Strange shudder under braking.

[eshuler]

For the first couple of laps, my mysterian M2 has an odd low frequency shudder under threshold braking. It goes away once the tires get up to temp.

It seems to be coming from the front, but not 100% sure. I put a dial indicator on the outside of the drums, thinking they are not round.

Both rear drums runout is about ~.010.
Both fronts are ~.005

Is there a spec for this measurement?

Any suggestions?

[Jon Jeffords]

Check out this recent thread:

http://www.apexspeed.com/forums/showthread.php?t=68929

[jpetillo]

Are they new brakes - different track?

[Purple Frog]

Are you parked on damp grass before you go out on the track?

Being a FV on bias tires. It could be a cold tire syndrome. Since it goes away when it heats up, it is probably not loose suspension, etc.

Does the brake pedal range of movement change after it warms up?

[problemchild]

Try adjusting your brake bias, and remove some from the front. I know that does not make a lot of sense based on your observations, but I have spent a lot of time trouble-shooting brakes on Mysterians.

[Jean-Sebastien Stoezel]

For a couple of seasons I had similar issues. Under braking the car would vibrate a lot, and the steering wheel would rock left and right.
Moving the brake bias towards the rear helped reduce the vibration but also decreased the braking power, which I believe masks the real issue. Whenever I would come back to the pits I'd often find the link pin fasteners loose, initially I thought this was the issue. It was in fact a consequence of the vibration.
I tried a few things, but only one modification cured the problem for good (thank you Stevan Davis for the insistent suggestion).

This is what didn't work for me, but it might work for you:
- Tried different sets of shocks
- Turned all 4 drums
- Bent droop horns to prevent bottoming of the lower torsion arms under braking
- Different toe settings
- Tightening the link pins over and over
- Different ride heights, some suggested the vibration was due to a loss of caster under braking
- Removed play in the steering box, over and over. I think the vibration stressed the steering box, which would keep on developing play. Which in turn added play in the front, and more vibration etc...

What fixed the issue:
Checking the micarta bushings in the beam for excessive play. In fact one bushing had been pushed further into the beam and wasn't even touching the torsion arm.
The issue went away once the micarta bushing got put back in place.

Your issue might be different, in fact I think any of the parts or settings I had checked could have been the cause of the vibration, should they have been faulty.
I'd check that the micarta bushings are properly located (I think mine are located about 6.25" from the outer edge of the beams - but just measure them all and compare), and that they are not worn out (check for excessive play of the torsion arms when installed in the beam without the spring pack and torsion bar).
And if all of this checks, move on to the next items.

JS

[Matt Clark]

I have had the same issue. Do all the checks that JS mentioned, but definitely follow Gregs advice. I started moving the brake bias rearward (surprised me how much) & adjusting the brakes a little better. When you set brakes, don't forget to spin the tires & hit the brakes hard to center the shoes. Less space between the shoe & the drum made a big difference on mine. Cold tires were mentioned, but it's not the tires, it's brake heat affecting it.

Also, when driving, be mindful of how hard you go to the brakes. I caught on that I was stabbing them, which created the shake. Easing into them slightly makes a world of difference.

[eshuler]

Thanks for all the advise, I will definitely follow your leads, and let you know how it turns out.

[budawe]

JST covered the main points quite well. It would be helpful if we knew what kind of shoes you are running. The high friction shoes being run tend to wear improperly. Due to the high friction, the shoe doesn't get flexed into full contact. This results in lopsided wear. If you look at the leading edge of the shoe on the primary or leading shoe, the wear will be multiples of what will be observed in other parts of the shoes. The leading edge of the secondary show will also show bad wear but to a lessor amount. This leads to shoes that no longer match the radius of the drum. Swapping shoes from top to bottom every couple of weekends will minimize this issue. Lopsided shoes will cause the issues you have mentioned. As the shoe gets hot, the issues is reduced.

If you observe dark spots on the braking surface in the drums, it is likely you have developed "hot spots". These hot spots create different friction responses and will cause the symptoms you have discussed. The spots will not be able to be ground out and the drums will have to be replaced.

Unlike disc brakes, our drum systems carry their own set of issues. Generally speaking chattering, noise and pulsing all are caused by the relationship between the shoes and drums. If there are wear or geometry issues in the suspension, the issue gets compounded.

Try applying the brakes very gently while rolling in the pits and observe the movement of the pedal. Don't push too hard or you will miss the pulsations. If you can feel the pedal going up and down, the drums are out of round most likely from heat warpage. If the drum is beyond spec and is too thin, it will warp much easier after being turned. The max allowed in VW drums is 9.114 and new is 9.060. In drum systems the shoes have to match the drum radius and arching shoes is a bit of a lost art due to OSHA and absence from street applications.

[Bruce Livermore]

Since this problem seems to go away as things heat up, this may not apply. However, maybe you are adapting to the problem without realizing it by applying the brakes more smoothly and maybe braking earlier and lighter as you do more laps (and the tires and brakes warm up).

My M2 has had front brake shudder on and off throughout my ownership of it (20 years now). Sometimes it has been extreme as in hopping from one front tire to the other. On researching this, I found out that it was quite common on M2's if not all Mysterians. For me, in every case, the shudder was eliminated by changing something in the rear suspension, usually adding some kind of rear roll damping or roll resistance. My theory is that the M2 rear suspension is very close to true "zero roll resistance", thus it cannot damp out any roll oscillation started at the front for any reason. Adding just a small amount of rear roll damping is like a magic elixer: the problem just goes away.

On a standard M2, the cure is simple: tighten up the rear rocker bolts until there is just a slight amount of resistance to movement. It takes very little resistance to cure this problem if it's what you are experiencing. To set it up, remove the rear shock and the pushrods from the rockers so they are free to pivot. Tighten each pivot bolt until each rocker just supports it's own weight. The rockers should move easily but not quite fall freely. Put it back together and go. You're done. You can try tightening or loosening at the track, but I've found it takes very little to fix the shudder (the "Mysterian Shakes" as it's been dubbed).

But while you have it apart, check the bushings and bolts for wear. With the bolt just snug (no friction but no end play) the rocker shouldn't have much free play front to back at the shock end. There will be just a bit, but the shock end of the rocker shouldn't be moving too much, say 1/16" at most if I recall correctly. The bushings are standard 7/16" ID flanged "oilite" bushings, two pressed into the chassis and two pressed into the rocker on each side (8 total on the car). The ones in the rocker need to be cut to length or carefully filed flush with the arms after installation. The bolt required is a 7/16" fine thread bolt with some hard washers and a nylock nut. AN grade stuff is probably best, but a grade 8 bolt is fine (even a grade 5 is OK but it might wear a bit faster). Just make sure the bolt is long enough so the threads aren't in the bushing. With a grade 5 or 8 bolt you may need to add some washers to make sure the nut doesn't bottom on the threads.

Do check all the things suggested above to make sure there are no basic problems especially with the trailing arm bushings. Of course if the trailing arms are loose, then the shudder shouldn't go away as things warm up either...

Bruce

[problemchild]

Ed, I have a bucket full of ARBs , dampers, and other gadgetry, I have built to combat the problem. I would be delighted to sell it to you, but just back off the front bias and go racing. I even had to change one master cylinder size to get the bias adjuster in the correct range.

[budawe]

As mentioned before suspension geometry can be an issue like the M2. The rockers are too close to the centerline of the chassis allowing the car to rock back and forth. Remember that the shuttering was likely started by the brakes.

[wyztek]

Cancel this! Didn't read the carefully enough. Doubt the shuddering would go away after warming up if it was a cracked frame.

This may be too far out in left field, but I had a cracked frame that caused an audible shuddering under braking. My problem was at the back under the transmission mount, but if nothing else reveals itself, do a careful inspection for any cracks at stressed joints.

[BeerBudgetRacing]

Cancel this! Didn't read the carefully enough.

Sometimes restating the same thing in a slightly different way gives the reader an ah-hah moment. Always good to share your experience.