Rear shock absorbers

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Re: Rear shock absorbers

Postby DavidL » Wed Sep 04, 2013 6:17 pm

Geoff Butcher wrote:If you have a 75lb/in spring in series with a 60lb/in, the rate of the combination will be about 33lb/in, until the short one goes coil bound, when it will of course jump to 75lb/in. :o

Are you sure? It's not an electrical circuit... ;)
I'd have imagined the combined spring rate to be between 60lb/in and 75lb/in. Can someone explain how you calculate the combined spring rate?
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Re: Rear shock absorbers

Postby Geoff Butcher » Wed Sep 04, 2013 8:14 pm

Yes. The spring equation is F=kx, where:

F= Force
k= Spring constant (rate)
x= Deflection

The spring constant of two springs in series is

1/k(total) = 1/k(one) + 1/k(two)

Putting k(one) and k(two) equal to 75 and 60, my maths gives 1/k(total) as 0.03, therefore k(total) = 33lb/in.

It's a bit like resistors, only the other way round, i.e. for springs in parallel, you add the rates.
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Re: Rear shock absorbers

Postby ACourtney » Wed Sep 04, 2013 8:59 pm

Geoff Butcher wrote:Yes. The spring equation is F=kx, where:

F= Force
k= Spring constant (rate)
x= Deflection

The spring constant of two springs in series is

1/k(total) = 1/k(one) + 1/k(two)

Putting k(one) and k(two) equal to 75 and 60, my maths gives 1/k(total) as 0.03, therefore k(total) = 33lb/in.

It's a bit like resistors, only the other way round, i.e. for springs in parallel, you add the rates.



Or put another way: A 60lb load will deflect the helper spring by 1 inch and the main spring by 60/75 = 0.8 inch, which is a total deflection of 1.8inches so the combined spring rate is 1.8/60 = 33.3 lb/in.
But as Geoff said, once the helper spring is coil bound then the spring rate is purely down to main spring and it is back to 75lb/in.
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Re: Rear shock absorbers

Postby DavidL » Wed Sep 04, 2013 10:29 pm

Thanks Geoff and Alistair. I do find that a bit counterintuitive though - a pair of springs of 75 and 60, I'd have imagined, act as if the whole thing is a 60, of which part has been stiffened - which would make the result somewhere between 60 and 75, if that makes any sense.

I'm not going to think about it for a while, I may just have to take your words for it...
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Re: Rear shock absorbers

Postby Alan D. » Wed Sep 04, 2013 10:32 pm

Maybe I need a 4 inch helper spring and a shorter main spring. I have a short ( 8 inch I think) main spring, rated at 90 pounds. I wonder what this would be like with the 60 pound helper spring?

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Re: Rear shock absorbers

Postby Stuart » Thu Sep 05, 2013 9:28 am

DavidL wrote:Thanks Geoff and Alistair. I do find that a bit counterintuitive though - a pair of springs of 75 and 60, I'd have imagined, act as if the whole thing is a 60, of which part has been stiffened - which would make the result somewhere between 60 and 75, if that makes any sense.

I'm not going to think about it for a while, I may just have to take your words for it...


I'm with you David, it makes no sense to me that the rate can be any lower than 60.
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Re: Rear shock absorbers

Postby ACourtney » Thu Sep 05, 2013 10:14 am

Alan D. wrote:Maybe I need a 4 inch helper spring and a shorter main spring. I have a short ( 8 inch I think) main spring, rated at 90 pounds. I wonder what this would be like with the 60 pound helper spring?

Alan


I think that would be going the wrong way and you will end up with a 90lb/in spring rate again, with the helper spring flattened.

If I have followed the thread correctly, then your problem is getting ride height with the softer 75lb/in spring.
The softer the spring the greater the deflection under static load, roughly around 4 inches with the 75lb/in spring. The 90lb/in spring would deflect less - around 3.33inches.

The helper spring will probably be coil bound (or near to it) under the static load - the corner load is around 300lb, which would deflect a 60lb/in spring by 5inches, but it is only 2inches to start with, so it follows that it is coil bound. That is okay because it is only there to keep the main spring seated under droop. However, if you have wound the spring platform up to raise the ride height you may have created preload such that the helper spring won't actually be called upon.

One answer would be to start with a longer 75lb/in spring - you get the same 4 inch static deflection, but the ends sit further apart. But thinking about it further that doesn't help, you still end up winding in pre-load to raise the ride height. You would get the same effect if you inserted a spacer sleeve between the coil end and the spring seat. The length of the coil just needs to provide enough travel to accomodate the static deflection - 4 inches - and then the free travel of the damper from the static loaded position - probably another 4 inches. So that's a total deflection of 8 inches, which sounds feasible with a 12 inch coil.

So if spring length isn't the answer what is? Well, logically you need to adjust the height at the top of the damper. That is, raise the body with respect to the top of the damper. As I mentioned before, if you preload the coilsprings to take out all of the static deflection you will have raised the top of the damper to its full extent. You won't get any more ride height and additional preload will just make the springs appear solid until the dynamic load overcomes the preload. Unfortunately, the scope for raising the ride height is limited by the length of the thread at the top of the damper. To gain more adjustment an extension piece would be required, something that would not be beyond the capability of someone who is handy with a lathe.
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Re: Rear shock absorbers

Postby Geoff Butcher » Thu Sep 05, 2013 10:27 am

It certainly is counterintuitive, but nonetheless true! It's much easier to visualise the other situation of springs in parallel.
Alan, the combined rate of a 60lb and a 90lb would be 36lb/in. I don't know how you'd go about deciding on the relative length of the two springs, although no doubt someone somewhere has a computer program to do it...You'd need to be very lucky to get it right by trial and error 8-)
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Re: Rear shock absorbers

Postby ACourtney » Thu Sep 05, 2013 10:27 am

Stuart wrote:
DavidL wrote:Thanks Geoff and Alistair. I do find that a bit counterintuitive though - a pair of springs of 75 and 60, I'd have imagined, act as if the whole thing is a 60, of which part has been stiffened - which would make the result somewhere between 60 and 75, if that makes any sense.

I'm not going to think about it for a while, I may just have to take your words for it...


I'm with you David, it makes no sense to me that the rate can be any lower than 60.



When you add springs in series you reduce their effective rate because the load is a continuum:

If you have two 60lb/in springs in line and apply a 60lb load, both will deflect by 1 inch.
That means your 60lb load has produced a total deflection of 2 inches - so your resulting spring rate is 60lb/2inches = 30 lb/in.

If you stick your two 60lb/in springs side by side and apply the same 60lb load, each spring will take half the load - 30lb - and consequently each will deflect by 1/2 inch. The 60lb load has produced a 1/2inch deflection so the resulting spring rate is 120lb/in.

Hopefully that helps.
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Re: Rear shock absorbers

Postby Alan D. » Thu Sep 05, 2013 10:49 am

First of all, thank you for all of your help. I've only speed read the post from Geoff. and Alistair. I'll read them more thoroughly tonight.

These are my thoughts from early in the morning.

If the 75 pound spring is fitted to the coilover with no pre-load and then fitted to the car. The car is then lowered to the ground with the full weight of the body on the spring, there is no deflection of the spring. This I think is because the spring is just supporting the body, which is relatively light. the radius arm below the spring is unsprung weight. So I don't think we need monster springs on the back of the Midas. When I sit in the car, there is a small amount of deflection. So we don't have 600 pound loads on the spring. The bathroom scales would measure the total weight on the footprint of the tire, but is that relevant to the whole spring loading?

Please feel free to correct me, I wish I stuck in at school!

Going up to the garages to start tidying up. I've not done any work up there for about two months. So time to make a start.

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