BMW Compact Build

Out testing again today. Car seemed to run well and even put in one 30-40 minute session without stopping, alternating between some full tilt laps and some 70% laps. My buddy Josh hopped out and snapped some pics during this sesson, so hopefully will have a few more pics to post in the near future.

Did find out one interesting tidbit about the check-engine light on the 95 BMWs. Most fault codes don't have any effect on engine output, so nothing to worry about with the "evap system" code. Unfortunately the "speed sensor missing" fault code does do something - namely the rev-limit is dropped to 6500 or so. Confirmed that this is the case today, so will need to figure out some sort of fix for that before I go out testing again.

Suspension was better and driving at 80% or so there were no issues at all. When I turned up the wick, I could get these to bottom out as well. Interestingly enough losing a passenger seemed to allow for driving ~90% before bottoming issues were encountered, so I think we're getting closer to having the right setup. I'm thinking some slightly stiffer springs are in order for the next test day as well.

Final issue I really need to turn my attention to is the steering. Now that I'm getting comfortable with the car and driving faster, its got some nasty handling traits that feel like they're coming from the front. I've bottomed out the tie-rod adjustments and still have some toe out, so I think that and the combination of CORE being so dusty are resulting in the front wheels fighting eachother a bit, alternating which ones's controlling the direction of the the car. This just makes for a nervous feeling car at high speeds. At one point when combined with a bottoming out of the rear suspension and subsequent jounce, it caught me off guard and led to an 'off' at ~85mph. CORE is great because as long as you just follow your momentum off the course, there's not really much to hit. Pulled into the pits to look over the car and no-harm, no foul.

Dave

Video from yesterday:





There's a quick clip in there at about 1:00 in looking back at the rear wheels, looks like they're moving around quite a bit.

Dave



Edited 2 time(s). Last edit at 04/24/2011 02:50PM by DaveK.

From the way the car looks/moves I'd say your suspension is pretty damn good.

I've seen people get all obsessed with preventing bottoming, some bottoming is ok. I alot of guys up the spring rates to achive "the car never bottoms" status and in doing so they removed all grip and destroyed the handling. Obviously you can't have the car being bucked like that to much but having the car get "upset" 1-2 times a stage for a split second each time vs. having max grip for 99.99999% of the stage is an excellent trade....just have to make jounce notes..

Nice work Dave, the car looks good, sounds good, and I'm sure is a blast to drive! Thanks for keeping this thread updated.

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DaveK
Video from yesterday:


There's a quick clip in there at about 1:00 in looking back at the rear wheels, looks like they're moving around quite a bit.

Dave

Suspension is 'spossed to move.
Looks very good.
You've been so much is turbo 4wd beasts like the Audi and Evo-bitchy I stink youse forgots that we have to make the rear more compliant to get grip

Example: Ford Sierra 2wd used about 177 lb/in on RAC or on Scottish progressive 171-199-410 but that mid number is the on one to look at , or on 1000 Lakes high speed and BIG jumps 216 ln/in---which the also used same rate and length on Ulster, Manx and San Remo tarmac

On the 4wd cars the used 197 or so on gravel. 1230kg car.
They said as time went on they kept going softer

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Gravel Spray
From the way the car looks/moves I'd say your suspension is pretty damn good.

I've seen people get all obsessed with preventing bottoming, some bottoming is ok. I alot of guys up the spring rates to achive "the car never bottoms" status and in doing so they removed all grip and destroyed the handling. Obviously you can't have the car being bucked like that to much but having the car get "upset" 1-2 times a stage for a split second each time vs. having max grip for 99.99999% of the stage is an excellent trade....just have to make jounce notes..

Cool, thanks for the thoughts! I'd say at the moment I'm pretty happy with it, but still feel there's a little room for improvement. FWIW, on the Evo I can rip that back straight at 115mph and it'll bounce around and catch air, but no bottoming out or sudden issues to toss the car off line. Not sure if I'm simply spoiled from that or not, but its my reference point, and I'd love to have just a little more confidence in how the BMW's going to react.

An interesting thing that I think I forgot to mention in my post yesterday was that in 4th gear full throttle is where the bumps were at their worst. In 5th gear pinned at the same speeds the car seemed to behave a bit better. Thinking that maybe in the higher gear with the reduced torque at the rear the car is simply squatting a tiny bit less?

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john vanlandingham
You've been so much is turbo 4wd beasts like the Audi and Evo-bitchy I stink youse forgots that we have to make the rear more compliant to get grip

Audi? Had a 323GTX before the Evo. Interestingly enough that would top out at ~90mph on that straight, so very happy to see that the BMW in just its first couple times out is almost there.

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john vanlandingham
Example: Ford Sierra 2wd used about 177 lb/in on RAC or on Scottish progressive 171-199-410 but that mid number is the on one to look at , or on 1000 Lakes high speed and BIG jumps 216 ln/in---which the also used same rate and length on Ulster, Manx and San Remo tarmac

I certainly don't want to make any big changes, we're pretty close. I think it may simply be a matter of running out of "up" travel just a bit too soon which is more of a chassis limitation.

Rear shocks have ~8.25" of travel
At full droop its 18" (wheel center to fender lip)
At rest its 15" (OEM E36 cars are typically ~13.75-14.25"
Bumpstops are ~2.5" tall so we start compacting them at 2.75" of up-travel

Any idea how the X-ratties compare? We know they've got longer arms and the attachment points are different resulting in different motion ratios...but any idea on how much up-travel those have before bottoming out?

I think the valving seems pretty good, and the back end is staying in contact with the ground like its supposed to so I can put the power down. I'm wondering would it be worth looking at a progressive spring, so that it'll stiffen up gradually instead of "bang" as it bottoms out? I think the DMS Golds (40mm street stuff) used 180 progressive springs...but no idea diameter or what the higher rate was.

Just thinking out loud here:

If under hard accelleration say in 4th gear the back squats 1" (IDK what the actual number is), should I consider raising the car so at steady state under throttle the car is relatively flat - or are we wanting the rear lower so there's more effective weight on the rear wheels? If raising the car a bit more out back is a reasonable idea, that could give me a little bit more up-travel before bottoming out.

Dave

Dave, I gotta admit I'm a little puzzled. I do have do say that ALL my experiencve is ALL with 4 cylinder cars, and that thang you got in that is really long and don't know if you noticed it ---I could tell by the sound---but its an INLINE 6. Now some people say I'm anti-inline 6 but if I am its just from watching every inline 6 I have ever seen ---almost---having latent pig-u-lar tendencies---something is making me keep coming back to the length and weight of the motor....

Also the Evo-bitchy maybe in addition to being excellently balanced---there's a reason that worldwide its a 20-1 ratio of Evo-bitchies to Sub-a-rats---and there's a reason Sub-a-rats are sub---anyway I always thing back to thing things I know best---which is bikes---and in bikes---same frame forks hubs wheels seat bars etc---500 class bikes were stable at 100, 125s were squirrelly---500 bikes had power left to drive forwards and there is an inflexible law in bikes :"Power on for balance---off for grip", maybe the Evo has power left and so some stabilising DRIVE forward and thus balance, maybe the Bavarian thing is not making similar balancing type drive..

Maybe.

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DaveK
should I consider raising the car so at steady state under throttle the car is relatively flat - or are we wanting the rear lower so there's more effective weight on the rear wheels? If raising the car a bit more out back is a reasonable idea, that could give me a little bit more up-travel before bottoming out.

Dave

That would give you more front rake. The extra hight of the rear roll axis would add too much oversteer tendencies. The only way you could do it would be if you had a panhard bar. That way you could raise the rear but then lower the roll axis with the panhard bar. Best approach is just better bump stops?

Dave, The XR has more than 10" of travel at the rear. Watch the F-cup and finnish Volvos, they are practically sitting t on the rear bumper under acceleration.

The car looks very good. I don't know how good of a representation of real stages core is, but I have not driven on roads like that in my short career.

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Cosworth

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DaveK
should I consider raising the car so at steady state under throttle the car is relatively flat - or are we wanting the rear lower so there's more effective weight on the rear wheels? If raising the car a bit more out back is a reasonable idea, that could give me a little bit more up-travel before bottoming out.

Dave

That would give you more front rake. The extra hight of the rear roll axis would add too much oversteer tendencies. The only way you could do it would be if you had a panhard bar. That way you could raise the rear but then lower the roll axis with the panhard bar. Best approach is just better bump stops?

bump stops!!! this is where the rieger comes in, clicks....clicks clicks ....adjustable dampers...magic 4way...5way with hydrualic bumpstops... no bunce/jounce ever again!

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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.

Car looks good to me. Since you're a bit limited on rear travel then stiffening the car enough to stop the bottoming will likely make it too stiff in the back.

I wonder if you're too familiar with the track? You are maybe driving harder or faster than on an unknown stage? If that is the case you might not see much bottoming out in real life situations.

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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)

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Gravel Spray
bump stops!!! this is where the rieger comes in, clicks....clicks clicks ....adjustable dampers...magic 4way...5way with hydrualic bumpstops... no bunce/jounce ever again!

lol yeah well, back when I worked at Peugeot the team manager used to say "if you dont have a dog then hunt with a cat" And we all know that the billies dont have clickers so theres not much that can be done besides revalve.

I'm a bit reluctant to chime in, given I'm no suspension expert... but we had similar issues. In 2007 at Baie-D-C near the end of an almost 2km, almost straight, were a couple of bumps, we didn't even notice them during recce... when gioing as fast as my balls would allow (maybe 110mph?), we stood the car practically on it's nose, due to the rears bottoming. I thought we were going end-over-end for a moment.

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.

Maybe this helps.

-Martin.

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MRWmotorsports
I'm a bit reluctant to chime in, given I'm no suspension expert... but we had similar issues. In 2007 at Baie-D-C near the end of an almost 2km, almost straight, were a couple of bumps, we didn't even notice them during recce... when gioing as fast as my balls would allow (maybe 110mph?), we stood the car practically on it's nose, due to the rears bottoming. I thought we were going end-over-end for a moment.

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.

Maybe this helps.

-Martin.

Don't be shy, man this is how we fine tune stuff. I was way concerned about lengths when Dave was busy caging the car---I know you'd gone over to the long ones and wanted to encourage Dave to raise the towers in the back.
As it happens the packaging with the horrible huge but weenie caliper Dave has makes missing everything a chore and a half but in the end maybe a short extension upwards and the dual rate thing is worth a try.. second hand 2.5 coil overs can be found and I can carve up a spacer easy enough.