Jounce bumpers & boots

I'm changing springs in the E21 soon. When I took them apart the first time the jounce bumpers basically disintegrated. I've been putting off installing the new springs so I could get some new ones. However it looks like the cheapest I can find a entire set is going to be around $75. That'll go a long way towards a lot of other things I need.

So how bad would it be to not run jounce bumpers on a rallycross car?

The stock struts are only going to stay on the car until I can get some Bilstien to replace them. And that will render my new jounce bumpers useless. However those are several items down on the list.

I have to renew my SCCA membership and that's $80 I'd rather spent the money on that.

Is there a cheaper alternative out there?

Any cheap bump stop that will fit on the shaft will be better than nothing. If it were me, and I had more time than money, I'd try to figure out about what length the stock bump stops were and head to the junkyard to salvage some from the newest car I could find with something similar. Bump stops can't cost much from a junkyard. Otherwise generic ones will work fine.

Run them, definitely run them on a car that is going to be exploring the limits of its suspension behavior. Ground Control has some inexpensive options.

http://www.ground-control-store.com/products/description.php/II=10/CA=1

Functioning bumpstops make a huge difference when you're actually bottoming out. Without them, it usually feels like you're breaking something (sometimes you are). With them, you have the confidence to push harder and maybe actually break something. yeehaw! I love bumpstops.

nicksta43 wrote: So how bad would it be to not run jounce bumpers on a rallycross car?

Depends on how much you like damaging your strut mounts and/or struts.

Were it mine, I'd go to Summit Racing's website, search for something like this: http://www.summitracing.com/search/stock-replacement/yes/department/chassis-suspension/part-type/shock-boots

...then find something close, see what the application is, then try to get that from the local auto parts store.

Well I'll have to do some research and see what I can come up with. I have a very limited budget and I just hate spending a large part of it on something that is going to be rendered useless in a little while. The junkyard Idea sounds good. If the wife wasn't working I would go today, the boy is too young to get in. I wonder how I could get them off in a junkyard without a spring compressor...hmmm.

OK went ahead and ordered the fronts for $33 with shipping. Can't find rears for under $25 each and I'm not spending that much, I'll have to figure something else out for those.

Why are the Bilsteins going to render these useless?

If they are like the ones on my Peugeot they have internal bump stops. On the front struts at least.

Yep, upside-down Bilsteins like HDs and Sports have internal stops in both directions.

Plus, even if they didn't, the shaft that you can see is over an inch and a half in diameter so a standard bumper won't work.

Huh, interesting. Thanks.

Generally speaking, you only find these on strut cars, so if you mainly surround yourself with Miatas you won't see 'em. HD shocks aren't upside-down, although they can be if you want to reduce unsprung weight.

The nice thing about upside down struts is that the strut shaft bears no side load. The shock body-to-strut housing bushings are what take the side loading.

Note: Most of the reason why I bought this GTI was because it had HDs on it.

Hey, I don't just have Miatas. I figured my M39 might have a few things in common with the E21, but the Konis I put on that still used external bumpers.

Oddly enough, when I think about it, the E39 is the only car I own with struts. Scratch that, the Vanagon and Mini have at least one pair each.

I don't see how the shafts can't take a side load simply because the strut is inverted. You've still got one end on the wheel and the other on the chassis.

Keith Tanner wrote: I figured my M39 might have a few things in common with the E21, but the Konis I put on that still used external bumpers. \

M39?

Keith Tanner wrote: I don't see how the shafts can't take a side load simply because the strut is inverted. You've still got one end on the wheel and the other on the chassis.

Well, I suppose if the bushings are really shot then the shafts could see side loading.

Picture an empty strut housing, just a tube with the top open. Now put a bushing set at the top of the tube and stick a shock absorber in it shaft-down, attaching the shaft to the bottom of the strut housing and the shock body to the strut-top. The side loads only go from strut housing to shock body, the shaft only sees up-and-down loads (as it should)

They're heavier than standard struts, but they're a lot tougher since the bushings that take the side loads will be far larger in diameter and can be spaced farther apart from each other. When you really want to get fancy, you make the shock body square and use needle bearings instead of bushings...

Knurled wrote:

Keith Tanner wrote: I don't see how the shafts can't take a side load simply because the strut is inverted. You've still got one end on the wheel and the other on the chassis.

Well, I suppose if the bushings are really shot then the shafts could see side loading. Picture an empty strut housing, just a tube with the top open. Now put a bushing set at the top of the tube and stick a shock absorber in it shaft-down, attaching the shaft to the bottom of the strut housing and the shock body to the strut-top. The side loads only go from strut housing to shock body, the shaft only sees up-and-down loads (as it should) They're heavier than standard struts, but they're a lot tougher since the bushings that take the side loads will be far larger in diameter and can be spaced farther apart from each other. When you really want to get fancy, you make the shock body square and use needle bearings instead of bushings...

Still not completely convinced - as long as the only thing connecting the suspension and the chassis is a shock/strut shaft, there has to be some side load. It might be able to deal with it better because of the size and locations of the bushings. Anyone got a diagram of how this goes together?

Found these, not really answering your question though.

Hope this helps. The side loads are borne by the strut tube to shock body interface. The shock shaft only sees side loads if the strut tube bends or the bushings between the tube and body get trashed.

I suppose, technically, since there has to be a small amount of tolerance for it to move, that the shock sees SOME side loads, but it's negatable compared to the loads a normal strut sees from cornering or braking.

Plus, they look sweet, and there's a small reduction in unsprung weight since the weight of the oil and shock body is attached to the car, not the wheel.

Movement's a result of side loads, not a cause. Like body roll

But I see what you're talking about, the shaft is basically encased inside the strut tube. That's what I was missing. Interesting.

But the shaft is not always inside of the housing and it is what attaches to the top mount.