
Divorced: 1385 * (0.576)^2 = ~460lb/in wheel rate

Big difference in the effective wheel rates. Feel free to do the math yourself, you need a very stiff divorced spring to get to the equivalent wheel rate of a 1000lb/in coilover spring.

[fsmtnbiker]

Just a +1 here on tsk94’s numbers. I have very slightly different numbers for each motion ratio due to slightly different track width (wheel size/offset) but the math is so minutely different it’s not worth mentioning.

FWIW, my car was great on 670/1230 and also 560/1120 currently, so these numbers and the front to rear split line up pretty well with what I saw in your video. I think trying a slightly softer rear spring will likely help calm down the inputs a bit and increase confidence on corner exit - probably enough that you may undo the shock change I suggested to loosen the car back up on entry but have more stability mid corner and corner exit.

[tsk94]

Originally Posted by fsmtnbiker

Just a +1 here on tsk94’s numbers. I have very slightly different numbers for each motion ratio due to slightly different track width (wheel size/offset) but the math is so minutely different it’s not worth mentioning.

FWIW, my car was great on 670/1230 and also 560/1120 currently, so these numbers and the front to rear split line up pretty well with what I saw in your video. I think trying a slightly softer rear spring will likely help calm down the inputs a bit and increase confidence on corner exit - probably enough that you may undo the shock change I suggested to loosen the car back up on entry but have more stability mid corner and corner exit.

That's a good point and I'm glad you mentioned that. The motion ratios will vary slightly depending on an individuals wheel and tire setup, but like you mentioned it's usually quite small.

I made sure to state these are 'rough' numbers as everyone's exact values will vary slightly, but in general I'm confident my calculations are pretty accurate.

[dogbone]

I just went out to my E92 M3 with divorced setup and did the measurements for myself.

I got 18.625" from the pivot point to the middle of the wheel.
I got 15.25" from the pivot point to the middle of the control arm bolt where the coilover attaches.

My wheel rate for coilover matches the tsk94's 0.819 almost exactly. I got 0.818.

All good so far.

However, my divorced wheel rate is suuuuper different. I tried measuring from the pivot point to the center of the divorced spring. Am I supposed to measure somewhere else?

[rhyary]

Can someone explain "wheel rate"

[tsk94]

Originally Posted by dogbone

I just went out to my E92 M3 with divorced setup and did the measurements for myself.

I got 18.625" from the pivot point to the middle of the wheel.
I got 15.25" from the pivot point to the middle of the control arm bolt where the coilover attaches.

My wheel rate for coilover matches the tsk94's 0.819 almost exactly. I got 0.818.

All good so far.

However, my divorced wheel rate is suuuuper different. I tried measuring from the pivot point to the center of the divorced spring. Am I supposed to measure somewhere else?

What motion ratio did you end up with for the divorced setup?

My calculations are as follows:
232mm/402.5mm = 0.576 MR

It sounds like you did it properly, which is why I'm curious what numbers you ended up with? You'd want to measure from the pivot point of the lower control arm, where it attaches to the subframe, to the center of the spring, divided by the same overall length you used earlier. Since you got the same coilover MR the overall length you used must be right and perhaps the distance from the center of the spring to the control arm pivot point was off? Because otherwise it sounds like what you did is correct.

Edit:
You seem to have gotten much larger numbers for the pivot points then I did, but it's offset but a larger distance to the center of your wheel - strange.
For the coilover calculation I got the distance from the pivot point to where the shock bolts up to the lower control arm at 330mm, much shorter then your calculation of 15.25" or 387mm
Using the same overall length as above (402.5mm), 330mm/402.5mm = 0.819 for the coilover MR.

[dogbone]

Originally Posted by rhyary

Can someone explain "wheel rate"

This link is a decent start to the idea.

http://performancetrends.com/Definitions/Wheel-Rate.htm

[tsk94]

Originally Posted by rhyary

Can someone explain "wheel rate"

Wheel rate is your effective spring rate at the tire. The location of the spring relative to the center of the wheel will vary in every vehicle, and obviously vary depending on whether the spring and shock are in the same location (coilover) or a separate (divorced).

The actual published spring rate means absolutely nothing if the motion ratio is not taken into account. The wheel rate is what is important as it is the spring rate the tire sees, which is what matters for determining handling balance and grip.

Like discussed above, a coilover and divorced spring rate will vary drastically in order to achieve the same wheel rate, and wheel rate is ultimately what one should be concerned about when choosing spring rates.

[Kelse92]

Is there any good place out there on the interwebs to do the calculations in a template format?
I've been wanting to run a few scenarios on my car and numbers but I am not math savvy and not really interested in doing a spreadsheet of my own right now, I look at too many spreadsheets at work already lol

[tsk94]

Originally Posted by Kelse92

Is there any good place out there on the interwebs to do the calculations in a template format?
I've been wanting to run a few scenarios on my car and numbers but I am not math savvy and not really interested in doing a spreadsheet of my own right now, I look at too many spreadsheets at work already lol

Are you wondering for wheel rates? The calculation is pretty simple and you could do it even on a phone. It's just spring rate * MR^2 = wheel rate.

Front MR: 0.956
Rear MR (divorced): 0.576
Rear MR (coilover): 0.819

I could make a public google sheets spreadsheet I could share if people are interested. You'd just need to input the spring rate and it'll spit out the wheel rate for you. Calculating wheel frequencies is much more difficult as it needs accurate corner weights, unsprung weights and wheel rates - I could also include that but people would need to know their corner weight and rough unsprung weights to make it accurate.

[rhyary]

Originally Posted by dogbone

This link is a decent start to the idea.

http://performancetrends.com/Definitions/Wheel-Rate.htm

Thx.
Now the discussion makes sense