Dealing with on-power Push

[codingparadox]

Hey, all. I managed to pick up Jeff's Elise early enough to run 3 full events before the season ended. They were all on quite cold days, though, so the tires only really got up to temp on one of the afternoon runs (this is with double-driving, with BWR bags). Either way, I'm wondering what the best way to deal with on-power push is.

The car has DA Ohlins, valved by Matthew Braun. In its current setup, it's certainly drivable, and drivable fast, but I feel like it would be a lot faster if I could remove some of the push.

The first question is "what is the current alignment". Well, I'm not entirely sure. Fronts are 0.8 degrees camber each, 1/8" toe out, rears are unknown camber, 1/8" toe in. The front bar is the BWR bar, set to full stiff.

The shocks are currently set to 9 ffs on the front rebound (out of 32 settings), I haven't tried any of the other settings.

The push is most noticeable on initial throttle application, ESPECIALLY on cold tires. You can see what I'm talking about in the opening slalom here. This is my first run (codriver, so 2nd run on the car) on a cold day. Watch the opening slalom section, where you can hear me get on gas and the car straightlines, get off gas, car tucks back in, no steering wheel movement:

David = Suck | Veoh Video Network

So, the first response would be "add more front rebound and rear compression". In steady-state, the car is pretty balanced, but still erring on the pushy side. Makes it reliable to drive, but hard to extract the max out of:

2008 WWSCC #7 - My PM Run #3 | Veoh Video Network

The car kept changing every run as the tire temperatures changed, so I didn't want to try messing with shocks much (also these 3 events were pretty poorly-attended, so it was quite hard to both prep the car for the next run and change shock settings before the codriver's time was up). I'm wondering what other people's experiences have been trying to work on this issue.

This is my first time ever in a mid-engined car, and I understand the whole weight transfer issue is always going to make it slightly pushy on-throttle. It just seems like it's a little extreme right now. Especially driving around on the street, I basically throw it into corners and do full lifts to get it to tuck in - any throttle application at all sends the nose skittering wide. That seems a little excessive.

Either way, the most fun car I've ever autocrossed, bar none, and I've already finished better at events than I've ever done in the RX-8 I'm jumping from. ;-)

[John Stimson]

Shock settings: theoretically, you would soften front rebound to reduce corner-exit understeer. Stiffening it would only make it worse. Stiffening rear compression should help, if you don't want to soften the front rebound.

Alignment: If this is generally a problem, try some more toe-out for the rear wheels. That is how I got my S2000 to balance out under acceleration with the 245/275 tire setup. I think it's a better solution than shock settings.

[codingparadox]

I originally thought softening it was what I wanted too, but Greg Fordahl corrected me. The idea, as it's been told to me, anyway, is that you're trying to limit weight transfer to the rear under acceleration. When you accelerate, the fronts will try to lift and the rear will try to squat. So, you can limit this by either raising rear compression damping (which will limit the rear squat), or by raising the front rebound damping (which will limit the suspension expansion and hence prevent the front lifting).

[Surferjer]

Welcome to the wonderful world of Elise autocrossing. There is a price to be paid for dialing anything out or in, and power-on push is (in my experience) what you wind with to make the car do other more important things correctly. Sorry.

Maybe Brian and Pat figured it out, but I doubt it. Matthew throws the car around with a vengence, the key being to toss the car so hard it lines up in the right direction and then power on doesn't make you push out. He's the best at it, though. Good luck...

[John Stimson]

I think you're confusing weight transfer with the body motion that accompanies it. When you accelerate, that puts a torque on the chassis that will lift the front and lower the rear. The full amount of weight transfer is the same, whether it builds up slowly and peaks when the spring reaches maximum compression, or increases more quickly because the shock is resisting the motion. By stiffening the front shocks to resist that motion, you are applying the lifting force more immediately -- accelerating the weight transfer off the front wheels.

It also accelerates the left-right weight transfer on the front end, which also contributes to understeer.

While increasing rear compression also accelerates front/rear weight transfer, it also accelerates left-right weight transfer on the rear axle -- the effects counter one another, but if you crank it up high enough, oversteer wins...but perhaps not in the smoothest, most controllable way.

Don't get me wrong, I'm a big fan of high rebound settings -- which is why in this case rather than reduce the front rebound, I would first try increasing the rear compression.

My first choice is still adjusting the rear toe.

[codingparadox]

Very interesting. Thanks for the info, John.

Yeah, my first step is going to be pulling out most, if not all, of the rear toe-in and seeing where that leaves me. The car hasn't been aligned in almost a year, so for all I know, the alignment is all kinds of whacked by now.

[Surferjer]

When you say Jeff do you mean Mr. Lundgren? What is he driving now??

[xtn]

You might try it a short while with the BWR front bar notched back one setting off of full stiff. Of course this will reduce under steer all around, not just on corner exit. But give it a try and see what you think. Unless you find it results in too much tail-happiness, it might be a good solution. But you said it was hard to rotate anyway, so I doubt moving it one notch is going to make you spin all of the sudden. It might be just the thing you need.

xtn

[Maypo]

John,

Your explanation makes sense, overall, but how does increasing rear bump settings accelerate front-to-rear weight transfer?

[xtn]

Originally Posted by Maypo

John,

Your explanation makes sense, overall, but how does increasing rear bump settings accelerate front-to-rear weight transfer?

The more stiffly damped in back, the faster is the weight transfer to the contact patch. Imagine if you replaced the shocks with steel bars. The weight transfer would be instant. Imagine if you took out the shocks completely. You would wait a couple hundredths of a second before the frame rails got down and smacked the axle into the ground. Stiffer equals faster response. Stiffer in the bump setting equals faster response as the chassis is coming down.

Reading back through that paragraph myself I see that it is not necessarily clear. Hope you can find some understanding hiding in it somewhere.

Here I'll try one more example: Imagine you've set a brick on top of your bathroom scale, and then you jump on top of the brick from your toilet seat. The scale would feel your weight immediatly. Now imagine you replaced the brick with some foam rubber and repeated the jump. As your foot came down onto the rubber and compressed it, the scale would see a slower ramp-up of your weight; more of a softly curved line on a graph instead of a digital looking table-top line. Make any sense here? The eventual reading on the scale is the same, but if you graphed the change they would look different.


xtn

[codingparadox]

Originally Posted by Surferjer

When you say Jeff do you mean Mr. Lundgren? What is he driving now??

Indeed. He's picked up a new 2009 WRX for DS. Looks like it could be a very strong car for the class.

[codingparadox]

Originally Posted by xtn

You might try it a short while with the BWR front bar notched back one setting off of full stiff. Of course this will reduce under steer all around, not just on corner exit. But give it a try and see what you think. Unless you find it results in too much tail-happiness, it might be a good solution. But you said it was hard to rotate anyway, so I doubt moving it one notch is going to make you spin all of the sudden. It might be just the thing you need.

xtn

What I've heard from a few people, including Jeff, is that if you back off on the front bar, due to the increased sway, and the not so great camber curves with limited camber, you actually end up rolling over onto the shoulder of the tire more, which lowers grip and increases understeer. I don't know if that's ultimately true or not, though.

[Maypo]

You had me at:

Imagine if you replaced the shocks with steel bars. The weight transfer would be instant.

Thanks for clarifying that.

[Surferjer]

Originally Posted by codingparadox

Indeed. He's picked up a new 2009 WRX for DS. Looks like it could be a very strong car for the class.

hmmm. Maybe. I would have thought a 135 would be a better bet. And the Mini might trump all in DS.

[codingparadox]

Anything's possible. The 135 has the power, but so far no one's been able to get it to go around corners to save its life.

[John Stimson]

Here's another note to cover my ass: What I've described is how shocks that are close to the proper damping rate behave when they're adjusted. If your rates are way out of whack -- such as if you're do under-damped that the suspension is hitting the travel limits hard, or so over-damped that the shocks are pulling the tires off the ground -- then my recommendations could be the opposite of what you should do to fix the problem.

It would be a good idea to try the extremes of adjustment on your shocks just to see what they do. That goes against the conventional wisdom of "only make small changes", but it will tell you what the extremes are like, good or bad.

[John Stimson]

Jerry, apparently the new WRX has a much stiffer suspension, like the new Lancer Ralliart. I've heard that the 135 won't balance properly in sweepers. Maybe they should try my toe-out idea.

[xtn]

Originally Posted by codingparadox

What I've heard from a few people, including Jeff, is that if you back off on the front bar, due to the increased sway, and the not so great camber curves with limited camber, you actually end up rolling over onto the shoulder of the tire more, which lowers grip and increases understeer. I don't know if that's ultimately true or not, though.

I've only got the factory Track Pack bar. I keep it in the middle setting and get good balance. Is the BWR bar a lot softer that you have to keep it full stiff to keep from rolling the tire over?

What's the worst thing that could happen? You'd have to notch it back up to full stiff? Give it a try.

xtn

[fpitas]

For what it's worth, BWR is selling a big front bar specifically for when you can't increase the front camber:

New BWR sway bar for Hardcore Auto-x and Track enthusiasts!

[John Stimson]

Originally Posted by codingparadox

What I've heard from a few people, including Jeff, is that if you back off on the front bar, due to the increased sway, and the not so great camber curves with limited camber, you actually end up rolling over onto the shoulder of the tire more, which lowers grip and increases understeer. I don't know if that's ultimately true or not, though.

I know that's the case for BMW Z3s with their limited static camber and strut suspension, but jeepers! Lotus has supposedly no-compromises double A-arm front suspension, their camber compensation should be perfect for any roll angle you are likely to bring about.