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Old 29-01-2020, 19:27   #1
plip1953
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Evo X - suspension discussion

Two particular points to mention and/or discuss.

First, rear spring rates. But not so much about what is optimal, but the fact that the outboard connection point of the damper is at the fully outboard position whereas on 4-9 model it's actually set slightly inboard. Even though I've owned Evo Xs for around 8 years and had six of them this is the first time I realised this deeply anorakish point!

And my second point is around bump and rebound travel. I have a particular coilover setup on my X RS that has 10cm of available travel between full droop and before the bump stop would come into play under compression. In one sense 10cm seems quite a lot (at least to me) but with the car in it's resting state the amount of available bump travel is no more than 3cm and therefore there is 7cm available for rebound. Intuitively I'd either expect it to more like 50:50, but I've yet to come across anyone that can confirm it or say otherwise. Thoughts anyone?
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Old 29-01-2020, 20:05   #2
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yes, you would want it at 50/50 or even less drop than bump (unless you have a very specific setup.. like modern WRC).. Small bump travel is bad for handling on rough roads, bad for comfort and can lead to damage if you have frequent bottomouts... When setting up the suspension we aim for 50/50 droop / bump travel..

as for rear motion ratio, yeah, it is different on the x than on the earlier model.. difficult to compare spring rates between the two generations..
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Old 29-01-2020, 21:58   #3
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A further slightly nerdy point relates to the helper springs on my particular coilover setup. By my reckoning I could dispense with them completely while still retaining exactly my current ride height. So is there any advantage is keeping them? I think I’m right in saying that as things are currently a reasonably chunk of the rebound travel will be under the (relative lack of) control of the helper springs, but I didn’t think that was in any way their intended function. If I get rid of them, whatever bump and rebound movement will be fully controlled by the main spring. Isn’t that a better thing?
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Old 30-01-2020, 03:35   #4
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usually a helper spring is there to control the drop when the stiff main spring extends completely.

which main springs do you run?
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Old 30-01-2020, 08:42   #5
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Quote:
Originally Posted by kikiturbo View Post
usually a helper spring is there to control the drop when the stiff main spring extends completely.

which main springs do you run?
It's a 9kgs/mm 200mm main spring. The helper spring is around 6.5mm extended, and 2.5mm when coilbound. And I have around 9mm of threaded tube length available to wind the spring perch up by. So by my reckoning I could easily dispense with the helper and still have full functionality. Or am I missing something.
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Old 30-01-2020, 10:43   #6
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are we talking rear or front suspension..?

I wouldnt remove the helper because you might end up with a loose main spring on full droop..
Having said that 6.5 mm sounds like an awfully short helper.. usually it would be 65 mm or more.. *I just remembered that you are talking extended helper when installed... ok. ok

I think your first move would be to get the dampers to sit somewhere around mid travel when car is on rest..

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Old 30-01-2020, 11:51   #7
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Quote:
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are we talking rear or front suspension..?

I wouldnt remove the helper because you might end up with a loose main spring on full droop..
Having said that 6.5 mm sounds like an awfully short helper.. usually it would be 65 mm or more.. *I just remembered that you are talking extended helper when installed... ok. ok

I think your first move would be to get the dampers to sit somewhere around mid travel when car is on rest..
Rear.

On reflection, you're right about the helper. There's certainly sufficient threaded section to permit it's removal, but I now realise that the ride height would either be too high or, as you point out, the main spring will be loose on droop.
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Old 30-01-2020, 12:31   #8
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A further thought/reflection - maybe around 30mm of bump travel (plus compression of the bump stop) actually is sufficient for what is essentially a track focussed setup? It would of course be possible to increase the ride height to give more bump travel, but I don't think the car is excessively low as it is now.
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Old 30-01-2020, 17:24   #9
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you do not have separate height adjustment.. like on ohlins? (I presume you do not..)
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Old 30-01-2020, 17:26   #10
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Rear.

On reflection, you're right about the helper. There's certainly sufficient threaded section to permit it's removal, but I now realise that the ride height would either be too high or, as you point out, the main spring will be loose on droop.
I wouldnt remove the helper..
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Old 30-01-2020, 18:04   #11
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you do not have separate height adjustment.. like on ohlins? (I presume you do not..)
Correct, no separate height adjustment.

On even further reflection I wonder if around 30mm of bump travel before hitting the bump stop should be sufficient on what is essentially a track focussed setup?
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Old 03-02-2020, 11:17   #12
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Correct, no separate height adjustment.

On even further reflection I wonder if around 30mm of bump travel before hitting the bump stop should be sufficient on what is essentially a track focussed setup?
Unless you know when you are hitting the bump stop when driving the car, an easy way to see how much suspension compression you are using is to put a thin tie-wrap around the lowest part of the stantion, when the car is at it's static ride height, and then go drive some flat out laps with warm tyres. The tie-wrap will be pushed to a position of maximum compression, and will stay there, so you can see what travel you are using. If you're using all travel and it's upsetting the car, you can either raise the ride height, or increase spring rates. Most bump stops are used as a progressive ramp up of spring rate at the end of travel, and hitting them isn't always a bad thing, especially if the car is not upset and no grip is lost.
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Old 03-02-2020, 13:25   #13
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Unless you know when you are hitting the bump stop when driving the car, an easy way to see how much suspension compression you are using is to put a thin tie-wrap around the lowest part of the stantion, when the car is at it's static ride height, and then go drive some flat out laps with warm tyres. The tie-wrap will be pushed to a position of maximum compression, and will stay there, so you can see what travel you are using. If you're using all travel and it's upsetting the car, you can either raise the ride height, or increase spring rates. Most bump stops are used as a progressive ramp up of spring rate at the end of travel, and hitting them isn't always a bad thing, especially if the car is not upset and no grip is lost.
Or might it be even simpler to slide the bump stop down the damper piston rod as far as it will go and then see how far it gets pushed back up?

Something else that I hadn't expected and therefore hadn't factored in is just how much effect the arb has. With wheels removed and car off the ground supported on the diff I loaded up one side (with a jack) and observed/measured the amount of compression of both helper and main spring ie to try to mimic how it would be with the car sitting on it's wheels on the ground.

What I hadn't expected, but probably should have done, is that the helper spring on the unloaded side was almost completely compressed simply because of the transfer of some of the load via the arb. I guess the next step would be to disconnect the arb and re-measure the amount of compression on the loaded side. However, in practice the arb will be connected at all times so I'm not sure quite what that might tell me - ie that would be useful to know?

But what I did conclude is that, in practice, while on either road or track, an unloaded wheel will never reach it's reach full droop position, and both helper springs will be permanently coilbound.

This then takes me on to conclude that the effective total amount of suspension travel that I have (on my particular setup) is around 60mm plus whatever the bump stop would allow.
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Old 07-02-2020, 00:00   #14
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Or might it be even simpler to slide the bump stop down the damper piston rod as far as it will go and then see how far it gets pushed back up?
You’ll want to remove the bump stop, if it’s fitted you won’t get an accurate measurement of travel due to how long it is.

Quote:
Originally Posted by plip1953 View Post
Something else that I hadn't expected and therefore hadn't factored in is just how much effect the arb has. With wheels removed and car off the ground supported on the diff I loaded up one side (with a jack) and observed/measured the amount of compression of both helper and main spring ie to try to mimic how it would be with the car sitting on it's wheels on the ground.

What I hadn't expected, but probably should have done, is that the helper spring on the unloaded side was almost completely compressed simply because of the transfer of some of the load via the arb. I guess the next step would be to disconnect the arb and re-measure the amount of compression on the loaded side. However, in practice the arb will be connected at all times so I'm not sure quite what that might tell me - ie that would be useful to know?

But what I did conclude is that, in practice, while on either road or track, an unloaded wheel will never reach it's reach full droop position, and both helper springs will be permanently coilbound.

This then takes me on to conclude that the effective total amount of suspension travel that I have (on my particular setup) is around 60mm plus whatever the bump stop would allow.
The helper springs are there to stop the main spring from coming off their seats when the suspension extends, or as part of the spring rate to make a linear spring progressive. With what you did, you are simulating droop whilst cornering.

If both rear dampers fully extend whilst travelling in a straight line, ie hitting a big bump whilst braking hard etc, the anti-roll bar won’t be doing anything, and the helpers are then doing their job.
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Last edited by Clivew; 07-02-2020 at 00:04..
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Old 07-02-2020, 08:34   #15
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Youíll want to remove the bump stop, if itís fitted you wonít get an accurate measurement of travel due to how long it is.

The helper springs are there to stop the main spring from coming off their seats when the suspension extends, or as part of the spring rate to make a linear spring progressive. With what you did, you are simulating droop whilst cornering.

If both rear dampers fully extend whilst travelling in a straight line, ie hitting a big bump whilst braking hard etc, the anti-roll bar wonít be doing anything, and the helpers are then doing their job.
All good points - thank you.

Further delving has identified that I'm running with rear rides heights (and quite possibly fronts too) that are 20mm lower than the coilover manufacturer advises/specifies as the minimum. So once raised by that amount my current 30mm of bump travel would become 50mm, and rebound (prior to unwinding of the helper springs) will be around the same amount. Those numbers seem much more reasonable.

So next time I have the geo checked/reset it looks like the ride heights should be adjusted too.
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