BMW Compact Build

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john vanlandingham

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Doivi Clarkinen

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DaveK
I think the DMS Golds (40mm street stuff) used 180 progressive springs...but no idea diameter or what the higher rate was.


Dave

The DMS springs are not true progressive springs. Just a regular linear rate spring with tapered ends. Think of the tapered portions as built in tender springs. The tapered ends are designed to be fully compressed at static ride height so whatever rate are stamped on them is what the rate is throughout the travel. Also, they are 70mm I.D. and made by King spring.

Dave, taper ground wire are TRUE progressive springs.
Straight wire can be Progressively wound like what most call "progressive" but a true "progressive is taper wire, very expensive to do versus straight wire.
Which is one reason they're so rarely used.
The two cars I've had true taper ground wire springs on, didn't like either---too soft (That's the Saab 96 S&R rears rally and the Xratty OE rear)

You're not paying attention, though. The way DMS uses them the "progressive" portion is just used as a built in tender spring. The effective part of the spring is linear. I'm not making this up, this is straight from Jamie Drummond's mouth. Something about it being to hard to match the adjustable valving to a truly progressive spring. Not that DMS have that much adjustment range...

Raising the shock towers would be a major PITA at this point, but something I could do if it seems like the best way. I could've welded in the towers up to another 3" higher, but the dampers you had earmaked for me to use at the time wanted the heights that I had the guys weld them in at. I knew a fair bit of stuff on this car would be based on guesstimates, so not unhappy that we're having to dial things in as we go. As long as I didn't have to drive something that looked like as funny as Grant's car.

The Stig dampers (I think were 1" longer than the ones I started with) could be another 1" taller, as the RTAs won't bottom out against the rear beam until ~19" fender/wheel center...but I don't know that being longer gets us any benefit on the up-travel.

I *think* if I had a more normal shaped caliper that I'd pretty easily find room to fit a 12" 175# and a 4" 300#.

Dave



Edited 1 time(s). Last edit at 04/25/2011 03:55PM by DaveK.

Dave:

What weight Kevlar did you use, and how many layers? Also, did you lay it so that it was removable, or did you epoxy it onto the bottom of the car? If it was removable, what did you lay between the chassis and the kevlar so that the kevlar wouldn't stick? How did you do it?

We're looking to do this with our BMW, and just need some guidance on how it is done.

Thanks in advance,

Wilson

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wvonkessler
Dave:

What weight Kevlar did you use, and how many layers? Also, did you lay it so that it was removable, or did you epoxy it onto the bottom of the car? If it was removable, what did you lay between the chassis and the kevlar so that the kevlar wouldn't stick? How did you do it?

5-5.5oz cloth if I remember correctly. Its a commonly available weave that's got a multi-axis break, so it forms pretty well around compound curves. We bonded 2 layers directly to the chassis, and even went so far as to epoxy strips into some of the frame gaps in hopes of keeping it from being able to peel off. Certainly an easier project if you can flip the car up-side-down.

FWIW, I have 4-5 layers on the Evo and after 3 seasons of hillclimbing, its just starting to wear through on some of the vertical surfaces that catch lots of rocks. I figured since the BMW doesn't have driven front wheels, there's not as much wear and tear from rock-blast. Time will tell if that was a good decision or not.

Dave

Had to get moving on getting the Evo ready for PPIHC this weekend, and first up was to replace the bushings on the car. That meant pulling off the 4 rear arms on each side and the control arms up front. I remember reading somewhere that on the Evo, the motion ratio of the rear effects the spring rates such that you multiply the spring rate by ~80% to get the "wheel rate." The spring rates offered in Ohlins rally dampers are as follows:

Gravel 55f/55r (315/315)
Rough Tarmac 70f/80r (400/458)
Tarmac 85f/90r (486/515)



From the pic here you can see how the shock arm attaches to the hub just under the brake caliper. The shock attaches about 2" inboard from the mount at the hub (inner of the two round lumps you can see).

I imagine that the motion ratio on a multilink rear is different from the RTA setups on the Merkur and BMW, but thought it got me thinking...dangerous, I know.

Got to run down to the car this weekend and get some measurements:

-----------------------------------------------------------Merkur--------BMW
RTA front mounts to shock mount----------------------22.5"---------19.5"
RTA front mounts to hub--------------------------------18.5"---------16.0"
Center of lower shock mount to inside of wheel-------0.75"---------2.75"
Center of lower shock mount to outside of wheel------9.0"----------11.0"

Some quick interwebz browsing turned up a 67% motion ratio for the E30, but I'm not sure that info is really worth anything since the E30 used an inboard spring whereas I've now got a coilover setup (spring now mounts further outboard and rearwards). In restrospect, I should've measured this change as well.

So, after all that, not sure if any of this information is worthwhile or will get me any closer to a solution, but the fact that the BMW wheel sits further from the shock than the Merkur wheel seems to indicate that stiffer springs and/or valving may not be out of line. Curious to hear what you guys think.

Dave



Edited 1 time(s). Last edit at 05/09/2011 12:46AM by DaveK.

I moved the mount back 1/2" and toward the wheel 1/2" on mine to get the spring to stop rubbing in the tower. I run the 150 lb 14" spring at the rear. On Wreck creek stage, which has some big G outs, and a couple jumps I felt the rear bottom once, and it did not do anything weird.

BTW, I still have the stock bump rubbers as well as the bump rubbers on the shocks.

I guess I should be saying "had" and "ran" now....

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phlat65
I moved the mount back 1/2" and toward the wheel 1/2" on mine to get the spring to stop rubbing in the tower. I run the 150 lb 14" spring at the rear. On Wreck creek stage, which has some big G outs, and a couple jumps I felt the rear bottom once, and it did not do anything weird.

Interesting - based on my thinking that would allow you to run softer rates than a standard setup. I talked with Grant and he said he's never had bottoming issues on his suspension at CORE and he's running 175 lbs springs.

Curious - any idea how much shock travel you have on your car? I'm on dampers with 8.25", but I sag 3" into it, and with 2.5" bumpstops, I'm pretty sure I have 2.5" of compression before the bumpstops come into play. Unsure how much those squish befort the bottoming out happens though.

Dave

FYI for folks looking to get Kevlar:

We ordered our Kevlar from Aircraft Spruce - they have a knockoff Aramid cloth at $13.40/yard in 38" width. I thought that was a pretty good deal.

W

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wvonkessler
FYI for folks looking to get Kevlar:

We ordered our Kevlar from Aircraft Spruce - they have a knockoff Aramid cloth at $13.40/yard in 38" width. I thought that was a pretty good deal.

That's not too bad and would be a great width for doing a front skid plate and could be just about right for doing the floor on each side of the trans tunnel. The place I was buying locally (Plasticare) had volume discounts and anything over 10 yards was ~$20/yard, 55" width.

Dave

I keep in touch with our old client/friend Dedo, he just uncorked his compact last weekend. More specs and pictures coming.

Bmw compact chassis
M3 e46 engine
Ohlins suspensions
sequential gearbox
4x4 trasmission from 325ix

"Just yesterday we went in race to its debut and we won 2 special stages on gravel in front of Trelles with official Lancia Delta Martini but we had just rear wheel drives!! It's nice" ciao Dedo

Dedo is rallying in a nice neighborhood there..

Sweet, look how far back the codriver is. So I'm confused, is it RWD or AWD? Says it has a 325ix 4x4 transmission.

Details and more pictures coming soon. Language...when Dedo was running with us there was a lot more caveman style grunting and motioning than talking. All I could really make out was "we need more turLbo boOost pEta"

I believe it's a AWD(4x4 ix), but he had "issues" with the front so ran a locked CD and RWD on the first event.



Edited 1 time(s). Last edit at 05/24/2011 07:42PM by Gravel Spray.

Do want!! (minus the whole AWD mess, LOL)

Peter - Any way to find out what sort of spring & valving rates he's using (if its been converted to coilover in the rear)?

Dave



Edited 1 time(s). Last edit at 05/24/2011 11:50PM by DaveK.

So now that PPIHC is done and I can put the Evo away for a few weeks, I'm turning back to the BMW.

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MRWmotorsports
We fixed the problem by going to a longer shock and double spring. The problem with a longer shock, as you have alreday noticed, is the car either sits way too high, or the shock has no travel and bottoms anyway... We had to raise the rear top mounts about 2.5"... basically allowed as much drop as the axles/subframe would allow, and raised the mounts to account for the rest of the shock height at full extension. I now run the same shocks as the Xrattys, but I use 2 springs per shock, 1 x 14", and 1 x 5" or 6". I've played around a bit with different rates, but mostly use 175Lb 14", and 300lb 6". This gives me a sort-of progressive rate, and 9+ inches of travel. shock does not bottom out, nor does it spring bind. Up movement is now limited by the tire rubbing the inner wheel arch, it doesn't rub much and only when landing big jumps.

I'm going to get some measurements of the limitations on the rear suspension this afternoon. Basically, I want to see how much the wheels can move before they come in contact with the wheel wells. Depending on the results here, I may look at adding some extra height to the shock towers.

One of the things I'm starting to explore is the idea of running stacked springs in the back. I've done a fair bit of interwebz searching, and the best info I found is from Hypercoils (http://www.hypercoils.com/PDF/dual_rate_slides.pdf). The interesting part of this to me is that when you stack springs, the combined rate (assuming both springs are free to compress) is LESS than the rate of either spring alone.

I was thinking of doing a 175#12" spring with a 350#4" spring, but after reading that stuff, I think that'll actually be worse than a 175#14" spring. According to a spreadsheet (link at bottom of this page: http://www.hypercoils.com/TechTips/?TipNO=1), my theoretical 175/350 setup would actually result in a rate of 117# until the 175#12" spring coil binds, and then I'd get the 350# rate kicking in.

Thoughts...theories...experiences?

Thanks,

Dave