JPS rear end shake

good thinking Tom--but its not possible

Two possibilities come to mind.....   Trans/Engine mounts or suspension /alignment problem...  Get ye to a high rise lift....

what year is the chassis?

what type of wheels are you running?

Any wheel adapters or spacers?

Also, check our tires for even wear? if there is a spot or two that looks to be worn more then the surrounding areas, you could have a tire coming apart (yes like the semi truck tires that you see pcs. of on the freeway) This will be fatal to your body work if the tire or tires come apart.

You might, as a first step, try getting all four wheels "Road Force Balanced".  A number of car dealers and some tire dealers have these machines (or you could get on the "Bear Automotive" site and find out where they've sold them) which apply pressure to the tire running surface while computing the needed weight to balance things.  Somewhat more accurate than the usual dynamic spin balancers.  the only problem might be that they won't have an adapter for your wide five wheels.  Another thing to consider is that so few shops do wide five these days that they aren't experienced at balancing them.  Should be simple to do, but when you do 4 wheels every 7 years you lose th experience.

After that, I would look to see if there is some sort of body shake going on.  A number of these replicas have this mysterious shake above 65-70mph, a result of less than adequate bracing of the body elements, allowing them to get into an harmonic rhythm at certain speeds.

Why not move the rear tires to the front and see if that changes things? If the front shakes, it is wheel/tire related. Broken belts in the tires will do the shimmy, shimmy shake.

Gary

JPS speedster-74 beetle chassis--new build-only 500 miles on the car--no adaptors-perfect tires------- Gordon-mysterious shake sounds like nobody has found the cure--what to look for?

I'd do this first.  If the rear is still shaking you can then cross wheels/tires off as a possible cause.

One thing might be reproduction wheels with excessive runout.  That's what I discovered when I had mine Road Force Balanced.

Lane--what was the fix-or wasnt there any-can road force balance correct the run out

Ron and Gwan---Dont know if the super wide rear wheels will run safely on the front

'fraid not.  The fix is new wheels.  I haven't decided if I want to do that yet or live with it.  It's only an issue as some speeds some times and I really like the look of what I have now.

It took me THREE SETS of wheels before I got a set that was actually round! For the first two years owning my VS I couldn't get a decent balance. What I have now give a ride that's smooth as silk---absolutely zero trace of any shimmy in the steeing wheel.

You would think that "roundness" would be a quality that is always part of a wheel---but it ain't necessarily so. 

Same goes for tires.  There's no such thing as a "perfect tire", even when new.

I have a set of real, Fuchs forged wheels from a 911.  I have checked them for wobble (less than .004") and roundness (less than .002").  Believe me, these things are both light and straight.

My first set of tires were Continental radials (mostly because that's what used to come on Porsches).  One wheel needed almost 32 oz. of wheel weights to balance it!  All of those tires needed at least 16 oz of weight per wheel, with two wheels needing over 20 oz each, but at those rates they balanced out (and I had several different places balance them over the years).  

After ten years I pulled the Conti's and installed a set of Michelins but before they were installed I asked the tech to run each wheel on their machines to check for trueness - all four were as noted up above and none of them (when spun naked) needed any weight at all so all of the weight that was needed for the Conti's was for the tires alone.

When the Michelins were balanced, the worst one needed 6 or 7 ozs of weight.  One of them they almost didn't put anything on but then opted for a couple of 2 oz at different angles to catch a minor shake.  I was (a.) really surprised and (b.) pissed that I had screwed around with the Conti's for so long, but swapping out a set of tires isn't cheap, so they stayed for ten years.

On the body shake, I've heard reports of this from more than several people with different models and different builders and have never found an obvious cause.  Considering that the body is an eggshell and poorly braced at best, it's not all that surprising, but I suspect it's somewhere behind the driver.  On CMC's at least, there isn't a whole lot holding the rear clip up so maybe that's a good place to start looking.  Also, many of us run such mushy shocks that if there is any amount of shake in either wheel back there the shocks won't do much to damp it out.  As Jack found, it could certainly be a wheel or tire causing the shake, and it often takes a concerted sleuthing effort to find and correct it.

I was thinking of wheel runout but hub runout is also possible.

lined up a road force balance for  early next week-- $13/wheel at firestone

Not all Road Force balancing is the same.  A lot depends on the tech running the machine.

I've owned a couple of Miatas and they are notorious for front wheel shimmies.  When I had them Road Force balanced I stressed that he (the tech) had to get the wheel/tires balanced as close as humanly possible.

I ended up paying $125, but it was worth it.  To do it properly, the tech had to rotate the tire on the rim a number of times to hit the 'sweet spot' balance wise.

ron u and vince are my Miata heros

Safest thing is to check with the manufacturer. Any input there?

linking me with Vince.....Oh boy...

had all 4 wheels road force balanced and now she rides very smooth

Yeah, these cars are so light tat it doesn't take much imbalance in the wheels to be very noticeable.

Glad that's all it was!

BTW:  $13 bucks per wheel is cheap!