Suspension Geometry question, What cause a car to wander under heavy braking? Options

[nine14cats]

I have a question for the suspension gurus out there. I am experiencing some wandering in my car during heavy braking out of high speed sections into sharp corners. Visualize turn 7 at Sears, turn 14 (CCW) at Thunderhill, turn 2 at Laguna Seca. When I get on the brakes hard, the car wanders and "twitches" a bit. The car doesn't jump and it's not out of control, but it keeps you busy, busier than I'd like. I'd like to refine the car to not exhibit this trait.

My thought (and others) is that under heavy braking the front squats and the toe out becomes excessive, allowing the car to follow the undulations in the track pavement.

What are possible causes of this as the front suspension goes through it's travel?

One possibility could be linked to my front strut steering arms on my car. I have coil over RSR style struts with "custom" bends in the arms. The shop that built the car put a custom bend in the arms when they fabricated the raised spindle. Can a non uniform bend to the arms contribute to excessive toe out under hard braking?

Thanks,

Bill P.

[groot]

Assuming you don't have any significant toe compliances (I'll get back to that later)... it's certainly possible that your front toe curves are causing this issue. It's pretty easy to verify, too. Run bump steer on both front wheels at the same time and see what your total toe is doing while the wheels are moving up. It could just as easily be the rear toe change, too. So, it's not a bad idea to check the bump steer at the rear, too.... but in rebound....since the rear should be raising up under braking.

My car had wander issues under braking. But, it was due to toe compliances that I was not aware of at the time...... the right outer rear suspension console was seperating itself from the chassis..... causing toe out under braking. Check for these things first.

[john rogers]

I agree with Kevin about the toe changes. On my car it was the rear causing some wandering so now we run a tad more toe in on the rear wheels and things are smooth.

[nine14cats]

Thanks guys. I'll check both front and rear. It seems like it would be pure heaven to have the car just hunker down and stay tracked under heavy braking. I know the car's body isn't tweaked because I had all of that checked in a PPI.

Thanks again!

Bill P.

[groot]

You can do a crude check for toe compliances by loading the suspension laterally (ideally through the tire patch) with the opposite side blocked and check to see what moves and how much.

[nine14cats]

You can do a crude check for toe compliances by loading the suspension laterally (ideally through the tire patch) with the opposite side blocked and check to see what moves and how much.

Hi Kevin,

If I do the crude measurement and I line up a "standard line" i.e. like the smart strings...for a base line, how much of a delta is acceptable or would not cause the wandering effect?

Thanks,

Bill P.

[groot]

It depends on a lot of things, but if you're getting more than 1/8" toe change with lateral loading you've got something to fix.

[Borderline]

Bill, I'm curious about the "custom" bend they put in the steering arm. Was it bent to move the end up or down with the intent of minimizing bump steer, or was it bent to move the end inboard or outboard to effect ackerman? Could you post a photo? I'm always trying to learn.

Thanks

[nine14cats]

Bill, I'm curious about the "custom" bend they put in the steering arm. Was it bent to move the end up or down with the intent of minimizing bump steer, or was it bent to move the end inboard or outboard to effect ackerman? Could you post a photo? I'm always trying to learn.

Thanks

Hi Bill,

That's a good question. My first thought is that the bends were made to minimize the bumpsteer, but who knows? (IMG:style_emoticons/default/confused24.gif) I have the car stored for the next few weeks while I get back to the honeydoo list, but when I pull the wheels off for my next safety check I'll snap a few pics.

Thanks,

Bill P.

[byndbad914]

just gotta ask... do you have an adjustable brake bias? Or did you remove the proportioning valve which is a popular mod/"mistake"? Upgrade the brake system but use the stock p-valve or no valve?

I ask because if you have a little too much bias to the rear brakes, the car will wander like a mofo on hard braking. I over adjusted my bias one time at the track (because I was dumb enough to think "what does it do?") and I thought I wasn't going to make it thru the turn I was going into pretty hot. The back end was all over and I couldn't stay on the brakes hard enough to get the speed down. Got lucky and was able to just get off the brakes and just drift the rear thru the turn without spinning.

Just a thought if after you check thru your toe settings you don't think that is it.

As for toe - have you ever checked your bump steer on the car? If you are going to race it, you really need to set the car at ride height, measure where the hub is at, remove the wheel and check bumpsteer thru travel. You may have really bad bumpsteer with whatever ride height you have set the car at, maybe not. If you have 1/8" toe-out bumpsteer at 1" compression, hard braking causes that 1" compression, then setting the toe at 1/8" toe-in would be the minimum toe-in you could run.

Cars get reeeaaaalllly wierd when they transition from toe-in to toe-out or vice-versa. Loosing some toe-in is one thing, but if you transition from in to out it makes the car a whole new animal to drive instantaneously.

I had my 914 so low, the bumpsteer was a mess, so yet another reason I went tube-chassis. I have majorly relocated the rack to fix all of that.

So long story short - my guess is you either have horrible bumpsteer and need to relocate the rack or deal with a ride height change to fix it, or you have too much rear braking.

[flesburg]

(IMG:style_emoticons/default/agree.gif) I would test for excessive rear braking first. Test it in straight line braking. Have an observer watch the car as you brake (from a speed fast enough that you can lock up the wheels). I believe your rear wheels lock up before your front wheels. If you have an adjustable proportioning valve turn it down to reduce rear braking effect. Turn it down until the fronts lock just before the rears, and I think you may find the cure for your problem.

[nine14cats]

just gotta ask... do you have an adjustable brake bias? Or did you remove the proportioning valve which is a popular mod/"mistake"? Upgrade the brake system but use the stock p-valve or no valve?

I ask because if you have a little too much bias to the rear brakes, the car will wander like a mofo on hard braking. I over adjusted my bias one time at the track (because I was dumb enough to think "what does it do?") and I thought I wasn't going to make it thru the turn I was going into pretty hot. The back end was all over and I couldn't stay on the brakes hard enough to get the speed down. Got lucky and was able to just get off the brakes and just drift the rear thru the turn without spinning.

Just a thought if after you check thru your toe settings you don't think that is it.

As for toe - have you ever checked your bump steer on the car? If you are going to race it, you really need to set the car at ride height, measure where the hub is at, remove the wheel and check bumpsteer thru travel. You may have really bad bumpsteer with whatever ride height you have set the car at, maybe not. If you have 1/8" toe-out bumpsteer at 1" compression, hard braking causes that 1" compression, then setting the toe at 1/8" toe-in would be the minimum toe-in you could run.

Cars get reeeaaaalllly wierd when they transition from toe-in to toe-out or vice-versa. Loosing some toe-in is one thing, but if you transition from in to out it makes the car a whole new animal to drive instantaneously.

I had my 914 so low, the bumpsteer was a mess, so yet another reason I went tube-chassis. I have majorly relocated the rack to fix all of that.

So long story short - my guess is you either have horrible bumpsteer and need to relocate the rack or deal with a ride height change to fix it, or you have too much rear braking.

(IMG:style_emoticons/default/agree.gif) I would test for excessive rear braking first. Test it in straight line braking. Have an observer watch the car as you brake (from a speed fast enough that you can lock up the wheels). I believe your rear wheels lock up before your front wheels. If you have an adjustable proportioning valve turn it down to reduce rear braking effect. Turn it down until the fronts lock just before the rears, and I think you may find the cure for your problem.

Hey Guys,

I have a cockpit adjustable brake proportioning valve. When I first purchased the car, we checked tire rotation on a lift on the 7 settings and at this last event I also had someone follow me on Thunderhill and another watch from track side. I then applied threshhold braking and got the fronts to lock up first. Your suggestions are logical and my track mates did the analysis real time. That is why I'm curious as to the possibility that the geometry is the problem.

We will be putting the car back up on a rack and taking the wheels off and running the car through suspension travel. At least we can see what it is doing.

Or could I have a bad front shock? one significantly different than the other under compression and causing the weight to be unevenly distributed?

There is no floating on straights or in turns, so I'm thinking a shock isn't bad. But I'm not that sophisticated of a driver, so I can't really tell.

Bill P.

[McMark]

IIRC, raising the spindles will necessitate bending the arm down. Raising spindles = raising steering arm, so lowering it will bring it back in line with the rack. The bends must be symmetrical in every respect. But if you measure the toe change under compression it will be easy to see if they are off. I'm not sure how you're planning on compressing the suspension, but I would theorize that checking the suspension in an unloaded state would illustrate a lack of symmetry between the two sides. It won't give you exact numbers or direction, but it would be a quick way to check. If it's symmetrical at ride height (after alignment), it would seem that it would be symmetrical throughout the range of motion (as long as the L & R are at the same point in the range).

[Brad Roberts]

just gotta ask... do you have an adjustable brake bias? Or did you remove the proportioning valve which is a popular mod/"mistake"? Upgrade the brake system but use the stock p-valve or no valve?

He is referring to a 911 NOT a 914. A narrow or wide body car with a 19mm MC does not need a prop valve in place when on stock calipers. I'll go to bat over this EVERY time. Close to 20 years now of putting 914's on the race track.. every time.. we end up with the bias wide open...so why run one? It doesnt increase the bias to the front when they install it on the rear line? Oh.. and these are not "back markers" I have put on the track (IMG:style_emoticons/default/smile.gif)

Back to the bump:

I dont feel that you can EVER bend a arm and get the bump correct for both sides. This is why the ERP design is so popular. It allows you too run the numbers and adjust it the best you can.

I'm betting this is a rear toe issue. The later 993/996 chassis all have a Kinematic toe arm that keeps the toe in check under braking.. you dont in the early 911 tub.


B

[smdubovsky]

Bill, if this is in the 911 - its something I know (a little) about. Its rear toe change. When the ass end lifts under braking it will start toeing out. I believe lowered cars make it worse. If you start w/ very little toe, it will eventually get to zero or even full toe out. My 911 will move 1 car width side to side under heavy braking over the bumps. Not uncontrollable but 'busier' than you like. Rear monoballs helped. I think the ultimate solution is to relocate the pickup points w/ boxes like smartracing sells and the 935-type rear spring plates (but I haven't done that yet). You can play w/ geometry more and it eliminates all compliance.

SMD

[nine14cats]

Bill, if this is in the 911 - its something I know (a little) about. Its rear toe change. When the ass end lifts under braking it will start toeing out. I believe lowered cars make it worse. If you start w/ very little toe, it will eventually get to zero or even full toe out. My 911 will move 1 car width side to side under heavy braking over the bumps. Not uncontrollable but 'busier' than you like. Rear monoballs helped. I think the ultimate solution is to relocate the pickup points w/ boxes like smartracing sells and the 935-type rear spring plates (but I haven't done that yet). You can play w/ geometry more and it eliminates all compliance.

SMD

When we get it up on the rack we'll have to take the car through it's suspension settngs. On Fritz (our 911 track car) we have RSR coil overs all around. In back I have the ERP 935 adjustable spring plates and Smart Racing Products 930 to 911 chassis Rear Control Arm Pivot Boxes. I also have 930 swing arms. But I can't recall if the inner control arm bushings are monoballs or not. They probably are, since the previous owner put this stuff on, but you never know.

I'm not moving over a car width under heavy braking, but it's busy. I'll probably run the car as is through August and maybe until the end of September depending on what events I run. this winter, I plan on a few upgrades such as checking the toe settings, monoballs in the rear if I don't have them on the control arms, and maybe popping for the adjustable Smart Racing Shocks. And while I'm in there...maybe I should put that ERP front suspension that was earmarked for The Beast into Fritz.....

Someone hold me back..... (IMG:style_emoticons/default/slap.gif)

Bill P.

[Brad Roberts]

Stephen is correct in his thoughts.

The 914 control arms have less toe change under braking than a 911 (John Rogers told about how he fixed it)

Typically (and I know who did your last alignment) this can be dialed out by overcompensating on the toe in adjustment (like Stephen spoke about) Run more toe in so it doesnt tow out as much under braking.

You nailed it with the thought of: running it through the ranges

This is exactly what I just did with the Boxster. I raised the rear and pulled down on the front with it on the scales and hooked up to the alignment machine to simulate braking. I didnt look for toe changes, I wanted to see weights. I also raised the nose to simulate "on the gas" This is where good pictures come into play. I know how far down the body is under braking for turn 1 at Willow Springs.

This is also where dataAcq would help out a LOT. 4 Shock pots and you would have all the info you needed to perfectly simulate the body movement.


B

[DanT]

Bill, Is this something you have just noticed happening?

When I drove the car at Laguna in December I don't remember having any real handling problems under hard braking for 2 or 11. At least not after the tires heated up (IMG:style_emoticons/default/biggrin.gif)

Who did you have follow you at TH to look at the car under braking?

Are they familiar with a car on slicks and its handling characteristics so they could give you good input?

Just wondering. (IMG:style_emoticons/default/smile.gif)

[nine14cats]

Hi Dan,

As I'm getting more comfortable with the car I'm starting to carry more speed into the braking zones. I was getting on the gas early enough from turn 2 to turn 1 that I could shift into 5th down THill's main straight because I would tach out in 4th... (IMG:style_emoticons/default/clap56.gif)

The turn 15 entrance wasn't the most uncomfortable, because I think entrance to turn 6 was bumpier.

I had Greg Adams follow me. He and I usually run at the front of the blue/green group with Chris Cox. The three of us usually pull away (well....Chris pulls away and Greg and I follow her.... (IMG:style_emoticons/default/huh.gif)) and we dice it up taking turns leading the other and trying different liines and seeing what our cars are doing. We can then debrief each other on where the other's car looked good. Greg's running Dot-R's, so he doesn't have that info, but he can see if I'm locking up under braking and that hasn't been the case. I've actually been working on my pedal control and I'm much smoother under hard braking since I don't want to ruin my pricey slicks!

I've been talking with Rich Walton and the collective thinking is either the "custom" bend and/or rear toe. I'll probably take the car down to JWE in the next 2 weeks and we'll go for toe settings first. At the very least we can document what the ends of the car are doing under load.

I want to wait until after the final Time Trial to change out shocks. I can then use the open track days during the winter months to dial in the new settings.

B,

I'm looking at some of the GPS data ack's right now...alot of options to choose from!

Bill P.

[DanT]

good! (IMG:style_emoticons/default/smile.gif) I would put my money on the rear toe. I would bet you are going to toe out at full suspension unload under braking...
Perhaps you need just a little more rear brake bias....get the rears to give you a little more bite....help to keep the rear in check.
Maybe with a little too much front bias the rear is unloading too much (IMG:style_emoticons/default/smile.gif) Giving you the unfriendly feeling.
This is something that I have experienced with heavier cars with more streetable suspension settings. Especially 944 turbos, turn two at Laguna is always fun as the car slews left and right trying to get a good bite....Actually can be advantageous to help turn the car if you release the brakes at the right time.... (IMG:style_emoticons/default/biggrin.gif) (IMG:style_emoticons/default/biggrin.gif)