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

[drew365]

My car was just aligned. They set it up with +1/16 toe all around. They told me I would get very quick turn in but it might be a bit darty under hard braking. I'm going to give it a try to see if I like it. Sounds like rear toe out might be the #1 suspect for what you're feeling.
Just think, we'll all be experts at this, by the time we're racing wheel chairs. (IMG:style_emoticons/default/biggrin.gif)

[nine14cats]

My car was just aligned. They set it up with +1/16 toe all around. They told me I would get very quick turn in but it might be a bit darty under hard braking. I'm going to give it a try to see if I like it. Sounds like rear toe out might be the #1 suspect for what you're feeling.
Just think, we'll all be experts at this, by the time we're racing wheel chairs. (IMG:style_emoticons/default/biggrin.gif)

I was thinking I'd set up with wheel chair pretty low to the ground with -1/16th toe in the rear and possible monoballs on the front footrests....I hear the monoballs allow more positive up and down action on the footrests when you stand up..... (IMG:style_emoticons/default/laugh.gif)

Bill P.

[John]

I concur with the rear toe idea. I believe what you are experiencing is the car transitioning from toe in to zero toe or even toe out.

If you are the only car getting squirrely in the braking zone, toe change is what I would suspect. Dial in another 1/32" or so toe-in at the rear and see how it does. Another option would be to stiffen up the front to help keep the car from "diving" so much during braking.

Another possibility is that the track is lumpy in the braking zone and the car wants to wander around a bit. I have felt all of these conditions in our track car at one time or another.

just my $0.02

[retrotech]

My car was just aligned. They set it up with +1/16 toe all around. They told me I would get very quick turn in but it might be a bit darty under hard braking. I'm going to give it a try to see if I like it. Sounds like rear toe out might be the #1 suspect for what you're feeling.
Just think, we'll all be experts at this, by the time we're racing wheel chairs. (IMG:style_emoticons/default/biggrin.gif)

Mine was set with toe, in degrees. How many degrees would 1/16 be?

[nine14cats]

Update on my wandering issues during hard braking. At the Buttonwillow event I was playing with the rear bias adjuster lever during the different sessions and braking hard as I hauled down from high speeds in the straights. I played it safe in the turns.
I used a set of Hoosier R45A compound tires with 19 heat cycles on them. There were past their stickiest life, but well within good lap time ranges.

I have not had the car's rear toe checked or adjusted yet, but I found that during hard braking it appears that my rears are locking ever so slightly even with the brake bias valve fully biased back or fully biased front. The Hoosier slicks have wear "holes" in the the face of the tire, 4 holes going across. I have a little stick I use to see wear.

I observed in subsequent sessions that the rears were "mini" flat spotting in at least 2 different places in the rear, equally across both rear tires. The car does not hop in the back, but the car wanders slightly as I've said before. I played hard on these tires for the day and I actually corded both rears by the end of the last session. Both rear tires were corded in the middle sections of the tires across the meatiest part of the tire tread, in a very even pattern.

My conclusion is that while the toe issues are unknown at this time, I still have a rear subtle lockup issue. Since the rear tires have 2 distinct flat spots, but cars that I have asked to watch from behind do not see a major lockup of wheels, I'm thinking I'm getting a slight lock / unlock / lock scernario.

My car is softer in front spring rate than my local race shop (Jerry Woods Enterprises) recommends. I am running a 350 lbs front / 550 lbs rear combo. For a 911 of my weight and braking capability, they recommend a 425 lbs front / 550 lbs rear combo or a 450 lbs front / 600 lbs rear combo. My old 914-6 had a very stiff setup compared to the 911. This was mostly influenced from Rich Bontempi's setup and bouncing from one area of the track to another. I have tried to be smoother and set up the 911 to absorb more of the track undulations and I am now much more comfortable with a softer setup.

My next steps would be to change out the front springs to the 425's to keep the car from diving as much under braking. This in itself may be enough to offset the weight transfer going on that allows the rears to lock up, even as slight as they appear to be. I'm pretty comfortable adjusting my AR bars, so I can dial out the change in turn in (the car should understeer a tad more with the 425's) between sessions.

I may also have another 1/32" dialed into the rear toe as well before my next time time trial.

Further, If the problem still persists, I can play with the brake bias with a new valve or mechanically with pads and or calipers.

Does this train of thought seem reasonable?

Thanks,

Bill P.

[J P Stein]

Your bias valve has no efect?
It's fucked up, me thinks.....stuck at one setting reegardless of adjuster movement?

My experience with mine is that it shows an effect with less than one full turn.....the knob type.

[nine14cats]

Hi JP,

I've got the "stick" type in Fritz. I had the "knob" type in the 914-6. I could feel a difference in fully biased / fully non biased but that's about it. I was still wiggling in either bias.

Hmmm....maybe I'll change out the valve. They're pretty cheap and it takes out another possible functional variable.

Bill P.

[J P Stein]

I had mine set at full open at one time.
While trail braking into a corner the back end would step out a bit more that I liked.
One turn closed was just about right. At the time....in my search for "the set up", that was about the only way I could get the back end out in tight turns. This is AX stuff of course....prolly wouldn't be much fun at 90ish (IMG:style_emoticons/default/w00t.gif)

Now days, I'm trying to tune out excess oversteer. (IMG:style_emoticons/default/huh.gif)
More front bar (again), softer rear springs....I have some Formula V on the way for the rears. We'll see....

[Borderline]

the weight transfer to the front is a result of the inertia of the car acting throught the center of gravity and the rate of deceleration. Increasing the front spring rate by 20% is not going to effect the weight transfer. I would suggest you work with the brake bias and toe settings. MHO.

[groot]

Bill,

It sounds like you're using a fluid restriction type of bias valve.....that's really not a proper way to bias your brakes.... as I'm sure you know. If you're positive it's not a toe issue you may want to go with a dual master cylinder and mechanical bias setup.

[nine14cats]

Hi Kevin,

Yep, I'm using the fluid restrictive valves from Tilton. And I've looked at the dual m/c systems. I'm just trying to see if I can get this refinement done without having to go to the expense of redoing the brake system. The brakes haul the car down great and I've never experience brake fade at all in 25 minute sessions at 110+ degree track temps, so I'm trying really hard to figure this out with the least amount of cash bleeding.... (IMG:style_emoticons/default/laugh.gif)

I'll probably give Fritz a little more rear toe and see what happens next.

Thanks,

Bill P.

[groot]

Not that I want you to open your wallet or anything...... I run a dual master setup with "M" calipers at both ends. You can implement the dual master setup without changing the wheel end parts.... and hence the dual master setup really has nothing to do with brake fade.

[nine14cats]

Not that I want you to open your wallet or anything...... I run a dual master setup with "M" calipers at both ends. You can implement the dual master setup without changing the wheel end parts.... and hence the dual master setup really has nothing to do with brake fade.

Hi Kevin,

Would I lose the power brake booster going to the dual m/c setup? Right now I'm still using the stock vacuum booster and I don't need to lose it since I can get to min class weight without touching it. I like the power brakes!

Thanks,

Bill P.

[groot]

To do it cheaply (relatively), you'd have to lose the booster.... but you can size your master cylinders to make up for the difference.


BTW... the reason the fluid restriction type of bias isn't as good it because it restricts flow rather than reduces pressure. With the dual master cylinder setup you can adjust pressure to each end of the car.

[byndbad914]

Would I lose the power brake booster going to the dual m/c setup?

Hey Bill, I ain't too good with the 911 stuff, but I am almost positive I saw a boosted dual master setup at last year's German Auto Fest at one of the displays. I also think there is talk of it in older posts on Birdboard if you search over there.

Not cheap I suspect, BUT many many racers seem to run in the opposite direction of bias valves and only want mechanical adj at the pedals, so I just play along and go with that. You wouldn't be the only person I have heard of having issues with those valves.

Otherwise, maybe there is something affecting a lack of pressure to the front (tho' less likely than the valve being a problem). Maybe your single master is having issues. Again, doubt it but if the valve isn't it...

[byndbad914]

oh yeah, sorry - as for the higher front spring rate - I don't exactly agree with worrying about weight transfer with spring rate. You REALLY have to tip the car to get weight transfer, so the springs won't help much if any with the braking issue.

Set spring rates based on how the car handles, not brakes. Anti-dive and such is a suspension geometry thing, not a spring thing (how's that for tech speak!).

If your car handles really well with the "softer than usual" front spring rate, then leave it alone. If you oversteer, then stiffen the front up, but don't do it because you dive under braking. You may have a good handling car now, go up in spring and then need the brakes bigtime to keep from plowing right off the track, and ultimately just wear out pads and go slower understeering. IMO of course, so I guess take it for what it's worth (see sig (IMG:style_emoticons/default/biggrin.gif) )