Car darts/hunts under heavy braking

E

Erickz

Well-known member
We're racing at Mid-Ohio this weekend: car darts or hunts under heavy breaking. Brake M/C and calipers are new, brake pads (Hawk black) are not. We're running 1/16 static toe out. This is the stock MG Midget brake setup.
Any thoughts?
Thanks
EZ
 
We had exactly the same problem. The driver was afraid to get too close to the track edge because he wasn’t sure which way the car would dart. I finally gave up and went to 1/8” toe out on each side. Totally cured the problem and in fact it seems to turn in better. Our car is bigger and heaver, 2nd gen RX7 in EP with 15” tires, but you should try a little more toe.

Mil
 
My customer called and said he was having the same issue off the back straight. His first time on TD's is part of the issue but is there something on the surface?
 
We run the disc/drum setup with slicks on both of our Sprites with 0 toe. Only time we get your symptoms is when we don't have enough rear brake dialed in. Worth checking out.

Bob
 
Bingo! Bob wins - RR drum brake was out of adjustment. We also tried to get zero toe. Probably a little of each gave us the problem. Thanks for all your input.
Thanks
EZ
 
In the absence of your other contributing factors......darting under hard braking is normally attributed to too much front toe out.
 
On the other hand............some toe out really improves initial corner turn in (no initial understeer)........ some say the car almost "jumps" into the corner but I think it's more subtle.

I always run a small amount..........say 1/16" per side....of toe out in the front so long as braking is not impaired too much. It's all a big tradeoff comparison.
 
Assume you have set up the car with 1/16" front toe out per side.

So why does toe out improve turn in? Example- As you start to make a right turn load transfers to the left outside wheel meaning most of the car direction control is on the left outside wheel....but the car (not the tire direction) is still going essentially straight. If the outside wheel is pointed straight then the inside lightly loaded wheel has ALL of the toe or 1/8" toe pointed to the inside of the corner. This generates some initial extra side force which pushes the front end of the car towards the corner apex. Thus the feeling of sharper turn in response and less initial understeer.

It all makes sense if you just take some time to think through it.
 
I always thought it possible that some production based cars could like the additional ackerman effect that is the inside tire pointing toward a tighter radius on a car that is toed out, in that brief turn in to set moment, before the inside tire is unloaded and irrelevant.
 
I always run as close to 0 as possible but stay on the toe-in side (0 to 1/32" toe-in). I don't have a problem with turn in. Braking can cause toe-in too. I know front wheel drive cars like toe-out because under acceleration the front end move toward toe-in.

I always though that toe-out on a rear wheel drive car would cause a wandering effect down a straight would stabilize under braking unless there was too much toe-out.

I know alignment specialist use the term toe per side but I don't understand the concept. I guess that term is used to make sure the steering wheel is straight. The front end doesn't know toe per side, it just finds a equilibrium of total toe with near equal on each side (that is without external force being considered).
 
Toe in either direction can cause wandering on straights and under braking as the tire with the most grip pulls the car in its preferred direction. This is more obvious over tracks with bad pavement or when you're just beginning to turn in and still hard on the brakes.

I had a lot of toe-in on the rear of my car for a weekend (knocked the alignment out of whack on saturday morning and didn't notice it until I got home. I thought I was losing a wheel bearing, but didn't have a spare in the trailer.) The car would wander really bad on the straights to the point one person following me came up after the race and asked why I was driving like a drunk..

If you have a lot of toe out in either end, it'll do the same thing much worse under braking. I've had the rear try to swap ends on me before when the same toe adjuster on the rear slipped the other direction and I had 1/2"toe out in the rear. (I've since replaced adjusters with beefier versions. problem gone.)
 
Some othe r places to look; Check the front bearings. the car will often pull to the side with the more snug bearing back lash. The loose side has more pad kick back.
A little grease on the back side of one rotor .

Often on old race cars the brake hoses get snug.
Very little play in a control arm bushing may add steer under load.

FWIW around 3mm (13in wheel)toe out is near zero scrub for - 3* camber. Zero scrub negates camber thrust and the car rolls cleaner.
 
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