Progressive Rear Springs?

[slomove]

I am in the process of ordering a new set of shocks for my Birkin from Protech in the UK and doing that wanted to uprate the rear springs from currently 150lbs/" to 175 lbs/" (I have ball-jointed radius arms).

 

For convenience reasons I could order those from ProTech, too and they suggested instead of a fixed 175 pound rate to use a progressive 150-225 pound spring, which they offer at same cost of GBP 20 per spring.

 

 

 

In my simple mind I thought that is a good idea, keeping the smoother ride when the road has only little bumps and the springs stiffening up when the prop shaft is in danger of hitting the hoops (which happens sometimes now in deep dips when I have a passenger).

 

 

 

Would anybody know why I should not do this (like vehicle dynamics on the track or something like that)??

 

 

 

Thanks,

 

 

 

Gert

[WestTexasS2K]

I like the idea of progressive rate springs and have had them on several street cars and the handeling was very good. But I have not had them on a car that I tracked hard. The rate seems about right. I run 225 on the back of my car but I have an IRS set up instead of a live axle. It would be interesting to see what some of the other guys think, that have tracked with a progressive spring.

[JohnCh]

I remember reading something about this a long time ago with regard to Porsche 911 Cup Cars. Apparently the progressive rate springs were a bit of a handful on track because the F/R spring rate ratio (and resulting balance) would change based on where the car was in the corner. When braking hard prior to turn in, the front springs were more compressed and so were at the stiffer end of their range, while the rears were uncompressed and at the softer end of their range. Accelerating out of a corner resulted in the opposite results. The teams then switched to linear rate springs which kept the handling balance more constant.

 

 

 

-John

[slomove]

Apparently the progressive rate springs were a bit of a handful on track because the F/R spring rate ratio (and resulting balance) would change based on where the car was in the corner. -John

 

 

 

Hmm, have to think about that.....

 

but on the other hand I heard that progressives are standard issue for some Caterhams. That means it can not be totally wrong?

 

 

 

Gert

[JohnCh]

Our cars don't have a lot of squat, dive, or suspension travel, so it probably isn't much of an issue for us.

[Birkin42]

On the front suspension of a typical 7 with out board shocks, the shocks are canted over so far that leverage ratio is actually fall rate. A progressive spring would help to counter act this. The rear suspension is of course as linear as it gets.

 

My opinion is that progressive springs are a nice compromise for the street and some track use, but prefer standard springs for competition use. I guess it really depends on what you are looking for.

 

For the progressive springs, are they offering you a choice of how much the rate changes?

 

Another way to look at it is a progressive spring is similar to a standard spring with a tender spring. I've seen these used on comp cars.

[R1 Seven]

Just to throw something else out there...most dampers are pretty much a linear rate. Since the damper and spring should be tuned to work together, wouldn't it make more sense to use a linear rate spring?

 

 

 

I had some progressive rate springs in my CRX once. I actually prefered the ride of the linear rate springs that were slightly higher rates. The same adjustable shocks were used for both. The progressive springs seemed to make the car bouncy and less settled especially over big dips and transitions. That is just one data point though.

[Birkin42]

In terms of dampening vs. spring rate, I agree that higher spring rates with less compression dampening feels better and handles better.

 

Vehicles with low spring rates that try to compensate with high dampening always seem harsh but blow through their travel on anything significant.

[roll a 7]

My Caterham orinally had 90/180 progressive springs on the rear. I wrestled with the cars handling for two years before I finally decided to try going in a different direction. In the end I bought new shocks and springs from Freestyle. This transformed the handling once we had the car set up. The rears are now linear 150 pound springs.

 

 

 

My front spring rate was also changed and the 24 way adjustable AVO shocks made it easy and fun to play with setup. At this point I believe that I could setup the car to oversteer at the limit on left turns and have terminal understeer at the limit when turning to the right. Can't think of any reason to do that, but it is fun to contemplate...

 

 

 

The current setup is neutral at the limit. The car does about a dozen autocrosses and a few track days per year.