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Like all issues, there's a story behind this vibration. I actually had all the wheels balanced first. The vibration was still there. Then I had it aligned. And it still had the vibration. Because it was a resonance at a specific speed, It had to be something with the tire balance. So back to the tire store. The tires were older and maybe 2/3 worn, so I had them replaced and told them to watch the rims carefully and if possible measure the runout. They weren't willing to measure the runout, but they did find a small dent on the inner side of the left rear rim. It was big enough to be visible spinning on the balancer if you were looking for it, but easy to miss if just watching. New tires and a new rim later, no vibrations! What doesn't make sense to me is this should have been a continuous vibration. The old tire balanced OK, but the rim made it physically not round. Wouldn't that cause a vibration at any speed?
Like all issues, there's a story behind this vibration. I actually had all the wheels balanced first. The vibration was still there. Then I had it aligned. And it still had the vibration. Because it was a resonance at a specific speed, It had to be something with the tire balance. So back to the tire store. The tires were older and maybe 2/3 worn, so I had them replaced and told them to watch the rims carefully and if possible measure the runout. They weren't willing to measure the runout, but they did find a small dent on the inner side of the left rear rim. It was big enough to be visible spinning on the balancer if you were looking for it, but easy to miss if just watching. New tires and a new rim later, no vibrations! What doesn't make sense to me is this should have been a continuous vibration. The old tire balanced OK, but the rim made it physically not round. Wouldn't that cause a vibration at any speed?
While this is maybe not the technical answer you are looking for, these cars are extremely sensitive to tire condition, balance AND pressure imbalances. My speculation is that the tire balances within the speed and variation of the machine. However, at a different/certain speed and condition, there was still an imbalance and you could feel it in your particular car.
last year I had a similar vibration in a new to me 99 996 Cab. After balance it made no difference I returned to the tire store and upon further investigation it turned out that there were two flat spots on the RH rims. They were able to straighten the rims and now the car is smooth *** glass at speeds up to eighty.
Usually out of balance or a bent rim will show up as a vibe around 65-70mph. Its a rotational frequency thing. Im sure there is a bunch of math that explains it but i just except it so i dont have to do the research. Driveline vibes show up at around the same speed too.
I completely understand how higher performance requires higher precision components and assemblies. And when they are complicated assemblies, the cause could be somewhat removed. The more interesting vibration that showed up shortly after this(maybe it was there all along, but I had not noticed it) was seemingly caused by the engine mounts. Hard acceleration from 60-90 and then holding at 90 would result in a similar type of vibration, like a wheel out of balance. But! This vibration would dissipate in a few miles at 90. This behavior was quite repeatable. From reading the boards, I decided it was likely the engine mounts. The thinking was that a hard acceleration would move the engine slightly, changing the alignment of the transmission and driveshafts. This would somehow create this effect. Sure enough, changing the engine mounts got rid of this vibration. The mounts were amazingly misshapen.. Maybe just a good guess. Now I have the problem of old interior rattles from better coupling of engine vibration into the cabin. It's been super fun to work through...
Sorry in advance for the poor Iphone photos...
Just picked up the car with the completed appearance items...I thought this was the best time due to COVID, and no track days
After 21 years of normal wear and tear, which included 4 years of solid track time, I thought it would be nice to spend a few $ on her appearance
*PDR all around
*Roof channel brackets replaced
*Exhaust Tips Aligned
*Front repainted..Hood, Fenders, Bumper
*Headlights sanded and clear coated
*Expel applied to headlights, hood, fenders, fr bumper
OK, time to get back to the use and (proper) abuse of this car!!!
Looks good, cleaning up the headlights alone can make a huge difference. My front end is pretty peppered but there is so much other stuff I wanna do first.
Picked up a set of Bilstein's B4 to replace the current ones of my M030 RoW Sports suspension. Will be putting them on next week . Banner was a present from the stockist I got them from , will be going on my garage wall
05-26-2020, 02:40 AM
