Shocks/Springs dimensions


Looking at shocks for my Tornado build. The car will be used on road but still capable to do a couple of track days per year.
Thinking of Protech shocks and springs, 2,25" and single adjustable 400 series.

A few questions that I would like to have your guidance on.

  • Open length? Shall I drop the wishbones to the bottom and measure c/c between mounting points?

  • Free length? What is a suitable length front & rear for a typical ride height?
  • Rate? Are 400 lb in front and 500 lb at rear a good starting point or something else?
Thanks in advance for your input/experience

Terry Oxandale

Skinny Man
I'm sure someone with a Tornado configuration will chime in. Until then, I believe a 10" length spring is pretty common, but all of the next information needs to take into consideration your specific geometry, angles, lengths etc. I wasn't sure about your query on dropping the wishbones. If I did that, they'd go full vertical because I have no stops, so that isn't a way that I could do this.
I determined my spring values by deciding what amount of travel is expected on my suspension. At ride height, some road cars have 3" +/- travel, which would be 6" total travel for a soft ride, but I would consider that excessive for our cars. I actually bias that toward the droop, and go with 2" bump, 4" droop, but my travel is more in the area of 3" or less.
I determined the sprung weight at each spring, divided by the ride height compression I want. But that has to include, the movement ratio (shock : upright). So for example, I have about 500 lbs on each rear wheel. Divide that by the desired compression of the upright at ride height. Mine has a compression at ride height of about 1.5", so 500/1.5 = 333 lb/in springs. My movement ratio is about .9, so that squared makes the spring rate higher, and now I'm looking at about 410 lb/in. I'm actually running 450 lb/in springs in the back. Coil bind in not an issue with that little amount of travel. If you want a softer, more compliant suspension, then softer springs with more ride-height compression can be used.
In terms of length, you will want to know where your max droop will be, and then ensure the shock is longer than that, and the full bump is hit before the shortest length of the shock. Again, with my 1.5 in compression, it allows a 6" total travel shock.
Front suspension is done the same way, but movement ratio on the front is less than the back (about .70), so even with much less weight on the front, I'm running 350 lb/springs on the front. With all of that said, I've never observed hitting the limits of my travel either on the road, or on the track. The springs do unload when jacking up the car, but that isn't necessarily a bad thing.

Sorry for the vague answer, but unless someone chimes in with direct relevance to your suspension geometry, you'll need to look at what you have (angles, motion ratios, unsprung weights, ride quality/height/compliance desires, and go from there. By the way, even a stiffly sprung car doesn't mean an objectionable ride if good quality shocks, and appropriate settings on those shock are made. You may even decide a different length spring is needed for your application.
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Thanks @blueovalz for you extensive feedback :)
Yes, it would be good to have some info from Tornado-guys that have built and tested their suspensions.

What I meant with "full drop" is the maximum drop I get with just the wishbones and uprights fitted - just dropping them in "free air". This is probably not the correct way of doing it. Therefore I would like some experiences from you guys - especially Tornado guys.

My "dropped" suspension as of today:

Just out of curiosity Joacim, as I don't see anything physically holding up your suspension, I assume that it's one of your ball joints reaching it's maximum angle offset that preventing the suspension from dropping farther? I'm looking more at the front suspension. If this is the case, I think that is something you might want to avoid, so full open of the shock should be before you reach that position.

Terry Oxandale

Skinny Man
I WOULD (just me), start of with positioning the axles at your desired ride height. Then measure distance between mounting points, then consider the amount of bump and droop from that point. Adding those two together (bump/droop travel from static ride height) should provide a reasonable value for maximum shock length at full extension, and minimum length and full bump, and thus provides anticipated stroke length. Obviously you'll determine which product offering best fits those dimensions. You may have the luxury of finding an appropriate shock/spring combo that will allow some degree of lowering the ride height for those track events.

Trevor makes an excellent point, which could prevent a serious failure.
^ double amen. Hopefully, shock has internal bump stop which should be the limiter in droop. Getting bushings/rod ends to the point of bind is bad bad idea. In your case, I'm guessing the rear is limited in droop by the lower radius arm contacting the chassis, which again, is a bad idea.

Mike Pass

Simple answer is ask to Andy Sheldon at Tornado.
You can see from the attached pic there are two things going on here which determine how the spring rate relates to the wheel rate.
The coil/damper unit attaches to the lower wishbone part way along and not at the outer end. From the yellow lines the ratio is about 3.2 : 6.2.This means that the spring rate will need to be 6.2 divided by 3.2 times more than the wheel rate required. e.g. to get a wheel rate of 100 lbs per inch the spring rate will need to be 100 times 6.2 divided by 3.2 which is 100 X 1.9 =192 lbs per inch.
Also the coil damper unit is not vertical but is angled at about 65/70 degrees. This means that the spring rate at the attachment point on the lower wishbone will need to be more to take account of the angle.
Taking both these factors into account an approximate calculation from the photo comes out that the spring rate of the coil should be about twice what is required at the wheel. e.g. a wheel spring rate of 100 lbs per inch will need a coil spring of 200 lbs per inch.
For most replica GT40s loaded and with driver the load on a front wheel is about 230 Kg or 500 pounds.
To find the normal ride height fit the wheels/tyres and set the front of the chassis to about 4" as this will be OK for sped bumps etc. and note the position of the lower wishbone at this height. Set the tyre pressures low to simulate the deformation that would occur when carrying the load of the car.


Terry Oxandale

Skinny Man
Mike, I believe the motion ratio impact is the square of the ratio. So if the shock only compresses half the distance that the axle (upright moves), the result is a spring 4X in rate, rather than 2X. Taking the spring rate (500 as an example) x motion ratio^2 = 500 x .516^2 = 500 x .266 = wheel rate of 133lb/in, which is about 1/4 of the spring rate (or spring rate is about 4X the wheel rate). OR. looking at it in reverse, a desired wheel rate of (300 as an example) x motion ratio^2 = 300 x 1.9^2 = 300 x 3.61 = a spring rate of over a 1000lb/in.

The force increase at the shock mount is multiplied by the rate decrease (spring compresses less than the upright, so the rate is less than the rate at the wheel), which is the reason for the ^2. I didn't bother measuring the shock angle, because if I measure the shock compression as compared to the axle movement, the angle and location mounting of the shock are all taken into consideration for an overall motion ratio.

This is why when I built the front arms on my project, I welded the shock mount immediately adjacent to the lower ball-joint (clearance due to suspension travel taken into consideration), AND, located the upper shock mount as much as practical outboard to make the shock as vertical as was possible. The combination of that is you decrease the movement ratio due to the arm itself, and decrease the movement ratio due to the inclined angle of the shock. Having the rear shock basically attaching to the rear upright isolates any motion ratio to the shock angle only.

All of this means a lot (to me) in terms of build design, strength, and having a shock that is really good.

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Here is a very easy to use calculator to do the sum for you, for a road only car i would suggest a wheel rate around to 110 - 120, for a track car raise that to 130-140. keeping in mind stiffer = more mechanical grip, but sacrifices ride comfort.
the other thing to consider is handling characteristics, stiffer front = under steer or push. rear stiffer than front = over steer.
welcome to the black art.
hope this helps
cheers John

Mike Pass

Hi Terry, You are of course correct. I was confusing the static state with the dynamic state.
In the static state the coil has to push with force twice the force at the wheel to balance.
However as you say a 200 lbs force at the wheel moving 1 inch will need a force of 400 lbs but only through a movement of 1/2 inch which will mean a spring rate of 800lbs inch.
Your method of moving the coil/damper attachment as close to the upright as possible is a great idea as is getting the shock as close to vertical as possible. This outwards moved shock mounting on the lower arm will have lots of benefits. Lower spring rate,lighter springs, less bending of the lower arm, better and more direct shock control of suspension movement and relatively softer shock setting.
I am building a GT 40 what is the part number for the Viking shock looking at these shocks THANKS!!!
I got from @Deno the following
Shock details fitted are :
Front Viking C202, a bit short for road use.
Comp / Extn 8.53" - 11.33". Stroke 2.8"
Spring 7" 400lb.
Should have used C204 and 8" Spring.
Had to wind 7" to Max. For 105mm Road Clearance at the front.
Rear Viking C212, a bit long.
Should have used C210.
Comp / Extn 12.47" - 19.22". Stroke 6.25"
Spring 12" 400lb.Sring is perfect.
Some additional information in AUS at:
Disclaimer: As always with advises on internet, you have to validate if you are satisfied/comfortable with the advice and then you decide :)
As I see it it is nothing strange about that and it goes for everything.

Good luck with your build!
Do you have a build log?

Terry Oxandale

Skinny Man
Disclaimer: As always with advises on internet, you have to validate if you are satisfied/comfortable with the advice and then you decide :)
As I see it it is nothing strange about that and it goes for everything.

Good luck with your build!
Do you have a build log?
No doubt. Suspension geometries could be substantially different, making some components useful on one, useless on the other for a chassis otherwise called a "GT40".