Tail wagging under hard braking?

[cygnusx1]

At the latest HPDE event I went to at Lime Rock Park last week, I really began to feel one with the car and was starting to play with later and harder braking. I began to feel the rear end start to wag around a little during real heavy brakes. Unfortunately, my motor blew the gasket before I could experiment with bias.

 

Those with track and setup experience, can you point me in the right direction? I figure first I need to get an accurate rear wheel toe reading, and then set some rear toe-in with the eccentric bushings I have. Secondly, would moving the brake bias around, have any effect on rear end stability? Thirdly, is there anything else I should know about it? Rear control arm uprights flexing around?

 

The car runs stock 280Z suspension hard parts, with 1" drop tokico springs, illuminas (#3 rear and #2 front), st sway bars 7/8" and 1-1/8", and eccentric alum/delrin LCA bushings f+r.

Edited April 12, 2011 by cygnusx1

[260DET]

You have it, add rear toe in and/or reduce rear brake. Actually a fair bit of rear toe in can give some rear steer effect too, particularly in slower corners.

[JMortensen]

Those with track and setup experience, can you point me in the right direction? I figure first I need to get an accurate rear wheel toe reading, and then set some rear toe-in with the eccentric bushings I have. Secondly, would moving the brake bias around, have any effect on rear end stability? Thirdly, is there anything else I should know about it? Rear control arm uprights flexing around?

AL/delrin TC rod bushings will help, as will the toe settings as mentioned above. I liked a pretty aggressive toe setting, at 3/16" total. I think Richard has mentioned running about double that amount.

[cygnusx1]

OK then I will work on the rear toe when I get the motor all sorted out again. I run Poly T/C bushings with OEM rubbers behind the T/C bucket.

[EvilC]

Too much rear brake. That is how my car felt before I put the prop valve in. Scary thing is that I got use to that feeling for almost 2 years but def isn't the way to be driving the car. Hopefully this year when I have the Z back together and back on the track I can play with different setups. Also I am running a slightly harder pad on my front brakes.

 

As far as rear toe, maybe look at TTT rear control arms. I can let you know what my alignment settings are.

 

 

Questions for the true racers: if you are running much wider tires, you wouldn't need as much front and rear toe as someone running wheels lets say 3-4 inches smaller?

[JMortensen]

Here is a new methodology for setting bias which I think is friggin awesome. From Neil Roberts, author of Think Fast:

 

I basically declared war on handling interactions in Think Fast. Ideally, each handling function would only respond to one driver input and not be affected by anything else. Of course not much in automotive dynamics acts like that, which makes the race car driver's job significantly harder. Since it is possible to make braking and cornering balance independent of each other, my technique for setting brake balance is to adjust the brake bias bar until the cornering balance stays the same with and without braking input. Here is a repeat of my technique for doing that, with additions to answer Stephen's question.

 

The cornering condition that I use for setting brake bias is steady state at roughly 80% of max lateral, and the speed is the most common turn entry speed for the track, or for the most important corner. I will do it during the out lap tire warming zigzag and during the first full lap. I will get the car stabilized while turning, then do a brief but firm brush of the brakes with my left foot at a constant steering wheel angle and a constant throttle position. While doing that, I pay very close attention to how the car reacts, then adjust the bias and do it again until the cornering balance is the same with the brakes on or off.

 

That setting also happens to be nearly ideal for simultaneous front and rear lockup, with the major advantage of never having to risk flat spotting a tire just to set the brake bias. Even if it weren't, I would sacrifice some straight line braking capability in order to separate cornering balance from braking input. That's how important it is for me.

 

The problem that I have with prop valves on race cars is that they make brake bias a function of hydraulic pressure, with a rear-heavy brake bias at low pressure and usually a front-heavy bias at high pressure. This makes it impossible to eliminate the interaction of braking effort and cornering balance. The '90 Corvette ZR-1 that I had was the worst car I have ever driven in this regard. Hard braking produced so much understeer that it didn't matter what I did with the steering wheel. During light trail braking, the car was loose until I released the brakes completely. What a disaster. Bias bars don't have this problem since the front to rear hydraulic pressure ratio is constant, but manually adjustable.

 

I had set my car up like this pretty much on accident. I was at a driver's school and we were doing a skidpad exercise, and they were trying to show us how the brakes affected the handling, so he had me get up to speed on the skid pad and then brake as hard as possible. He said: "Now watch: When you step on the brakes, the car will spear off on a straight line." I stepped on the brakes and it just hugged the cones and stopped. The instructor was pretty impressed, but I really just lucked into that because I set the bias the normal way. What I like about Roberts method in addition to it handling better under braking is that you don't have to flat spot the tires to set the bias. He mentioned this method in his book, but he was pretty vague about it, just basically saying that properly set, applying the brakes shouldn't cause oversteer or understeer. This is much more explicit as to what to do.

 

EDIT--One more thing I suppose I should mention is that no matter how the bias was set, under heavy braking the tail wagged pretty hard.

Edited April 12, 2011 by JMortensen

[cygnusx1]

Very cool article. I always noticed that I could "steer" my car in corners with the brake pedal. Brakes on = oversteer, Brake off = understeer. I always attributed it to the loading and unloading of the front tires.

[johnc]

I began to feel the rear end start to wag around a little during real heavy brakes.

 

Perfectly normal. When you get the brake balance right the back end should move a round a bit when you're at the limit. IMHO, don't change anything until you do some more testing. Next time give it a little more brake to make sure the fronts lock first. If the back locks first, then move bias forward a little bit and try again.

[1 tuff z]

i can remember doing some late braking runs into the busstop at watkins last summer and the tail waggin' back and forth. sawing the wheel to keep it back there and having fun doing it. the guy riding with me asked if it was normal. i told him it was an old car, dated technology with no electronic nannys and, most importantly, an absolute workout to drive-i love it.

 

great excerpt you included jon, will print & use that to fine tune my bias next month at wgi.

Edited April 13, 2011 by 1 tuff z

[jt1]

Neil is a smart guy and I have a lot of respect for him and his work. However, most of his background is with formula type cars that have a lot of aero downforce. For average guys like us running Z cars at trackdays, I would approach that method with a good bit of caution. Mark, Tom and I have talked about this a good bit, and done a good bit of experimentation. Z's are notoriously loose under hard braking, and it's real easy to be sliding backwards thru the grass hoping you don't hit anything. Been there, done that, many times.

 

jt

[mark]

jt is right. I remember the last time we discussed this we both agreed that we end up modifing our line to leave a foot or two on either side of a braking zone because the car is going to move a bit.

 

My last attempt to improve the rear brakeing on my Z car left a nice dent on my right rear quarter pannel from hitting the braking markers in turn one at VIR. They are normally on the left side of the car. However when going backwards.................

[johnc]

Its also helps to have the rear end dance under braking because it usually scares the guy behind who's trying to decide where to make a pass. Sometimes they will just stay back there waiting for you to wreck and that's where they finish the race.

[260DET]

Very cool article. I always noticed that I could "steer" my car in corners with the brake pedal. Brakes on = oversteer, Brake off = understeer. I always attributed it to the loading and unloading of the front tires.

 

Haha, yeh I've done that, back it into a corner using rear brake. But Neil Roberts in his book says you should not do that because its a slow way to negotiate a corner. Bit of a spoil sport NR is

[johnc]

As mentioned above, Neil comes from an open wheel background. Sometimes you have to do things as a driver in a production based sedan that you wouldn't do in an open wheel or sports racer. I've raced all three and you don't trail brake a sports racer or an open wheel car (or a Toyota MR2 for that matter) because of their low MOI. They will snap around. A high MOI car like a production based car often needs some "rotational help" in the form of trail braking.