Higher spring rates w/o rear bar or not?

[illinipo]

On street tires with lower max G, we can get away with a bit higher roll gradient, so 600 no bar works fine. (And, IIRC, this thread is about DE with low end R compounds close to the grip we have on 140 UTQG streets... hence the recommendation.) If I had to change classes to BSP tomorrow and run R compounds, I would certainly consider a bar, but first would try more spring alone to see if the tires could handle the extra rate. Of course this would not be a shot in the dark on which springs to use, I'd have the spreadsheets to point me in the right direction and then the testing to see if they were right

[95maxrider]

Hi everyone, sorry to bump an old thread but it seemed to be the most relevant to my question, so I figured I would give it a try.

I rallycross my 98 M3/4/5, and I need some advice about what to do with my spring rates when I remove both sway bars. The car is on JVAB offroad Bilstein-based shocks, and has/had stock sway bars. Front springs are 12" 250 lb, rears are 7" 350 lb (although I'm probably going to try and squeeze some 8" springs in there). I recently removed my FSB in an effort to get rid of understeer when on course (it helped), and I think I would like to also remove the rear to help me put down power and get better wheel articulation. I believe that I will need to raise my spring rates to compensate for the lack of sway bars, but I'm not an engineer and can't calculate how much I should change. If I had to guess, I would start with 300f/400r, but it's just a guess. Can anyone comment on how I should go about this? I can't exactly buy a bunch of different springs and try them all out, because that would be too expensive on my tight budget.

Thanks in advance!

[jim13x]

My 98 M3/4/5 is asphalt-only. Removing my rear bar seem to make 100 - 150 lb difference in effective spring rate (with 600 lb rear springs). I ended up increasing the rear springs to 800, I may even go to 900 now that I'm comfortable with a livelier rear end. Front is 750 with a big bar. I have no idea about setting up the car for rallycross, but it sounds like you are on the right track.

[95maxrider]

My 98 M3/4/5 is asphalt-only. Removing my rear bar seem to make 100 - 150 lb difference in effective spring rate (with 600 lb rear springs). I ended up increasing the rear springs to 800, I may even go to 900 now that I'm comfortable with a livelier rear end. Front is 750 with a big bar. I have no idea about setting up the car for rallycross, but it sounds like you are on the right track.

This is exactly the type of response I've been looking for, thank you! I take it you're still running a FSB?

[jim13x]

Correct, the front has a big Ground Control bar. Rallycross sounds fun, you should post a video.

[95maxrider]

Correct, the front has a big Ground Control bar. Rallycross sounds fun, you should post a video.

Here you go, don't judge too hard, I still haven't figured out how to drive this thing yet!

[jim13x]

That's pretty bad-ass. The exhaust sounds good too.

[95maxrider]

That's pretty bad-ass. The exhaust sounds good too.

It's a whole lotta fun! Thanks, the car came with some no name cat-back.

[jakermac]

Here you go, don't judge too hard, I still haven't figured out how to drive this thing yet!

What steering rack do you have? We can't see your hands in the video but I imagine that if you're still using the OEM M3 rack that your hands are flailing about pretty wildly on the corners. Have a look at a Z3 1.9 steering rack. That'll make a huge difference in the steering speed/lock to lock. And if you want to go even further (this is where I'm at) you can combine that with the SLRSpeed Roll/bump steer correction mini kit that boosts steering speed even more. I'm at about 2.3 turns lock to lock (I think, memory's failing me). Makes it super easy to catch a drift/oversteer and really minimizes the steering inputs.

Correct, the front has a big Ground Control bar. Rallycross sounds fun, you should post a video.

Which bar? .120, .155 or .188"?

I'm on the .120" with 800fr/900rr no rear bar, and I think I need a bit more front bar. I'm getting inside rear wheel lift (usually at the same time as the front, so bicycling).

[jim13x]

Unfortunately I'm not sure, I'll have to measure it the next time I'm at the garage. The bar was on the car when I bought it.

I wish I had that Z3 steering rack. This is me flailing about:

95maxrider: I have the SLRSpeed Roll/bump steer correction mini kit new in the box in Silver Spring if you are interested. I was thinking of building another car but the plans are on hold for now.

[95maxrider]

What steering rack do you have? We can't see your hands in the video but I imagine that if you're still using the OEM M3 rack that your hands are flailing about pretty wildly on the corners. Have a look at a Z3 1.9 steering rack. That'll make a huge difference in the steering speed/lock to lock. And if you want to go even further (this is where I'm at) you can combine that with the SLRSpeed Roll/bump steer correction mini kit that boosts steering speed even more. I'm at about 2.3 turns lock to lock (I think, memory's failing me). Makes it super easy to catch a drift/oversteer and really minimizes the steering inputs.



Which bar? .120, .155 or .188"?

I'm on the .120" with 800fr/900rr no rear bar, and I think I need a bit more front bar. I'm getting inside rear wheel lift (usually at the same time as the front, so bicycling).

Yup, stock rack, hands flailing! The Z3 rack is the last big item on my to-do list, but with the money that went into replacing the motor it got pushed down the road a bit. What's the deal with the bumpsteer kit, any side effects or trade-offs? I put one of those on my other car at one point and removed it after 12 hours because it made the car darty like a go cart and unnerving above 50 mph. Would you run it in your DD?

EDIT- Just looked up the kit, looks nice, but very pricey.

[jakermac]

I put one of those on my other car at one point and removed it after 12 hours because it made the car darty like a go cart and unnerving above 50 mph. Would you run it in your DD?

Was trying to find an old post I made about this but alas, the search function on this forum sucks. I don't understand people that have issues with steering feeling darty. I have no issues with this in my car. It is my daily driver (though not at this particular moment, the engine is out and in many many pieces). Even at 120+ MPH it is not darty. But then I'm not a fidgety crack whore!! ;p

[95maxrider]

Was trying to find an old post I made about this but alas, the search function on this forum sucks. I don't understand people that have issues with steering feeling darty. I have no issues with this in my car. It is my daily driver (though not at this particular moment, the engine is out and in many many pieces). Even at 120+ MPH it is not darty. But then I'm not a fidgety crack whore!! ;p

Good to know. I'll start with a Z3 rack and see if I need more.