Strut question for 74 super

[Humble]

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.

[Eatoniashoprat]

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

[evilC]

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

[Eatoniashoprat]

Originally Posted by evilC

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

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

[evilC]

Mike, I think both the '86 and '87 have 23.5mm TBs but it won't harm taking the later model ones. Hint: check the diameters and the stamp on the inner end (Links & Rechts) as it is not unknown for the TBs to get swapped out for the wrong one. They will be identical but will have been loaded differently so it is imperative to put them in on the correct side.
I understand your need for the stiff front spring rates and of course your static front weight is higher than originally(?). One thought that occurred to me is that if you are using front 2 1/4" coilovers then it would be worth investigating motorcycle monoshock hydraulic adjusters that appear on touring orientated bikes as they could provide an easy, convenient way of adding preload to get you away from the bumpstops.
As an aside, I was scanning some Porsche racing data on suspensions for their 935/76 race cars (as you do on a boring Sat night!) and came across the spring frequencies of 147CPM front and 137CPM rear - well stiff. I don't think they carried a front ar bar but the rear was variable but typically 22mm with a blade type adjuster that had a variation ratio of 8:1 and was adjustable by the driver to account for the reducing fuel load centred over the front axle. the weight distribution was a dry static 40/60 that increased to 47/53 with driver and full fuel load. Also, the brakes were identical front and rear with twin m/cs of 17mm dia front and 22mm dia rear that gives a front bias 1.67:1 although this could be adjusted through the balance bar arrangement.
I've raised these Porsche figures only because of the similarities in layout and weight distribution to a well sorted bug.

Clive

[Eatoniashoprat]

The springs on the front are 2.5"

For interests sake I weighed my beetle on the weekend at the trucking scale on the highway, with me in it and probably half a tank of fuel. Weight saving was not a huge factor when building it, but from the looks of it maybe it should have been!

front: 420 kg
rear: 650 kg

39.3 % front
60.7% rear

[evilC]

Thats over a tonne! I thought the typical weight of a bug was around 850kgs so I would go back to the trucking scale and ask when they last had it load checked! Seriously, did you check it with two wheels on at a time? If so, did you get a total weight as well? the last time I weighed a vehicle on a weigh bridge the sum total of the axle weights exceeded the total weight. If you use the weigh bridge then only use the front/rear weights to show distribution % and calculate the actual from the total weight.

Clive