Front spring rating help needed.

My KVA "B" chassis came with 220lb front springs.
Its basicly a XJ6 front setup ( wishbone, brake & hub) using a single XJ rear shock each front side.
Rear setup is dual OEM coilovers each side as on XJ6 / 12 / E-type. ( yes, four shocks on the back.

Its a GAZ adjustable coil over using a 2.25", 10" lenght spring.
Spring rating is 220lb which is to soft.. front sits on the ground with just 3cm ground clearance

If I pre-load the spring 2" it gives me the proper ride heigth., 10cm ground clearance up front fully loaded car,

So I need stiffer springs. But what rate?

With preload I indeed mean I compress the spring in its seat using the adjustable cups.
With a free (unsprung) adjustable cups are on the end so there's never a free play of the spring.


Thanks in advance
Did test it using ten 25kg cement bags to simulate two full gas tanks & two 80 kg persons in the car.
 
My KVA "B" chassis came with 220lb front springs.
Its basicly a XJ6 front setup ( wishbone, brake & hub) using a single XJ rear shock each front side.
Rear setup is dual OEM coilovers each side as on XJ6 / 12 / E-type. ( yes, four shocks on the back.

Its a GAZ adjustable coil over using a 2.25", 10" lenght spring.
Spring rating is 220lb which is to soft.. front sits on the ground with just 3cm ground clearance

If I pre-load the spring 2" it gives me the proper ride heigth., 10cm ground clearance up front fully loaded car,

So I need stiffer springs. But what rate?

With preload I indeed mean I compress the spring in its seat using the adjustable cups.
With a free (unsprung) adjustable cups are on the end so there's never a free play of the spring.


Thanks in advance
Did test it using ten 25kg cement bags to simulate two full gas tanks & two 80 kg persons in the car.
My GTD weighed 2760 lbs. with a load of fuel and 180lbs. in the driverseat with Bilstein shocks i found that 350lb springs in the front and 325 lb. springs in the rear worked very well. That is use the heavier spring in the front and the lighter one in the back. Good luck.
 
Unfortunately, the KVA may need different springs from the GTD due to motion ratio differences between them. There's nothing wrong with preloading the springs to get the ride height you want, provided it gives you the ride you want, from a comfort and handling perspective. Does the car bottom out during normal driving? Does it have a bump stop on the shock or front suspension? If you do decide that springs are significantly too soft, the front dampers will not match up well with the new springs. Hopefully, some KVA owners can chime in with their observations and recommendations.
 
Using this calculator https://www.hypercoils.com/spring-rate-calculator/
And these numbers ( corner & unsprung weight are not verifiet but an estimation)

Corner weight 500lbs
Unsprung weight 100lbs
Dimension A, 7“
Dimension B, 11“
Spring angle, 80%
Shock ride height from extended height, 4” travel, 25% = factor 1

I should need 630lb springs up front.

So I need to buy a wheel scale or caravan weight scale to do the math real time.

The front shock is a shorty on the B type chassis, without the bump stop it has 4" travel. With the bump stop on the rod its 3½" travel.
 

Ian Anderson

Lifetime Supporter
Is the weight of the spring,the amount of force it takes to compress the spring 1 inch?
so if you compress it 2 inches to get correct height is that not 440 lbs? Or do you have to add the original 220 to your compressed 440 to get the 660lbs?

get two of these per wheel 8 will do all 4 corners
out on a piece of wood / plywood over the pair and then add the two weights together!

cheap way to do corner weights

ian
 

Mike Pass

Supporter
Using this calculator

Front corner weight (from SGT) 260Kg = 570 pounds
Suspension frequency 1.7Hz - fast road
Motion ratio 7 "; 11" = 0.64
Spring rate = 413 pounds per inch

Use Ian's corner weight method with wood beam and scales to get the exact corner weight. Set the other front wheel at the same height with blocks.

For 4" total movement use 2.5" bump and 1.5" droop
From no load to supporting the car weight the spring will compress 1.4"
Set spring platform to middle of threaded section. To get the length of spring needed find the compressed distance (spring platform to top mount) at your desired ride height. Chassis to ground of 4" is about right to allow for for bumps etc. Add 1.4" to the distance from spring platform to top mount of shock. The spring platform at the middle of the threaded section will allow some final adjustment.

Hope this helps
Cheers
Mike
 
Hi I am racking my brains trying to remember my springs on early KVA/GTD suspension. I don`t think I used springs any heavier than 300lbs on the front with around 450lbs on the rear. It rides well on the road and on circuits but I have never raced it. The use you intend for it dictates spring choice and many people on here can advise on that better than me.
 
Ian thanks for the scale tip. Ill get on with next friday

The question indeed is, if I preload my 220lbs spring for 2", will that make it a 440 or a 660lbs..
Rally Design did not have an answer to that also.

Mike, thanks again for your input. As said, Linda & I get on with it next friday, using two home scales and some timber.
If Ian first thoughts are right, 2" preload makes 440 and thats very close to your math.
 
^ first inch of spring compression requires 220 lbs of force. Second inch requires another 220 lbs, so 440 lbs total. 660 lbs force will compress the spring 3". But that is not the wheel rate - the effective wheel rate will change as the angles in suspension change. I'm sure there is some software that will give you your instantaneous wheel rate throughout suspension travel.
Mike's citation is exactly what you need - get the spring that gives you the ride frequency that suits your use. Just be aware that your old dampers may be outside their adjustment range if you decide on upping the spring rate significantly (assuming the dampers were matched to your old springs).
 
Just be aware that your old dampers may be outside their adjustment range if you decide on upping the spring rate significantly (assuming the dampers were matched to your old springs).
Brand new GAZ adjustable dampers. Custom made to order.
Project came with new Spax but they where leaking allready so I had GAZ made the same.
I am still in the build but encountered this issue.
Project came from the Ridley stable. Most will know why Mick couldn't finish his projects.
 

Howard Jones

Supporter
A spring is rated in pounds per inches of travel, the normal convention being Lb/in. So a 300-pound spring will compress 1 inch if a 300 pound weight is placed on it. If you add another 300 pounds it will compress another inch and so on until it binds. This example is for a linear wound rate spring. Progressive rate springs are similar except they follow a spring rate change and the required load to do so at a predetermined profile. Then there are dual-rate springs and have two spring rates stacked on top of each other such as 200-300. Once the 200 is coil bound the 300 section takes over.

My GTD uses 350 on the front and 450 on the rear. Just about right for open track on street tires. A little stiff for street only. 50 pounds less on each end would be slightly stiff but better on a street only car. This is something you will need to figure out for yourself. Remember the suspension geometry is different on a KVA and GTD
 
""preload my 220lbs spring for 2", will that make it a 440 or a 660lbs."".

Preloading the spring will not change the spring rate, only the ride height. if you want a different spring rate you need to get different springs.

Regards

Fred
 
Remember the suspension geometry is different on a KVA and GTD
In fact, mine is differend from the regular KVA also. I have one of only three "B" type chassis ever made. KVA "B" type is fully Jaguar XK / XJ based.
Have two scales now, 130kg each. Will get along with it on friday using Mike's calculator. Ill keep this updated.
 
Well as my Scania was booked in for MOT, I had a short working day. As my workingdays normaly starts at 4AM, I deliverd the goods, got the truck at the dealer at 9,30 and was back home at 11.00.

Using Ian's corner weight method with wood beam and scale, & several attemps we got an average of:
275 Kilogram total mass corner weight.
About 40 Kilogram unsprung mass (wheel including nuts and spinner @ 20 kilogram, Jaguar XJ brake assembly and hub @ about 15 kilogram, wishbones etc about 5Kilogram)
So thats a total sprung mass of 235 Kilogram corner weight.

With a frequency 1.7 & motion ratio 0.64 the calculator comes up with 374 lbs/inch. I think thats a good starting point.

I suppose, the twin shock per side jaguar XJ rear setup has 250 lb/inch springs as OEM, so thats 500 lb per side.
 
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