Spring rate and length questions

Non-engine, eg: aerodynamics, gearboxes, brakes, suspension
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chrislandy
Posts: 29
Joined: November 17th, 2008, 2:13 pm
Location: Cheltenham, UK

Spring rate and length questions

Post by chrislandy »

Hi,
Currently I have 6" 2.25ID 700lb/in springs on the rear with a 2:1 ratio but want to experiment a little without it costing the earth, has anyone had experience with different spring manufacturers (are there any to avoid?)

I have a couple of other questions (Appologies if they are basic, elemetry questions but I am very new to suspension setup/tuning)
1) At what point when increasing the spring rate should you change the bump:rebound ratio and how do you work out what it should be?
2) at what point would you use a shorter spring with 2ndary "helper" springs?
3) does a 25lb rate change effect the handling substantally?

My current dampeners are single adjustable but I am unsure of the valving ratio (I will have to try and find the dyno plot for them). Initially I am looking for a "fast road" setup, but next year I would be looking to purchase another set of dampeners and springs (plus slicks) for track/sprint use as the Shelsley only qualifies in the "sports libre" class

Thanks
Chris
TR-Spider
Posts: 132
Joined: June 23rd, 2006, 8:37 am
Location: Rekingen / Switzerland
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Re: Spring rate and length questions

Post by TR-Spider »

Chris

I use KW racing springs. They have a huge range with 60mm ID. Check their webside.

ansering your questions:

A: Adjust the bump/rebound after every change until the car feels good (cannot give you numbers here). Be sure you changed bump/rebound already with your existing setup and you will be able to notice the difference in car response. Then change springs only, test drive. Then start adjusting dampers bump and rebound individually. Take some notes on each change to keep track...

B: when moving to harder springs, you will loose negative suspension travel, because the car will not drop as low on its own weigth.
Therefore you will most likely use shorter springs to maintain the cars height (if possible use height adjustable spring bases, thats very helpfull, also available at KW)
Now, with the shorter harder spring you have 2 options to prevent the spring from falling out with the suspension fully expanded.
1) you shorten the damper or the rope, whatever limits the negative (outwards) movement of the suspension. That is quick and easy, but it may make the car twichy and its response difficult to predict. Because now body movement (leaning into the turn) can cause the inside wheel to lift off the ground, thus reducing traction aprupt and significantly.
2) You maintain the negative travel of the suspension and use a helper spring to keep the spring package under tension and in place. The helper sping should be designed to be fully compressed under the cars weight. Then it will be 15-30mm high and uncompressed 60-80mm. I would advice using helper springs...

C: 25 out of 700 makes a 3% change. I dont think it will change substancially. From my experience 15% makes a notable but not exterme effect. I would rather start with big steps.

Just try with your dampers. If you find they are at the limit of adjustability, get them revalved or new ones.

Hope that helps.
BTW, what car is it?

Thomas
GC_23
chrislandy
Posts: 29
Joined: November 17th, 2008, 2:13 pm
Location: Cheltenham, UK

Re: Spring rate and length questions

Post by chrislandy »

Hi Thomas,
Thank you for the reply, I suspected that would be the process for the spring selection and helpers would be needed on the shorter springs. I know of one car that uses 3000lb springs on the front and 5000lb on the rear, but that is pure track only.

My car is a Shelsley T2 kit car which I have been building for over 6 years now and finally got it registered and road legal this week, the chassis was designed by an ex British Midland Hillclimb champion so I am expecting good things from it.
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