Spring rate for rear coilovers

[spannermanager]

There are some very high spring pressures being discussed here, something is not right, i ran 150lb front/ 300lb rear on my 850 kg 1303 with f/r arb's and 8" slicks. i always run the lowest spring value to give me the balance i want, this leaves the dampers with a good adjustment reserve and margins for fine tuning on the days conditions. I cant see how you've arrived at 400 at the front Wally, it must understeer big time, even for the rear thats a lot of spring, even with a G50 and a T4, hope this helps anyway

[Wally]

Nope, essentially no oversteer (also to my own surprise) and only lower speed understeer if I push it on the track.
It seems the measurement might be different?
I use AVO front springs as on this page:
http://www.avouk.com/Downloads.htm
Those springs are rated from 0-1600 lbs, so 400 seems not out of range for an axle without an engine on it?

[owdlvr]

...but guuuuys, I wanted a hard number!! ;-)

I knew well before I started on this project that I was going to be buying a variety of springs. Ideally the first set ends up somewhere in the middle, so I'd have a good base to start from. If you're far too soft, or far too hard, on the first set of springs, it's really tough to figure out how much further in one direction you should move. Not to mention the fact that even if one of you had "the perfect gravel setup" using XXXlb up front and XXXlb in the rear, chances are I would put them in my car and decide it was all wrong anyways! It's as much about feel as it is armchair math...though laptimes (when available) should be the ultimate deciding factor.

Taking both Clive and Wally's suggestions above I ended up going with a 300lb spring as my starting point in the rear. Based on Clive's math these should probably work out well for my road setup. If they're too stiff, they might have use in a track setup. If they're too soft, as Wally may suspect, then they may work well for either my gravel or snow setup. And hey, if they're completely wrong for everything...that's what Ebay is for.

-Dave

[owdlvr]

It still wonders me how a 944 with the engine in the front runs the 23,5mm stock in the rear and how that can be an acceptable upgrade for a sporty beetle with the rear engine?

A 944 weighs in at around 2700-2900lbs, with nearly 50/50 weight distribution.
A 1303 weighs in at about 1950lbs, with about 40/60 weight distribution.

So, roughly 1400lbs static weight on the 944 rear vs. 1140lbs static weight on the Beetle rear.

-Dave

[Wally]

Originally Posted by owdlvr

A 944 weighs in at around 2700-2900lbs, with nearly 50/50 weight distribution.
A 1303 weighs in at about 1950lbs, with about 40/60 weight distribution.

So, roughly 1400lbs static weight on the 944 rear vs. 1140lbs static weight on the Beetle rear.

-Dave

Never calculated it through like that, but if your numbers are somewhat accurate, thats a good point and would explain a lot
Mine's a 'little' heavier, but your point is well taken!
Thanks!

[evilC]

Originally Posted by owdlvr

A 944 weighs in at around 2700-2900lbs, with nearly 50/50 weight distribution.
A 1303 weighs in at about 1950lbs, with about 40/60 weight distribution.

So, roughly 1400lbs static weight on the 944 rear vs. 1140lbs static weight on the Beetle rear.

-Dave

Dave, if my memory is correct Porsche's 911 2.0ltr rally cars ran 23.5mm (standard type3) rear TBs for gravel as an 'upgrade'

Clive

[spannermanager]

Guys, its time to ask your self why Porsche fitted skinny little torsion bars to the aircooled cars? it wasn't for ride comfort, it was to reign them in, they cant be selling oversteer for a road car.
A swing axle bug can be fitted with 29mm short tb's and handle very well as long as the front is sorted, do the same trick to an IRS car, either super or b/j with IRS, and it will be undrivable, HUGE oversteer will result, not good unless your building a drift car, or maybe autotesting, i know because ive tried it. You just cannot make the rear very stiff...
i'm now running stock 1303 22mm tb x 20mm rear arb nowdays, with gas spax on full hard it oversteers mildly, i can still trail brake into hairpin or other turns that need it, 24 mm tb's just tip it into the danger zone where it just oversteers too much for comfort in combat, one slip and it was gone, tho i could do a quick lap time with them, you may be o.k on the street with them,
its possible to get away with 24mm rear bars if your front grip lets you, ie, you already have understeer, the hardest part of a quick track bug is to get the front working. Clive's table above is bang on and something is wrong if your a long way removed from it spring rate wise, damper or alignment problems would be suspect, and you need to be LOW, you wont beat a Porsche unless you are as low as one, and that takes a lot of work as your beginning to find out..

[owdlvr]

Originally Posted by evilC

Dave, if my memory is correct Porsche's 911 2.0ltr rally cars ran 23.5mm (standard type3) rear TBs for gravel as an 'upgrade'

Clive

Which is similar in size, if memory serves me, to the bars the Huebbe brothers' are running.

Moot point, I cut out a 6" section of TB splines on mine

-Dave

[evilC]

I've found some data that is food for thought;

Car: Rear engine, rear wheel drive.
Weight: 970kg dry 40/60 split front/rear; 1145kg wet including fuel and driver, split 43.6/56.4 front/rear
Wheel rate front; 6.5kg/mm (368lb/in) = 368lb/in spring rate for a 500kg front axle load max
Wheel rate rear; 8.5kg/mm (482lb/in) = 616lb/in spring rate for a 645kg rear axle load max
Damper rate front; 180/80kg
Damper rate rear; 300/90kg
Frequency front; 147
Frequency rear; 137
Note that the weight will vary due to the aerodynamic loads. In this case the front lift was -0.05 this gave a down force of 20kgs @ 125mph and 45kgs @ 187mph with rear lift of -0.26 giving a downforce of 105kgs @ 125mph and 235kgs @ 187mph.


The car? This was the all conquering Porsche 935/77. The circuits it ran on were invariably billiard table smooth so the suspension ought to be considered at at the very stiffest end of the spec. I was interested to see how close to the same weights some of our bugs were.

Clive

[piledriver]

Hate to bump an ancient thread, but there needs to be some clarification from the 935 specs and how those might translate to a beetle or 944T rear suspension.

If the shock/spring motion ratios and weights are all known, you can actually reasonably translate the springs/damping.

I'm currently trying to pull some reasonable numbers out of the available calculators for spring rate and damping.

The trailing arm length on a 911 and its children are IIRC much longer, so a given dia torsion would effectively be softer.
The shock//coil over attaches out BEHIND the axle.
The motion ratios for the rear shocks/coil overs is thus >1.0

The motion ratio of a 944T is supposedly ~.94 up front and .42 out back (pulled from Rennlist etc)
(I suspect similar for the early steel T1/T3 arms)

The front rates ~match up as the strut angle is close.

The rear spring rate and damper rates on a T1 or 944 suspension will have to be >2,5 X the 935 rates to have the same effect at the wheel.

Also, at least for the 934 (assuming same trailing arms), the published rear damping specs are more like 2200/1200.

Note that the base recommended Bilstein damping settings for a baja bug prerunner/race app are almost EXACTLY what is spec'd in the previous post for the 935 (including a 2x motion ratio correction for the rear, again, the weights are about the same, the front ratio corrected damping would be softer on the baja by about 30%)

Note Bilstein makes small body coil over-ready shocks in 6" stroke length, 17" overall (S6G) that take 1 7/8" springs, <100 for the shocks and $40 for the slip on coil over kit. (+ springs) They can be bought in 21 std digressive valvings or custom valved when built for only a little more.
Too long if the car is slammed, but about right if the stock shock length still works.