#16
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The calculations you did I believe were for Humbles car at 1900lbs, whereas my car is about 2200-2300lb with me in it. And actually the factory springs were measured by topline to be around 63 lbs, and the maxx springs I have right now are 71.5lbs. Without punching the numbers in *guess* tells me I should be in the 105-125lb/inch spring range. For the rear it's either weld in the 2 extra bars to make mine a 5-bar (do these with the motor/tranny installed?) and go coilovers OR, go the cheap way and put in 944 bars, although with 125lbs/inch springs this is probably going to have a similar CPM to the 125lbs/inch and will need to be even stiffer. Or go 100lbs/inch springs to be on the low end of the spectrum and do 944 bars. Time to crunch some numbers! *grabs calculator* not to mention the fact that I don't have a rear sway bar (yet?)
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1969 VW Bus 2.2L suby - Driving Daily 1302 EJ20 turbo Last edited by Eatoniashoprat; May 28th 2010 at 11:33. |
#17
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on a macpherson front end shouldn't the motion ratio be measured to the center of the tire? So the lever arm would now be something like 1:1.1 or so?
Also, using a lever arm of 1:1.277 in the back is giving me goofy numbers. I can't seem to replicate the ones you produced in the other thread. I'm getting CPM values that are huge (150+). This is using the spring rate values you gave for the stock and 944 TB at the shock position (186 and 216, respectively). This is fun Good info here at this link also http://www.swayaway.com/TechRoom.php#MOTIONRATIO
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1969 VW Bus 2.2L suby - Driving Daily 1302 EJ20 turbo Last edited by Eatoniashoprat; May 28th 2010 at 12:13. |
#18
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At the rear I was working on 1.27 as the motion ratio and using one of the webs torsion bar calculators the standard 22mm TB gave a notional spring rate of 186lb/in with a wheel rate of 145.5lb/in (95CPM). The 23.5mm bars gave 216lb/in, 170lb/in wheel rate and a CPM of 103. Staying with Humbles figures if you use 100lb/in springs at the front the CPM is 93 and with standard 944 TBs the CPM is 103 that is the 10% variation we are looking for. You could also retain the standard TBs and add 30lb/in springs as coil overs. Clive |
#19
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In all this appreciate that the calculated spring rates are in fact only the starting point to achieve the desired handling and ride parameters so dwelling on CPMs in anything other than as an indication of the rate will be counter-productive. It might be of interest that in the early '80s after a great deal of testing the very competitive Talbot Sunbeam Tarmac Rally Car was using CPMs of 128 front and 133 rear and that was much less than the circuit racers. It's Friday and a long w/e beckons so I intend to cool the brain down with some cold beers - Bye Bye! Clive |
#20
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Thanks Clive this has really helped a lot.
If I input my weight into the calculations this is what I get: 100lb/in front CPM 85 170lb/in rear wheel rate (23.5mm) CPM 95 ~10% difference. If I go higher rate in the front I'd have to go coilovers or even bigger TB otherwise the CPM's become very close. Have a good weekend! Have a cold one for the germanlook forum (I may have several) Mike
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1969 VW Bus 2.2L suby - Driving Daily 1302 EJ20 turbo Last edited by Eatoniashoprat; May 28th 2010 at 13:06. |
#21
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I think I just found an error on the Eibach springs worksheet page.
In step 2 it shows WR = C/MR^2 (assuming no ACF) But I'm pretty sure it should be WR = C*MR^2 Otherwise your wheel rates would be higher than your spring rates since the motion ration is less than 1. 'Tip2' on that page also contradicts 'step 2'.
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1969 VW Bus 2.2L suby - Driving Daily 1302 EJ20 turbo |
#22
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Sorry for the delay, my comp at home ate it's hard drive a couple weeks ago :P
I run the 200# springs up front because I had tried the 300# setup which was far too stiff. Wally runs 300# up front and seems to like it just fine. My street bug is 2280 w/ full tank and driver (and stereo ), and running anything less than 200# put the front on the bump stops in hard corners or more often if I took a passenger. It was so bad that even with 120# (a good bit stiffer than the normal maxx springs) I murdered the bump stops on both sides and blew the struts on both sides in the front. This was at the maximum ride height the maxx struts offered. I haven't started adjusting the rear spring rates yet but I have a pile of springs to sort through. For coilovers I was looking at 500#-600# on the street bug, but I need to get a cup brace in the car before I can make the switch. TB wise I started with 23.5mm bars then 25.5mm bars but with the heavy alu case and sub box in the back I think I need a bit more. I've got single adjustable qa1's waiting to go on and that will make finding spring rates a bit easier. I tend to go by feel and work my way back to find the hard numbers. I also push my bug harder daily than most, so I tune it to that level. What I consider soft, others might find jarring or vise versa. Admittedly, the feel I'm going for is the E46 M3 because I found it to be one of the best compromises between track and street. |
#23
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I just know right now the 71.5# max springs are waay too soft, loaded up with gear for a weekend, with no one in the car I can easily move the front bumper down an inch and hit the bump stops Do you think its necessary to run the helper springs up front with the 200# springs? Also, have you test fitted the QA1 coil overs with the 2.5" springs in the rear to see if the springs physically fit in the trailing arm?
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1969 VW Bus 2.2L suby - Driving Daily 1302 EJ20 turbo Last edited by Eatoniashoprat; May 31st 2010 at 11:38. |
#24
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I'd suggest starting with the 200# springs in front and go from there. They might feel a little stiff without gear or a passenger but won't bottom out when you do.
Is it a helper or tender spring? A helper spring is usually very light (around 10#), easily compresses, and is only used to keep the main spring seated at full droop. A tender spring is a fair bit heavier and used to make a dual rate spring basically. A tender spring might be 50-70# and easily compress during high load but would smooth out the jarring ride of the main spring around town. Now with my setup I put coil over sleeves on the maxx strut body and with a 10" spring I found I need a helper spring to keep the spring seated. Using a 10" spring also gave me a fair bit of ride height adjustment. I haven't had a chance to fit the qa1 coilovers on the rear but it's a fairly simple affair. Just drill a 2nd 1/2" mounting hole higher in the cup, and locate the lower shock mount with spacers. For the top, drill out the upper mount to 1/2" (kafer brace too) and put the shock in place. I think sandeep but up some pics of the lower mounts for the qa1's. |
#25
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1969 VW Bus 2.2L suby - Driving Daily 1302 EJ20 turbo |
#26
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FYI, I did some calculations for something closer to Humbles setup (stiffer) to see what it looked like, this is what I found. Also, I made a little spreadsheet that I could email if someone wanted to check it out.
Front: 175lbs/in CPM = 113 Rear: Stock TB combined with 200lbs/in coilovers CPM = 126 F/R CPM difference = 10.5%
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1969 VW Bus 2.2L suby - Driving Daily 1302 EJ20 turbo |
#27
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Here's my street car setup w/ full tank and driver:
LF 488 RF 437 LR 691 RR 633 Front: 925lbs. 41.1% Left: 1179lbs. 52.4% Cross: 1128lbs. 50.2% Total: 2249lbs. I run the 200# up front and i'm not sure what the CPM would be but I'd like to match it and add 10% or so CPM on the rear. You might be in the same territory with the subie swap. |
#28
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I think that's very close to my weight, I might be 50-60 pounds heavier but that's about it.
I went autocrossing for the first time last night and it was a blast, managed to turn some decent times but the front was on the bump stops the whole time and it understeered pretty bad unless you already had the back end coming around. It was a very tight course For 200# here are the numbers from my spreadsheet. For the coil over with the torsion bar I'm just adding the wheel rates, hope that's the right way to do it. I'd take them with a big grain of salt but here they are anyway. Front 200lbs/inch 121 CPM Rear stock TB 250 lb/inch coilover 134 CPM 10% difference Rear w/944 TB 250 lbs/inch coil over 139 CPM 13.1% difference
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1969 VW Bus 2.2L suby - Driving Daily 1302 EJ20 turbo |
#29
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Mike, Do you know what your damper rates are? Are you running a rear anti-roll bar? You might find that stiffer dampers plus a rear ar bar might dial out your initial understeer. Personally, I intend to set mine up as a soft(ish - 100lb/in front 944N/A rear) with stiff damping as most of the roads around here (that are used for Road rallys) are fairly bumpy and I don't like the thought of the front skipping across the bumps particularly under initial braking.
In all of this of course lies the variation of conditions/use/driving styles that will give wildly different results. Clive |
#30
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Clive, The front dampers are Golf GTi inserts and the rear are KYB gas-adjuts. I haven't got into figuring out damper rates yet. I like your approach though, I think I need to try and get a spring that will keep the front end suspended enough that I can have >2" of travel and they will keep off the bump stops and then stiffen with dampers. I guess stiffer dampers will also keep it off the stops to a degree just need to try and balance that. I don't have a rear anti-roll bar but it is in the plan. I've found a guy locally with an 86' 944 NA and an '87 944S. I've found conflicting info around what torsion bars are in those but I'm gonna scoop the TB's from the '87 and if either has a AR bar I'll get it too. I haven't settled on a spring rate yet but before I jump into coil-overs I'll put in 944 TB's and rear AR bar, adjustable front dampers (recommendations?) and right now I'm thinking around 125-140 lbs/inch front springs. For the front springs I have to keep in mind that if I load up to go somewhere for the weekend I could be adding an extra 250 pounds (including passenger). Its definitely going to be a balance! The roads here are also brutal, huge pot holes, wash outs, etc. With the freeze/thaw its near impossible to build a lasting road. Mike
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1969 VW Bus 2.2L suby - Driving Daily 1302 EJ20 turbo |
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