Lowering standard coilovers.

[v8bloke]

I have bespoke double wishbone suspension with standard style coil overs ( an eye at each end ).
Extended length 305mm closed length approx. 230mm. Springs are 7inches long. This equates to 4inches total suspension travel.
With 300 lb springs I had 20mm of bump and 85 droop. I would like to go a lot stiffer say 400 to 450 lb springs but this will just give more (too much) bump travel and not enough droop.


How can I go stiffer while still having control of bump and droop travel?


Cheers
Mark

[stevieturbo]

20mm of compression if that is what you're saying before it hits the stops, seems incredibly short and poor suspension travel.

Harder springs of the same length will lift the vehicle higher if nothing else changes. But that's why coilovers will have adjustable platforms to allow some adjustment. Presumably your current springs are not just installed loose as such ? They have some pre-load applied ?

If you call some of the spring manufacturers you could run through what you have and they could advise.

[v8bloke]

Yes 20mm. I bought these springs as a starting point, I wouldn't consider using like this.
I think with a 400lb spring I will have too much bump travel and not enough droop.
Must be a way round it?
The only adjustment I have is to add preload which just takes the spring closer to binding on full compression.

Edited by v8bloke on Sunday 6th January 11:48

[GreenV8S]

The available suspension travel is limited by the damper end stops. You need to choose the damper and end stops to give you the bump and droop travel you want around your chosen normal ride height. Some cheap damper suppliers will offer 'one size fits all' damper valving with a huge range of damper rate adjustment. That isn't ideal because it means the adjustment is relatively coarse. Better dampers are designed with a narrower range of adjustment chosen based on the vehicle weight, damper leverage and spring stiffness - if you don't get asked about those things, make sure you know what you're getting.

The normal ride height is determined by the spring rate and length. With the dampers chosen, you need to spec the spring rate to give you the desired suspension stiffness and spec the spring free length to give you the desired ride height based on the spring compressed length and damper spring seat positions.

[stevieturbo]

v8bloke said:

Yes 20mm. I bought these springs as a starting point, I wouldn't consider using like this.
I think with a 400lb spring I will have too much bump travel and not enough droop.
Must be a way round it?
The only adjustment I have is to add preload which just takes the spring closer to binding on full compression.

Edited by v8bloke on Sunday 6th January 11:48

Adding pre-load will not change the spring compressed height, unless the vehicle weight changes. It will lift the vehicle a little higher though offering you more compression with less drop.

And if you think a 400lb spring will have too much bump vs a 300...then use a 325, 350, 375...whatever.

The platforms are adjustable for various reasons

And you've already made a statement of having only 20mm compression...there is no way any spring will coil bind at only 20mm of compression.

[v8bloke]

Thanks for the replys.
yes I understand that the shock stops and spring rate will set the bump, droop and travel. But I don't see how spring length can alter ride height.
Say i'm using a 9inch 300lb spring and that corner of the car weighs 600lb. The suspension will settle by 2 inches.
If I use a 7inch 300lb spring and the corner weight is the same then the suspension will also settle by 2 inches.

How do I get a higher spring rate while controlling bump travel? my brain hurts.

[v8bloke]

stevieturbo said:

Adding pre-load will not change the spring compressed height, unless the vehicle weight changes. It will lift the vehicle a little higher though offering you more compression with less drop.

And if you think a 400lb spring will have too much bump vs a 300...then use a 325, 350, 375...whatever.

The platforms are adjustable for various reasons

And you've already made a statement of having only 20mm compression...there is no way any spring will coil bind at only 20mm of compression.

Ive got 20mm of bump compression 105mm total so the spring is compressed by 85mm when at ride height.

[v8bloke]

Thanks again, I will try a 375lb spring and see how that works out. When I can get it weighed I can work out the wheel rate and calculate settings better.

[stevieturbo]

v8bloke said:

Thanks for the replys.
yes I understand that the shock stops and spring rate will set the bump, droop and travel. But I don't see how spring length can alter ride height.
Say i'm using a 9inch 300lb spring and that corner of the car weighs 600lb. The suspension will settle by 2 inches.
If I use a 7inch 300lb spring and the corner weight is the same then the suspension will also settle by 2 inches.

How do I get a higher spring rate while controlling bump travel? my brain hurts.

No. What you seem to be asking is can you run a stiffer spring without it affecting the current ride height. And again 20mm compression seems massively inadequate, you need to raise that vehicle up anyway

If you need extremely stiff without affecting ride height, then a shorter strong spring with a short soft helper spring could achieve that, bit it will also affect how drop occurs too.

You're asking mixed questions here.

The title is about lowering the car, yet you've already said you have only 20mm of compression...so if you lower it you'll almost be sitting on the bump stops. That is in no way whatsoever good. If anything you currently need to raise the car.

[v8bloke]

stevieturbo said:

No. What you seem to be asking is can you run a stiffer spring without it affecting the current ride height. And again 20mm compression seems massively inadequate, you need to raise that vehicle up anyway

If you need extremely stiff without affecting ride height, then a shorter strong spring with a short soft helper spring could achieve that, bit it will also affect how drop occurs too.

You're asking mixed questions here.

The title is about lowering the car, yet you've already said you have only 20mm of compression...so if you lower it you'll almost be sitting on the bump stops. That is in no way whatsoever good. If anything you currently need to raise the car.

Thanks and sorry if my questions are misleading.
The spring that I have at the moment is just the starting point. I aim for about 35 to 40mm of bump travel with stiff spring. ( 250 to 300 lb wheel rate ).

I think I will just have to do it by trial end error.

Thanks

[SlimJim16v]

It sounds like you've used up all your bump travel at this ride height. So you need shorter dampers.

Plus, 305 - 230 = 75mm, so 3" of travel.

[v8bloke]

75mm travel at the shock equals 105mm at the wheel.
Finding this really difficult. I think trial. Will see how 375 x 7 springs work.

[SlimJim16v]

v8bloke said:

75mm travel at the shock equals 105mm at the wheel.

Yes, you're right. I keep forgetting that it's not the same

[GreenV8S]

v8bloke said:

I think I will just have to do it by trial end error.

It's relatively easy to calculate it out if you can cope with simple arithmetic.

You need to know / measure the weight on each corner, and know / measure the mechanical leverage between the road and the damper i.e. how far the damper compresses when you have an inch of bump on the wheel. This lets you calculate the normal load on the spring at rest. From that and the spring rate you can calculate the deflection. You'll have already chosen the normal ride height you want and chosen a damper / spring seat combination to achieve that. This gives you the compressed length of the spring. Add the deflection and this gives you the free length.

If the spring seats are adjustable, you only need to get the length in the right ball park.

[v8bloke]

My previous setup which was separate spring and shock had a mechanical ratio of 0.34 and a 650lb in spring which equals a wheel rate of 221.
I want to be at least as firm as this.
New coil over mechanical ratio is 0.53 so to get close to my original wheel rate I need a spring 425lb in.
The new coil overs have a stroke of 75mm and with a 7in spring with minimum preload has about 30mm of threads below seat.

I am at a disadvantage as I don't know yet the weight of each corner so cant calculate the spring deflection at ride height. I suspect that I will have more bump travel than rebound. Next step is to get a 425lb spring and measure where it sits at ride height.

[gordmac]

Your wheel rate depends on the motion ratio squared. A motion ratio of say 0.5 will give a wheel rate of a quarter of spring are.

[v8bloke]

gordmac said:

Your wheel rate depends on the motion ratio squared. A motion ratio of say 0.5 will give a wheel rate of a quarter of spring are.

Yes I have seen several formulas. can you explain why the result is squared?
I Would have thought that on a wishbone suspension the distance from fulcrum to shock mount divided by distance from fulcrum to centre of tyre would give an accurate Motion Ratio? Is this right/


Cheers

[GreenV8S]

v8bloke said:

can you explain why the result is squared?

Leverage of half means the spring only sees half the travel of the wheel, so you get half the deflection and force at the spring. The leverage also means that the force at the wheel is only half the force at the spring.

[Mr2Mike]

v8bloke said:

I have bespoke double wishbone suspension with standard style coil overs ( an eye at each end ).
Extended length 305mm closed length approx. 230mm. Springs are 7inches long. This equates to 4inches total suspension travel.
With 300 lb springs I had 20mm of bump and 85 droop. I would like to go a lot stiffer say 400 to 450 lb springs but this will just give more (too much) bump travel and not enough droop.


How can I go stiffer while still having control of bump and droop travel?

Are your 20mm bump 85mm droop measurements with zero preload added to the spring? If so it's trivial to work out the load on the spring, and from there you can calculate the spring rate you require.

75mm (damper travel) / 105mm (wheel travel) = 0.714

Static sag at wheel = 85mm. 85 * 0.714= 60.7mm static sag at spring.

Convert to inches and multiply by spring rate

60.7/25.4 * 300 = 717lbs load on spring.

You want to achieve 40mm bump travel at the spring, so the spring should be compressed by (75-40)=35mm under static conditions. Convert to inches and divide into the spring load (since we are mixing units all over the place)

717 / (40/25.4) = 455 lbs/inch with zero pre-load.

However this is a high spring rate, maybe too high for road use so you might need to get a less stiff spring and add preload to set your desired static sag. e.g. say you want to use a 400 lbs/inch spring. Applying 717lbs load to this spring will move it by 717/400 * 25.4 = 45.5mm, which is 10.5mm too much. To fix this you need to apply 10.5mm of preload.

[v8bloke]

Thanks to all for your help!

I am starting with a 450lb spring , will update the results.

Cheers