Trail brake shake

[showa]

A friends car we are trying to sort. Has us scratching our heads.
Not a PT car but a tube frame road race T Bird built in 1980's.
Under hard straight braking car is fine. Under soft brake, like trying to set suspension or if trail braking, the steering wheel shakes hard.

Doesn't do it until heat in tires and running hard.

Have replaced all bushings, ball joints, tie rods, bearings, upper control arms, rod ends, tires, brake discs, steering rack, and added reinforcement to rack mount.

1/16th toe out, can't remember what the caster/camber is but it's not much, 650 lb front springs (tagged as 650, came with chassis), bilstein shocks also revalved by factory (I can't remember %). Chassis scales almost perfect 50/50. Can't remember total car weight but I think it's about 2800#.


Tires are 14" wide Goodyear slicks.

Looking for ideas as to what else to check.

[showa]

Thanks for any thoughts on this.

[Bryce]

You could be getting a harmonic at the natural frequency of the chassis, with this specific input frequency and amplitude. Google modal analysis.

[exwestracer]

I think it's important to identify whether the car ONLY has the problem when turning and braking. Have the driver repeat the barking effort in a straight line. If it IS only when turning, I think you may have a bump steer issue compounded with natural harmonics, as Bryce pointed out. Keep in mind that you WILL have some bump steer anytime the wheel is turned, but if it's excessive (rapid change) it can set up tire shake.

Is the car on !5" wheels, or larger?

[showa]

Thanks gentlemen.
The wheels are 16".
I will check with him on straight/turning question.
I got some reading to do....

[exwestracer]

Thanks gentlemen.
The wheels are 16".
I will check with him on straight/turning question.
I got some reading to do....

So you do have a fair amount of sidewall. Probably exaggerating the problem.

[silver69camaro]

What is the wheel offset? If it is as little as I think it is,.you have a large amount of scrub radius and are feeling tire chatter or a harmonic through the wheel.

Edit: Reading my post, it makes it seem as if I have the answer - I don't, but I suspect it has something to do with sidewall or component deflection magnified by the large scrub radius creating an extreme force feedback situation. Did the car always do this, or just recently?