rear end dancing under braking

[tiagosantos]

I think I may already know the answer to this, but here it goes. Now that (with some major help from a few people) I'm starting to get rid of the major issues with my car and getting closer to a decent pace, I'm finding smaller things that I'd like to fix.

I noticed the past two races that my car likes to dance around to both sides under braking. I set up the brake bias statically in the garage (ratchet strap on the pedal until the fronts locked up and making sure I could still move the rears). Rear toe was set as close to 0 as I could get it. The first weekend I adjusted the brakes myself, then for the second race, the local FV super hero adjusted the brakes while he had the car in his shop checking something else. So I know they were adjusted as well as they'll ever be!

Going through the data, I can see a "wave" on the lateral G graph on every hard braking zone, around +/- 0.15-.2Gs. That seems pretty excessive.. I added 1/16" (or thereabouts.. what my untrained eye considered "a little bit") toe-in in the rear that I *think* might have helped, but to be honest, I don't think I was braking as hard for the subsequent sessions. For reference, the braking G forces seemed to be fairly consistent at 0.91Gs.. I expected them to be much closer to the max lateral G forces, so I'm either braking like a little girl, my brake bias is way off, or both. Actually I still haven't ruled out a potential alignment issue.. But I'm fairly confident of my toe in readings. Does rear toe typically change any noticeable amount under braking on a caracal D type rear end?

Any thoughts?


fake edit: when I strap the brake pedal down and check the front brakes, they seem to be fully locked - but if I try really, really hard, the wheel will start moving. And once it starts moving, it doesn't seem to require that much effort to keep it going. I remember learning about static vs kinetic friction in high school, so is this normal?

[hardingfv32-1]

Check the toes in a heavy braking rake. Weigh done the front and jack up the rear. Maybe something is odd.

Your shop balance procedure is fine for ballpark, but there are just to many you are missing in a static condition. It must be evaluated on the track. There is a very good reason for the cockpit brake balance adjusters.

Brian

[problemchild]

Just add a turn of front brake every time you go on track until it gets better or something else gets worse.

If that does not fix it ....
Try taking out rear rebound
or
Letting the rear droop more (smaller number)
or
Tighten up rear suspension pivots
or
Combination of above
or
Something else
or
Don't brake so hard

[CitationFV21]

I sometimes run front toe near zero, but never rear.

I would set rear at ride height, around 1/16 to 1/8 to start and then check the bump steer. Compensate so that with 1" travel either way you don't go negative.
If you mark the nuts as you adjust, you will know (that 6 flats = 1/32 for example) so at the track you can take a little out at a time. Just never start a race with an unknown setting.....

When you get it where you like it, measure it again and keep it there.

Follows my rule - As much as needed, as little as possible....

ChrisZ

[jpetillo]

I agree with Brian that setting the brake bias statically is only a good starting point, and that it's a lot different under racing conditions. Greg and Chris make good recommendations. Also, it's typical that you can corner harder than you can brake. I think close to 1 G is what most people see for max braking in Vees. And yes sliding friction is lower than static. So, judge by the sliding friction. John

[tiagosantos]

Thanks for the tips and ideas guys. Everything makes sense, gonna go through all of this for the next weekend!

Edit: ok, actually I do have another question.. How do you setup the brake bias at the track? I know this may sound incredibly dumb, but I don't think I can tell what's happening with my rear tires. I can see when the fronts lock up (they don't normally lock up in the dry..) but not the rears. Do I just mount some old tires and stomp on the pedal at the end of the straight and adjust the bias until the fronts lock up?

[FV80]

Tiago,
If the rear is 'dancing' back and forth, then you are almost certain to be locking up the rear brakes, unless you have a suspension problem.

You can expect to get about 80+ % of your braking effect from the Front brakes in the dry - however, the fronts will not be NEARLY as effective if you have NO rear brakes. I'd suggest that you move the bias towards the front more - if you have matching (same size) master cylinders, set the bias so that the 'ball' is almost all the way to the front - more than 3/4. and try it. ( You'll want to start using early braking so you have a little extra distance to work with <G>). Hit the brakes as if you mean it (in a straight line) and watch for front lock up. If only 1 locks up, then press a little harder or work on adjustments until both lock up (you may have a 'cross car balance' problem - you might also want to go to the scales and make sure you have equal weight on both fronts). If you can't lock up the fronts, then move the bias ball further forward till you can. Once you can lock up the fronts, back of the bias until you don't lock up the fronts and you'll be about where you want to be. It does 'waste' sessions to go through this process - but the only better way is cockpit adjustment...

Adjustable brake bias in the cockpit is a wonderful feature - even if it's a 'crass' adjustment - go to the junkyard and get a speedometer cable and housing from an OLD car (60's or early 70's - newer ones don't have cables) - or order one from somewhere. attach it to the bias bar somehow and route to someplace you can reach it. You can also use the alternate method and BUY one of the 'pressure adjusters' - available from Pegasus or Summit (and probably some of the vendors at the top of this page) to put in the line to the rear brakes. The pressure adjustors have MUCH less effect but are better than nothing.

Adjustable bias will be a true JOY when you get caught on a 'iffy' rain day and you need to adjust on the fly. If it is raining, you'll need more like 50/50 adjustment since you won't be able to get any weight transfer to the front.

Obviously, when making these adjustments, you want to be careful of flat spotting or cording your tires - don't LEAVE them locked up - just get it there and then let off a bit till they quit locking - then make the adjustment in the pits and try again. If you have cockpit adjustment, remember that you cannot ADJUST it with your foot on the brake - only between braking points - I sometimes even lightly tap the brake pedal (through it's free play) while I adjust to make it easier on the cable. If you put lousy tires on for this setup, it won't work unless all 4 tires are equally lousy .

Good luck,
Steve, FV80

[problemchild]

Edit: ok, actually I do have another question.. How do you setup the brake bias at the track? I know this may sound incredibly dumb, but I don't think I can tell what's happening with my rear tires. I can see when the fronts lock up (they don't normally lock up in the dry..) but not the rears. Do I just mount some old tires and stomp on the pedal at the end of the straight and adjust the bias until the fronts lock up?

Like I said .... Just add a turn of front brake every time you go on track until it gets better or something else gets worse.

[tiagosantos]

Hehe.. Sure thing Greg. My worry is that I've been trying to drive around these little things ever since I got the car, so I'm not sure I'd recognize the perfect braking condition if I stumbled onto it. But yeah, I guess it should be pretty obvious once it's working right, eh?

Thanks for the detailed reply Steve! I do have cockpit adjustable brake bias - the car came with an hydraulic pressure reducer thing for the rears which I got rid of over winter and replaced with a cable bias adjuster. Seems a lot more intuitive to use - even if I've only ever used it in the garage hehe.

[FVartist]

fake edit: when I strap the brake pedal down and check the front brakes, they seem to be fully locked - but if I try really, really hard, the wheel will start moving. And once it starts moving, it doesn't seem to require that much effort to keep it going. I remember learning about static vs kinetic friction in high school, so is this normal?

Having read this part I think you may have air or a leak in the system. Have you physically checked every aspect of the brake system? You need to start at the beginning and then proceed from there. A leaky brake system can contaminate your shoes and cause them to grab. No amount of adjustment will relieve this. Since you are talking about your front brakes losing pressure, it may be caused by a defective master that has an internal leak. This will cause your braking to be biased towards the rear and can give you the same problem.

Bruce

[CitationFV21]

Hehe.. Sure thing Greg. My worry is that I've been trying to drive around these little things ever since I got the car, so I'm not sure I'd recognize the perfect braking condition if I stumbled onto it. But yeah, I guess it should be pretty obvious once it's working right, eh?

Thanks for the detailed reply Steve! I do have cockpit adjustable brake bias - the car came with an hydraulic pressure reducer thing for the rears which I got rid of over winter and replaced with a cable bias adjuster. Seems a lot more intuitive to use - even if I've only ever used it in the garage hehe.

One trick is to put a series of stripes on the front and rear tires, toward the inside of a heavy braking turn. Position one of your crew, or a volunteer and have them watch as you brake. They should be able to tell you which end is locking up first.

We used to do this on the straight in front of the pits, but since the "line" is now all over the place, the Stewards tend to frown on this

ChrisZ

[Campbell Motorsport]

When you have the brake pedal fully depressed it should not go over center, and the balance bar should be at a 90 degree angle to the pedal. At rest, depending on the master cylinders you are using, the balance bar will likely not make the 90 degree angle.

Larry Campbell