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Forces exerted on control arm mounts under braking
Can anyone confirm or correct my thought process here regarding the forces exerted upon control arm mount points under braking...
When brakes are applied, the caliper (mounted on the rear facing side of the knuckle on a C6 Corvette) is forced upwards due to the rotor's forward rotation.
The caliper is fixed to the knuckle, the hub (to which the rotor and wheel/tire is mounted) rotates on an axis in the middle of the knuckle.
This results in the knuckle rotating forward (torque) on the axis of the hub.
The control arms mounted at top and bottom of this knuckle prevent rotation and translate this torque into two vectors, one forcing the upper control arm forwards, one forcing the lower control arm rearwards. The lower control arm vector being (by far) the largest due to the (rearward) travel of the road upon which the tire is gripping (and the assembly is thus moved.)
Annnnd... that's why this happens (pic below)?
Since I am able to flip the mounting pin so that the solid flange is on the front side of both the forward and rearward upper control arm mounts, is this an acceptable way to avoid this problem?
You are correct in that the upper control arm is trying to move toward the front of the car due to forces applied.
There is a discussion on the Autocross and Road Racing page that covers stock bushing distortion and folks are flipping the stock bushings to achieve the same thing as you are talking about with your problem.
The problem is that the front leg of the arm is taking all of the braking forces and it is bending (as you have found) and if you put bushings in that don't spread the horizontal load between the two bushings it's going to happen no matter what kind of bushings you have.
The only thing you need to consider is that with bushings that you have allow the arms to slip in one direction. No big deal for track duty if you flip them, but expect that they will slip aft (and make noise and change alignment) if you back up and put on the brakes. This isn't an issue with the rubber bushings since they are strong enough to hold the bushings in place even after they've been flipped since you generally don't see much braking force when you are backing up.
You probably need to put on new arms anyway to get rid of the bent effect, flip the aft bushing if you stay with rubber, or flip your greased bushings and install some kind of snap ring or some means of stopping the arm from moving aft when you back up.
The only thing you need to consider is that with bushings that you have allow the arms to slip in one direction. No big deal for track duty if you flip them, but expect that they will slip aft (and make noise and change alignment) if you back up and put on the brakes. This isn't an issue with the rubber bushings since they are strong enough to hold the bushings in place even after they've been flipped since you generally don't see much braking force when you are backing up.
You probably need to put on new arms anyway to get rid of the bent effect, flip the aft bushing if you stay with rubber, or flip your greased bushings and install some kind of snap ring or some means of stopping the arm from moving aft when you back up.
Thanks for that. I'll have a search through this forum for the posts you mentioned.
These are poly bushings, FWIW, with a washer/plate and circlip on the side opposing the solid flange that is part of the pin, so shouldn't have a problem w/ reversing.
Curious how one determines when new arms are needed... I can't observe with the naked eye any kind of deformation and they slot onto the mounting studs with little persuasion (flipping the pin on these actually puts the mounting holes slightly closer together by ~2mm.)
If the control arms bend, you'd be able to measure an alignment change, right?
Even if you could (sounds reasonable), how do you eliminate all other possible causes (e.g., wheel bearing play, ball joint play, minor camber plate shift/turn, rounding of the measurement tool/device, the way the wind is blowing, how you hold your tongue, etc)?
How do you narrow down to upper and not the lower control arm?
I have a bigger issue with the front rubber LCA coming out, i.e the arm appears to move backwards relative to the car and there is a loss of caster. I gather it can be flipped but it is not symmetrical and I wonder if it will still allow the LCA to fit in the car.
That bushing setup in aftermarket. I see two things that are causing problems. 1) The snap ring always goes on the opposite side of where the force is being applied during a stop. That way the force is on the solid washer and not the snap ring. 2) There is way too much play in that bushing setup as shown in the picture. Is the extra play just because the snap ring popped out of it's groove ?
That bushing setup in aftermarket. I see two things that are causing problems. 1) The snap ring always goes on the opposite side of where the force is being applied during a stop. That way the force is on the solid washer and not the snap ring. 2) There is way too much play in that bushing setup as shown in the picture. Is the extra play just because the snap ring popped out of it's groove ?
Because it popped out - this is just how I found it after the last track day.
I've flipped the two rear-most pins on both front uppers per Ernie's experience. The circlip will be more than fine to hold any reverse braking, and I can't even remember the last time I reversed the car other than to park under my canopy.
10-03-2015, 11:51 PM
Forces exerted on control arm mounts under braking
