The Rear Grip Thread: Ride Height, Spring Rates, Anti-Squat

[blake261]

Well, I found out why my car was straightening so easily, I thought it was the grip, and the grip likely had a lot of influence, but one of the couplers on my intercooler had slipped about 1/2 way out of the clamp, which made it look OK from the top, but a pretty big boost leak at the bottom [banghead]. Explains a lot of the lag. I noticed that the turbo sounded different, but I had just put a new turbo on so I thought it was just quieter.

I'm still going to go down to a 3.23/3.15 in the rear to lower the wheel speed.

[Liquidity]

That will raise wheel speed, not lower it. If you’re running 3.91’s now, you’re gonna need 4.10’s to lower wheel speed. The smaller the number, the taller your gears. The longhand version of the ratio is 3.91:1 - driveshaft rpm:axle rpm.

[blake261]

That will raise wheel speed, not lower it. If you’re running 3.91’s now, you’re gonna need 4.10’s to lower wheel speed. The smaller the number, the taller your gears. The longhand version of the ratio is 3.91:1 - driveshaft rpm:axle rpm.

I'm running 2.93 now.

[Liquidity]

Ah. Well I’d jump right to 3.38 if you can find one.

[blake261]

Well, I couldn't get my hands on a 3.15-3.45 in time for July 4th at AMP, so I threw my 3.91 back in.

It was much better with all the grip. 4th bogged a bit out of boost but made it through, and only ran out of gear on a long straight after a HUGE sweeper trying to no not manji and link them. I guess I could've shifted to fifth, but it lead into a tight 3rd gear hairpin and was too much to do in the cockpit while playing with the handbrake and foot brake to keep it sideways.

Thanks for all the help. The car feels much more controllable, but still fast. I also took a ton of caster and camber out of my front suspension, and softened it up overall. This made the front feel much more planted.

[blake261]

Update time.

Here's what I have found. If anyone is reading this still, lol.

3.15 is the best diff for me. Can clutch kick third gear at low speeds and not bog.

Grip should be split into two types, forward bite and side bite.


Forward Bite is affected by rear toe, ride height, tire pressure, dampening/compression, spring rate, weight transfer to the rear (front spring preload).

Side bite is affected by rear track width, tire pressure, dampening/compression, rear sway bar, front sway bar, rear ride height (Anti-squat)

Basically, I had way too much side bite and not enough forward bite. I took out side bite by raising rear tire pressure, raising my rear compression/dampening, and I will be adding my rear sway bar back in. This simultaneously gave me more forward bite, and more traction in the run up. with the rear sway bar in, I may be able to reduce rear dampening without inducing too much side bite. Before, I was rolling over Accelera 651's (stiff sidewall) and wearing the lettering off the side.

Most importantly, these changes allowed me to stay on throttle more. My car had so much grip in the front and the back that any correction/change I would make in drift would unsettle the car and i would have to come off throttle. even though I had tons of grip, the car was really difficult to drive. It is possible to have too much in our chassis! I guess this is what the pro's call "tight"

Along with the changes in the rear, I raised the dampening in the front a ton, and jacked the front tire pressure up to 35psi. I took out almost all my caster and camber to decrease camber gain and increase lock. This "numbed" the steering and made my car SO. MUCH. BETTER. it squats down in the back and stays there, allowing me to make small corrections without unsettling the car (transferring too much weight) and staying on throttle, always. It finally feels fast, but controlled and not violent. The steering self steers because I left some ackerman in the front, but have the SLR kit on the lowest setting with the control arms all the way forward in the well, with a little toe out. Rack stops keep me from hitting the sway bar or control arm.

Next, I will start messing with KPI to push the leading wheel into the ground and lift the trailing wheel to have less drag in drift.

[protomor]

Interesting to hear a numbed steering feeling making the car feel better. Generally you hear that causing issues with self steer. Seems like you found a good middle ground. What caster did you wind up with?

[blake261]

Interesting to hear a numbed steering feeling making the car feel better. Generally you hear that causing issues with self steer. Seems like you found a good middle ground. What caster did you wind up with?

I'm not sure, I've been trying so many changes that I bought toe plates and a camber gauge. I've been stringing up the car to zero and starting there. if I were to guess, maybe 3 degrees.

I have the SLR top plates, but their set like a wisefab setup. all the way out (toward the fender) and FORWARD one screw set (like many wisefabs). Wisefab uses kingpin offset to induce "fake caster".

that being said, I think with regular tires, my front would wash pretty bad. but the melting pattern on the front wheels looks like I'm getting a lot more contact in the center of the tire than I used to. this also may contribute to the self steer.

[protomor]

Kingpin offset meaning mechanical trail? I thought the wisefab kit lowered it while the SLR kit kept it the same as factory?
It does seem like mechanical trail does more for you than caster angle. At least in terms of keeping tire contact patch at angle.

[blake261]

Kingpin offset meaning mechanical trail? I thought the wisefab kit lowered it while the SLR kit kept it the same as factory?
It does seem like mechanical trail does more for you than caster angle. At least in terms of keeping tire contact patch at angle.

Yes, Mechanical trail. as I understand it, the Wisefab kit increases offset to be able to reduce caster, but I've just been looking at my friends 240's lol. SLR should keep the same as factory since it uses factory knuckles. I've read that '95 knuckles have more offset, which is why '95 did not get offset upper strut mounts, whereas 96-99 got reduced offset knuckles, and offset upper strut mounts.


Edit: I was confused.

KPI and Mechanical trail are different.

KPI is kin pin inclination (Fake camber)
Mechanical Trail is Kinpin offset (Fake Caster)

Changing the KPI relative to the camber makes the wheel turn in a negative parabola shape.Causing steering jacking (car raises when you turn). The other way to think about this is the car is pushing down on the wheel. if you have ackerman, it means the leading wheel will be pushed down more than the trailing wheel because it is not turned as far.

This is how the RTR's lift their trailing wheels. they also use strut mounted sway bar links to increase the effect.


Edit #2: I was confused, but right

Missed the offset post when I reread, so the information was correct before I edited lol.

But Good info NTL so ill leave it.

[protomor]

Yea I was a bit confused by that wording lol. IIRC the wisefab kit for bmws has the same bolt on style as SLR but they physically relocate both mounting points. Which causes a loss of mechanical trail. I'm not sure how either kit changes KPI. I think SLR doesn't. *shrugs* Without all of them in front of me, I get confused quickly.

[Piner]

As far as Mechanical trail. E30, E36 non m, E36 m3 '95, E36 M3 96-99 and E46 knuckles all have different geometry when it comes to trail.

For simplicity sake the E36 non M has the most mechanical trail built in due to spindle and balljoint position. The '95 M3 knuckle is next in line then 96-99 then E30 and e46 bringing up the rear. This is why e30 guys are always looking for double digit caster numbers where non-m e36 guys are happy with around 6*.

Its also why putting E36 non-M knuckles on an E30 brings your wheel position back so far that it causes rear clearance issues with your fenders/bodywork. Also why my vert having e46 knuckles and 5.5* of caster feels like numb and disconnected despite that number being fine with e36 non-m knuckles.

We do not change trail on the SLR steering plate but you certainly can by swapping your knuckles since the steering plates fit every combo.

[protomor]

What about KPI?

But yea, that's why I went from an e46 knuckle to an E36 M3. Kept the brakes but changed mechanical trail.

[E36M3LX]

so more mechanical trail is better?

I have been thinking about changing the knuckles on my e46 (with e36 struts) from 96-99 e36 m3 knuckles to non m e36 knuckles. My SLR lolli pops are adjusted way out to reduce the caster from the later m3 knuckles. I also have caster adjustment at the top hat and have that adjusted at its lowest caster setting.

[protomor]

Non M knuckles use smaller bearings and smaller brakes. I don't think you want those. "More trail the better" is... right I think? I don't know at what point there's too much.

[blake261]

Non M knuckles use smaller bearings and smaller brakes. I don't think you want those. "More trail the better" is... right I think? I don't know at what point there's too much.

More Trail means you can run less castor, which means you can have less camber gain > ~45* without sacrificing self steer.

[beat328i]

Non M knuckles use smaller bearings and smaller brakes. I don't think you want those. "More trail the better" is... right I think? I don't know at what point there's too much.

You're correct about the bearings, but 330 brakes are a direct bolt on to non-m knuckles so then you get more braking and more mechanical trailing... oem++

[protomor]

Are they? I thought it was only bolt on for M3 knuckles. Because I have E36 M3 knuckles on my 330 E46 right now. I didn't think E36 M3 brakes fit on non M knuckles.

[blake261]

Are they? I thought it was only bolt on for M3 knuckles. Because I have E36 M3 knuckles on my 330 E46 right now. I didn't think E36 M3 brakes fit on non M knuckles.

E36 M3 brakes do not fit on E36 non-m knuckles

E46 330i brakes, which are the same diameter as e36 m3 but thinner, fit on non-m e36 knuckles.

[protomor]

huh. that's an interesting compatibility combination. So 330 brakes will fit on both M3 and non M3 E36 knuckles but M3 E36 calipers won't work on non M3.