Coil Over conversion input?
 

I am ready to upgrade my suspension on my 85 Carrera and am interested in learning about going to coil over with progressive springs in an effort to keep a somewhat reasonable ride on the street and hopefully significantly stiffen up and reduce the amount of body roll to reduce camber loss.

Dose a coil over set up a better going to achieve this than going to say 22/30 torsion bars.

Any thoughts on what spring rates I should be looking at?

Not counting extras like re-valving shocks or other upgrades it looks like solid torsion bars would be about $500 and coil overs about $1100.

Keeping rubber my new bushings, have HD/Sport shocks, 22/21 Carrera sways, 7&9/16's w 225/265 VictorRacers' or 225/245 R888's. Car weight about 2700lbs.

Any input, recommendations, and or thoughts appreciated.

Thx. :)

Ben/M&K has this setup. Contact him for info.

JMO, you are opening a can of worms when you convert a t-bar car to coil overs. It might be worth it in a pro series race car but I just can't see it for the usual amateur. Those that like a technical challange may do it just because it's possible.

If you have the usual dual use car take a long hard look at the costs and issues. I'd rather put the $ elsewhere, correct o/s 8 & 9(.5)s hollow t-bars, adjustable sways. T-bars are more than adequate for 99.99% of anyones use, they keep the weight low, are well known and easy to tune around, there are lots of good bushing options

either optimise and eliminate stiction in the existing suspension or ditch the whole car and start w/ a 964, 993 or 996 chassis

The other can of worms that you are opening is the progressive spring rates. You really need to understand the wheel-rate of the car and then pick the progressive spring rate to offset the change in the wheel rate as the suspension swings through it's full travel. This is what Porsche did on the 906. The end result is a linear wheel rate, which means that the increase in spring pressure is linear across the wheel's travel. In the absence of ground affects, this is generally desirable since it makes for a car who's handling is consistent.

The problem is that if your objective is to make the springing stiffer in bump, that it is very difficult to due this without a very careful understanding of the car's suspension dynamics. Both Lotus and McLaren tried increasing rate suspension in their F1 cars of the early '70's for the exact reasons that you are, and discarded the concept when they found that it made the car's handling unpredictable. It's because of the following scenario.

1) As a wheel progresses through it's travel, because the lower coil spring mount travels through an arc while the coil spring compresses in a linear fashion, you will find that for each increment of wheel travel, you will not have a consistent increment of spring travel. So while the first 1" of wheel travel may compress the spring by 1", the 2nd 1" of wheel travel may only compress the spring by .9", the result is that the spring rate at the wheel will build at a decreasing rate. This is a regressive wheel rate. This is the most common scenario and the degree of regressiveness will depend on the angle of the spring. In the case of of the 911's T-bar suspension this is neatly side-stepped by using torsion bars, so X degrees of wheel travel will result in the same X degrees of T-bar twist across the full range of travel.

By moving to progressive coil springs, things can get flakey if the progressiveness of the springs does not match the regressiveness of the suspension geometry. This is not a big deal if you have stiff springs and things aren't moving much. But with softer street springs where there can be significant wheel travel during cornering, things can get weird. Without taking you through the math, you may find (in this hypothetical case) that as you corner that the front end will get stiffer at a different rate then the rear end, the result is an increase in understeer. But if you increase the rear roll bar to compensate, you will (in this case) find that it causes corner entry oversteer leading to mid-corner understeer as load transfer shifts from front to back and vice-versa as the relative roll stiffness front-to-back shifts during the cornering process. At a different corner you may find it handles differently. Now add into the confusion that this balance may change as you raise or lower the car.

My point is make sure that the progressive spring rates are well thought out before you put them on the car, lest you change your 911's handling in an unpredictable fashion.

I am in agreement with Bill, If you are doing this just because you want to, go for it, I did.

A big advantage is in being able to change spring rates from track to track, if you go to the same track most of the time set the car up for that track with T bars. Do you need this?

Another advantage is being able to tune suspension between track sessions, do you need this?

If this is mostly a street driven car it will be coolness factor.

Don't forget to factor into the cost the need for chassis reinforcement in the rear. Consensus (at least on this board) is that you need to gusset the rear shock towers.

All good points.

I am learning there is options such as liner, progressive and stacking two different type of springs.

Something like one very short lower rate or progressive lower rate spring of say for this example about 200 lbs/inch with a very short travel under compression of say 1".

Then put this on top of say a 450 lb liner rate race spring.

This might give an ok ride on the street with smaller bumps, then almost act as a full race system in a corner limiting body roll.

However, sounds like the complications and or cost might not pan out. ;)