violent brake vibration on Stohr F1000

[Ivinsea]

We tested both of our cars yesterday at NJMP Lightning in preparation for the Regional. My brother's car has been plagued with a vibration under braking since we bought it; sometimes it is better than others. He has replaced all 4 rotors and all 4 calipers. I drove his car yesterday after 2 sessions in my car and the vibration is as bad as ever. It is so violent that the steering wheel chatters enough hurt your wrists.

There are 2 heavy braking area on Lightning, entry to T1 and T7.
Here are some notes:

Approx 20% of the time, the brakes are fine.
Approx 30% of the time, there is not vibration under initial heavy braking, the brakes feel really good, but at the end of the braking event, the steering wheel suddenly vibrates.
Approx 50% of the time, the steering wheel vibrates upon brake application.

Hot or cold does not seem to matter.

Any ideas guys?

[racerdad2]

Check rotor run out while mounted on car ? Brake pads properly bedded in ? Any worn parts in steering box or links ?

[Jnovak]

Fastener check on everything, look at every rod end too.

[Brian.Novak]

Yes. I'd look for a rod end failing or a loose fastener. If the rotors are good and no real runout, then I'd look there.

Are the rotors floating rotors? Are they actually floating like they are supposed to?

[GAC]

Make sure your fastening method holding your upright together is properly torqued too. Also check that your calipers are mounted parallel to the discs and fastened properly. Check for cracks in the caliper mounts.

[JeffW]

If the cars are identical models, why not switch out with your brothers car and yours one upright complete with rotor and caliper. If you wanted to get fancy you could switch them one at a time.

If it persists then its either a bearing or chassis (rod ends etc) issue.

JeffW

[Charles Warner]

Remove the uprights and brakes from your car and put them on your brother's. Try it. This will eliminate or confirm a chassis related issue. If all is better then try replacing one corner at a time to verify the culprit. If things don't get better then look for cracks in bulkheads, rod end issues, steering rack play, etc. I know this is a convoluted way to narrow down the problem area but it may prove the most efficient. One test day should do it.

[S Lathrop]

Are you getting any pulsing in the brake pedals? If not, then the problem is likely not in the brakes but the suspension system.

How does the car handle otherwise?

Have you checked the shocks?

Do you have any excessive movement in the wheels or steering system?

How carefully was the alignment done?

[R. Pare]

Is the "vibration" actually a particular front tire jumping up and down?

If so, you may have a dead shock on that corner. Combined with a too-soft spring, it is possible that the wheel rate and the tire rate ( at a particular pressure) come too close to matching each other, allowing a self-energizing harmonic bounce.

Haven't actually seen that happen on a race car before, but have on street cars - really bizarre when it first happens to you!

[yellowesty]

After replacing virtually everything (one piece at a time) on the front end of a friend's vintage TransAm car for the same reason and not having any success at eliminating an intermittent steering wheel shake under braking that was so severe (when it happened at all) that it wore holes in his driving gloves, I think (hope) we've now solved the problem.

His front brake pads on one caliper extended, radially, a small bit past the outer edge of the rotor -- probably due to a sloppy rebuilding of the caliper mount by the previous car owner after a shunt. When the pads were new or only slightly worn, there was no shake. As the pads wore, the outside edge of the rotor could interfere with the unworn pad material. (At least that's our current theory. And radial runout of the rotor edge wasn't recognized as a critical parameter.) Simple solution: brake pads beveled on the outer edge. Correct solution (other than rebuilding the caliper mount): slightly larger rotors. So far, that seems to be working. Fingers crossed.

Suggestion: check that your brake pads don't overhang your rotors in the radial direction. Even by "just a little."

[Ivinsea]

Thanks for all the suggestions.

We were not able to diagnose the problem so my brother sat out the race.

We had the shocks tested and they are OK, so we are calling in the professionals. We are taking the car to Eric Langbein's shop this week.

I will report back.

[BURKY]

Thanks for all the suggestions.

We were not able to diagnose the problem so my brother sat out the race.

We had the shocks tested and they are OK, so we are calling in the professionals. We are taking the car to Eric Langbein's shop this week.

I will report back.

Your car will be in good hands.

[Blair Robertshaw]

Hi Ivan,

I think you are running a Stohr and if so, I would bet you have many cracks in the frame. The Stohr, in our experience, is soft to start with, our chassis was cracked in so many places that we finally had to strip it down to bare frame, we would find one, then another and as we chased the flex we exposed even more cracks. so a sand blast, exhaustive inspection, crack repair and then on rebuild we added many gussets to many areas forward the mid bulkhead, a few other frame strengthening plates and so on were also added. The car now is very stiff and does most everything we ask it to.

Its a big job as we all know but it may be what lies ahead. Good luck

[Ivinsea]

I think you are running a Stohr and if so, I would bet you have many cracks in the frame.

Its a big job as we all know but it may be what lies ahead. Good luck

Yes Blair, we have two Stohrs. Thanks for the suggestion.....but I hope you are not right! I had not entertained the problem being quite that severe.

[Charles Warner]

One assumes (maybe a stupid thing) that you have changed wheels and tires with known units.

[Ivinsea]

One assumes (maybe a stupid thing) that you have changed wheels and tires with known units.

Yes, we ran my car, then swapped my wheels onto his.

[DFR Dave Freitas]

I'd have another look at the shocks. We have seen cars run with horribly worn racks, wheels that aren't close to true and flat spotted tires, and the drivers didn't complain.

But get a bad set of shocks on the car and forget it, the thing is un-drivable.

[James Michael]

On the front of the Stohr, the lower front A-arm fwd rod end has a very thin flange to pick up the rod end on the bottom side. I might suggest checking that to see. It was thin and cantelevered out a few inches away from the front bulkhead of the chassis. I reinforced this area on my car by making the flange thicker and fitting in a triangular piece to fill in the gap. Worth checking.

James

[peteytoo]

Tried to aim cameras to capture the event for analysis. Not sure framerate is high enough to really do it, but you can get an idea even though this is not a heavy braking turn.

http://youtu.be/QSYbQ75XJqU

Thanks to all for the awesome responses so far!

[Daryl DeArman]

I am thinking/hoping much less expensive and simpler fix.....

Fingers crossed.

Front caster going too negative during dive.

My redneck engineering methodology would have been to check static caster and if okay then when under acceleration give slight brake pressure to see if the vibration is there. If so, I'm looking harder at brake system and a-arm mounts. If not, I'm looking at the front a-arm geometry to see if something has "moved"/"changed" so that I am getting too much caster loss under braking.

[JakeL]

Ran into this on my Stohr WF-1 DSR back in the day. Problem was twofold:

1.) The bobbins in the Stohr rotors are too tall, so there is an ASSLOAD of float to them.
2.) the bobbins are different material than the rotors and heat up/expand much faster than the rotor does.

This leads to the bobbins expanding at the end of the braking event and locking the rotor in a sideassed arrangement which will vibrate the wheel right out of your hands. My symptom was the car would be fine for the first 1/2 to full lap, then as the rotors came up to temp, the vibration would start and not go away.

Solution for me was:

1.) drilling the holes in the rotor a few thou larger than delivered.
2.) Wayne provided some spacers to reduce the float down from .050 to the .005 or .010 range. just barely enough to feel by hand.

Post on JL.com from back in the day:

http://www.jakelatham.com/2011/06/

"Bobbin Spacers – as mentioned below, Wayne has some .032 spacers that will fit on the floating rotors to reduce the float down into the .005 to .010 range. They can go either directly under the “hat” of the bobbin, or on the back side of the rotor, just under the head of the bolt that secures the hat to the rotor. I found that putting them on the back/under side of the rotor was a better location, as otherwise the rotor is moved inboard by that .032 thickness (give or take). In the rear, that was enough to put the new vented rotors uncomfortably close to the nut which holds on the bolt that attaches the caliper. Putting the spacers on the rear floating rotors definitely helped, as I had far fewer issues with knockback coming into Canada corner after installing them."

The bobbin spacers appeared to be sourced from McMaster-Carr, I think they were just steel shims of a certain diameter and thickness, enough to clear the OD of the bobbin.

Hope that helps,

-Jake

[Blair Robertshaw]

Ivan, watched the video a few times, take a look at the thread boss thats welded to the frame on the lower right wishbone aft, where the A arm attaches. Take a real close look at it, that was what let go on our Stohr and then on closer inspection we found more and more and more cracks. It may reveal a hairline crack and that, if it is cracked, may be the cause. Good luck Ivan, hope you sort it soon.

[Rennie Clayton]

Ivin:

When's your next outing at NJMP? I'm bouncing back and forth from NJ to the Stohr / Dauntless factory in Cali these days, but if schedules align I can drop by the track to have a look at the cars.

Meantime have a very close look at the mounting points for your front lower a-arms. Have seen this behavior before when the a-arm mounts are moving around. My Ralt would vibrate eyeballs out of sockets if you touched the brakes and the front lower a-arm mount clevises got even slightly loose.


Cheers,
Rennie

[Ivinsea]

Ivin:

When's your next outing at NJMP? I'm bouncing back and forth from NJ to the Stohr / Dauntless factory in Cali these days, but if schedules align I can drop by the track to have a look at the cars.

Cheers,
Rennie

Rennie,
That is a nice gesture.

We dropped Pete's car off at Eric Lanbein's shop yesterday.

The next member days at NJMP are:

Fri June 26
Sun July 5
Thurs July 16

Then the next race is the Majors July 24-26.

I'll let you know when we are testing.

[Ivinsea]

After swapping out parts from my car to Pete's, we isolated the problem: rotor & hat assembly.

What threw us for a while was that with 8 brand new rotors and 4 brand new hats, it STILL vibrated.
We drilled out the rotor holes to allow the bobbins more space to expand - no change.
We tried running the bobbins with and without .030 hardened shims - no change.
New hats - no change.
New bobbins measured to within .001 to .002 length - no change.

We run the 8 year old rotor/hat/bobbin assembly from my car: presto, no vibration, perfectly smooth.

We still do not know why my rotor & hat assembly works.

We wanted to run the brand new parts assembly on my car to confirm that they vibrated, but my car has an issue.

After we get through the Majors this weekend, we will measure my assembly and see if we can ID the difference (s).

[yellowesty]

Are you CERTAIN that, when using the rotors that cause vibration, there can be NO overlap of ANY of the brake pads radially past the edge of the rotor, either toward the axle or away from it -- no matter how the pads reposition themselves in the calipers? If there is, vibration can ensue once the pads experience wear, as the unworn portion of the pad irregularly contacts the unmachined inner or outer edge of the rotor's wear surface. This vibration under braking can be intense yet devilishly hard to pin down. Ask me how I know.

[ExKartracer]

A couple of ideas come to mind. New rotors may not be machined parallel - mike the rotors at various points on the surface. I have seen rotors that warp as soon as they start to get hot and return when cold. Hard to trace without replacing one rotor at a time and have a lot of track time to do it. Possibly in a large parking lot?

JMR