Rake information

[aeronaut]

Just to be clear, it's not perfect. There are several turns at the tracks I visit (Summit Main, Shenandoah, Jefferson, Dominion, VIR) that cause understeer no matter what I do. Off-camber, decreasing radius, bad technique, etc, I have to manage some understeer.

[Bobkid]

Great info - Do you like the way your car handles?

Mine, with that much rake, won't get off of the corners - instead of gripping and leaving, it just goes to oversteer. It's also not great in transitions as the rear wants to come around a lot.

Of course we all know mine's a 328 and something is wrong with the front.

When I played with the rake on my coupe, where I could really feel the difference was in how the weight transferred front to back. Too much rake (>1.5") and the weight was slow to go rearward with power inducing some nasty oversteer; too little rake (<1/4") and it transferred too quickly and I got bad mid-corner understeer. Anything in the middle (1/2" - 1") and it was predictable and fairly linear in how it transferred. Too much rake also made mine very tail-happy under hard braking, making it tougher to get the car stable entering the corner and taking a set.

[JBasham]

I do like the way it handles. In steady state (long constant-throttle corners) it's dead neutral. On corner exit I can 'easily' get throttle oversteer on 2nd gear turns. On 3rd gear exits under full throttle, I can feel the rear end 'set' out just a little (as if all the slack in the suspension is preparing for oversteer), meaning if I had more power, I could easily get manageble oversteer in 3rd gear corners too. I'd like it to be a LITTLE easier to rotate on trail brake entry, but it's all a trade off.

FYI, the car was predominately an understeer car until I moved the front UUC ARB from full stiff to full soft. That change really made the car balanced.

Rough numbers:
95 E36 M3 used for HPDE/instructor (not race)
Weight is OEM minus rear seat stuff.
-3.5 camber front, -2.5 camber rear.
0 toe up front, 1/8" toe in rear.
Toyo RRs.
TCK SA shocks.
550/650 springs.
Front UUC ARB at full soft, rear OEM ARB.
All bushings in the suspension are OEM except for the LCAB, which are VAC poly.

Great info -- can I ask you specifically about your experience on the turn 6- turn 7 stretch on Summit Main?

I am trying to set up an E36 325is with stock LCAs. Like you, I needed 2mm washers on the strut/spindle bolts to get my negative camber out to ~ -3.25; my rear is probably a little more negative than yours, like -2.8, but not hugely different.

In that sweeper it feels like I get my choice of terminal understeer, terminal oversteer, or slow travel.

I'm used to being at about 40 mph at the apex of 6 and up to about 60-65 mph at the apex of 7, in two other cars.

With this E36, it's more like starting at 40 and then apexing 7 at something in the low-to-mid 50s!

Are you picking up the usual 25mph on your set-up?

Maybe I need more spring.

[aeronaut]

The big question, what tire are you running?

Looking at my data, this is from a 1:28.x lap, so one of my quick ones. On Toyo RRs.

Slowest speed in T5: 46.3
Fastest speed between T5-T6: 58.3
Slowest speed in T6: 43.5
Speed at apex in T7: 61.3
Speed at apex in T8: 72.4

T6/7 at Summit Main is certainly a picky bastard. Lots of patience needed to get neutral car dynamics through there. My car's now pretty neutral in that section. Assuming you're on line and not rushing, sounds like you may be able to tweak suspension for better time. Also, small changes in the line in T6/T7 make a big difference.

If by chance you happen to be there tomorrow for FATT, I'd be happy to ride with you (I'm an instructor for FATT) or give you a ride in my car.

[CoMZ302]

When I played with the rake on my coupe, where I could really feel the difference was in how the weight transferred front to back. Too much rake (>1.5") and the weight was slow to go rearward with power inducing some nasty oversteer; too little rake (<1/4") and it transferred too quickly and I got bad mid-corner understeer. Anything in the middle (1/2" - 1") and it was predictable and fairly linear in how it transferred. Too much rake also made mine very tail-happy under hard braking, making it tougher to get the car stable entering the corner and taking a set.

This is awesome thanks.

I'm watching the video from the car behind me (for four laps) and lots of power on oversteer.

As (I think) I noted earlier, my tow-hook-measured rake is about 1.25"

I think the mid-corner push is something completely different - in fact I'm pretty sure it's Caster, but that's another thread

[=BA=]

This is awesome thanks.

I'm watching the video from the car behind me (for four laps) and lots of power on oversteer.

As (I think) I noted earlier, my tow-hook-measured rake is about 1.25"

I think the mid-corner push is something completely different - in fact I'm pretty sure it's Caster, but that's another thread

Raising the rear has very little to do with the amount of weight transfer to the rear. Think about it, increasing rake by 1/2" will raise COG by 1/4" or less as most E36 are front biassed.

What you are actually doing is raising the rear roll centre. Ie the rear gets less prone to rolling and similar to stiffening up the rear springs, it will get more oversteery.

[CoMZ302]

Raising the rear has very little to do with the amount of weight transfer to the rear. Think about it, increasing rake by 1/2" will raise COG by 1/4" or less as most E36 are front biassed.

What you are actually doing is raising the rear roll centre. Ie the rear gets less prone to rolling and similar to stiffening up the rear springs, it will get more oversteery.

Couldn't agree more. Also when the front and rear roll centers differ greatly, as in, the front has the rollcenter correction, the rear is jacked up, the problem is even worse.

I think I know which direction I'm going - thanks for the feedback.

[JBasham]

The big question, what tire are you running?

Looking at my data, this is from a 1:28.x lap, so one of my quick ones. On Toyo RRs.

Slowest speed in T5: 46.3
Fastest speed between T5-T6: 58.3
Slowest speed in T6: 43.5
Speed at apex in T7: 61.3
Speed at apex in T8: 72.4

T6/7 at Summit Main is certainly a picky bastard. Lots of patience needed to get neutral car dynamics through there. My car's now pretty neutral in that section. Assuming you're on line and not rushing, sounds like you may be able to tweak suspension for better time. Also, small changes in the line in T6/T7 make a big difference.

If by chance you happen to be there tomorrow for FATT, I'd be happy to ride with you (I'm an instructor for FATT) or give you a ride in my car.

Thanks for the offer. Sorry I missed it; I was over at Shenandoah that Friday for BMW instructor day. I'm having these issues on sticky-ish tires and your times are much better than mine through that section. And I don't think I'm blowing the arc through the section. I'll be there Tues-Weds with Drivers Club and a box full of springs, so I'll see what I can work out.

[fbirch]

If your front and rear lift points are perfectly level (same height above the ground) will the front and rear tow hooks also have the same height off the ground? Reason I ask is that I'm getting some strange results when trying to measure rake at the lift points. I think some of the metal there may have deflected a bit from so many lifting cycles. If the tow hooks are level, the longer separation would make them an easier reference point to measure anyway.

For example, if the lift points are 4 feet apart and we're supposed to have a 1/4" height differential there, the the 14.5 foot vehicle length would mean we should measure a 0.9 inch height differential at the tow hooks.

[aeronaut]

Just going by a picture, relative to the center of the wheels, the rear tow hook looks to be several inches lower than the front tow hook.
I've never read anything that uses the tow hooks as a datum reference. I *think* both tow hooks are part of the bumper mounts (they certainly are up front), and are not 100% fixed relative to the frame.

EDIT: But, as long as you know your car, you could use the tow hooks as a *relative* datum to make adjustments. Understanding that bumping something, or actually getting towed, might change that reference point.

[fbirch]

Thanks for the info. Without knowing the nominal offset for tow hook height, I can't use them as a reference. I've read other threads that that use wheel-center-to-fender-lip distance as another way to quantify rake. Because the rear fender dips down lower relative to the wheel, the front measurement should be about 1 inch to 1.25 inches higher than the rear, if I remember correctly.

I'd appreciate it if someone could tell me the rake on their car at the jack points and also the rake as measured from wheel center to fender lip. Knowing the difference in separations between those measurement points, simple geometry would allow me to work backwards to rake at the lift points.

[aeronaut]

My car is on the trailer headed to the track, but I can get this for you early next week if no one else has.

[fbirch]

Thanks! I found one reference in an old thread - see post #3.

https://www.bimmerforums.com/forum/s...r#post21963848

Would appreciate an indpendent confirmation.