Inside rear wheel unloading

[sailaholic]

If the spring was stiff enough and the unsprung mass light enough, wouldn't the spring prevent the shock from reaching its maximum droop?

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No because the spring only stop compression. It's not fixed to the holders do just ends up loose / not touching the top of the shock.

I don't think helper springs would improve the situation if the issue is the wheel unloading to the point the diff opens up. You don't need the wheel in contact with the ground. You need the wheel in contact with about 1/2 the force in it of the other drive wheel.

Ie if the outside drive wheel has 6000N force on it, the inside needs 3000N of force to stay within the bias range of the diff (assuming a max bias of 2:1)

So as madjak pointed out. The wheels probably still on the ground, just not heavily enough loaded for the diff.


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[hks_kansei]

If the spring was stiff enough and the unsprung mass light enough, wouldn't the spring prevent the shock from reaching its maximum droop?

How though?

Remember the spring isn't actually attached to the spring seats, it simply pushes the top seat apart from the bottom.

if you had a short extremely stiff spring as the wheel unloads the shock would simply extend and leave the spring un-captive.



I can only assume that with a stiffer spring the car sits higher with it's weight on the ground (ie: parked) and so as load is taken off there is less distance for the wheel to droop from that static point.
vs say no spring where when parked the car is resting on the frame rails and as it's lifted the wheels drop a much greater distance.

The end droop point would be the same for each, if measured from a point on the car (say the hub centre would be 50mm below the sills)

But the actual distance the wheel has travelled would be less for the stiff spring, the spring having essentially held the car at X% through it's available travel already.

Or something.


Maybe I should just shut up before this gets too confusing...

I'm not one to suggest setups, i'm currently on blown rear shocks and worn front ones.

[smy0003]

Thanks hks and sailaholic for pointing out the massive hole in my logic hahaha.

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[plohl]

It's probably more of an issue with lines than anything else... you might have to changed how you go through there - may not be the faster line, but you will have traction instead.


Another suggestion...
Remove the aero. You should be able to match the times without it. Stop dicking around with setup until you're times are consistent, once they are, play around with sway bars. If sway bars don't do what you want after another 4 events, look at springs.
Hopefully after a year you have an idea on how you like the car to handle less aero. Then add aero.

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[Magpie]

Good points Shane and well worthy of being listened to. Agree the focus should be on getting consistent on times first.

With respect to the aero at the speeds for T4 on the sprint my aero would have little impact.

Again thanks for all the replies!

[madjak]

I wrote a long post then deleted it. I've just gone through all this on my car, first ordering 8" springs for the front only to find that I couldn't get the car low enough, so I had to order 7" springs instead. I should have ordered a 6" spring with a 2" helper.

summed up it goes like this:

1) Regardless of the rate or length of the spring it must be preloaded slightly to stop it falling of it's perches.

2) Ride height is set by the combination of the amount of compression of the spring (less with high rate) and the total length of the shock. So you need a much shorter shock with higher rate springs. This often causes problems because you may need a much shorter spring so there is room to shorten the shock.

3) Droop has nothing to do with the ride height and pretty much only equals the amount the spring compresses under the weight of the car (multiplied by the control arm leverage). So the higher the spring rate, the less droop you get. This is why tender / helper springs are used on race cars, so that you can get more droop without the spring becoming loose on the shock.

[madjak]

btw.... here is a really clear example of what Magpie is talking about: go to 39 seconds



In this case the track was slightly damp and very cold. Grip levels were low and at 39 seconds where the diff slips I was only just getting back onto the throttle after corner apex. I had to back it off slightly to re-engage the diff then get back on the power and you can see the car doesn't step out at all so there is no wheel spin. It feels almost like the clutch is slipping but I know it isn't. The next corner was opposite with the back stepping out under power, all up a bit of a messy run, but still good for showing and hearing whats happening.

And yes my entry into the left hander was completely wrong... I should have been out a lot wider after the chicane and apexed later, which I did in the next run. Didn't help the diff slip but it was a faster run by 1 second.

[project.r.racing]

Lets be real here also. The diff is partly to blame. They are a cheap diff designed to work on a reverse engineered car. Once you start messing with the boundries of that engineering and going forward again, it will never work as it should.

[madjak]

Lets be real here also. The diff is partly to blame. They are a cheap diff designed to work on a reverse engineered car. Once you start messing with the boundries of that engineering and going forward again, it will never work as it should.

I'd have an OS Giken over a torsen any day. It's a shame they are so expensive.

I am pushing a fair bit of power through it, but with the HP turbo cars it must be far worse... Maybe they have enough power to break away both tyres together rather than that slippage I'm getting. It might be Magpie and myself are falling into a specific power range where we have enough power under throttle to slip the unloaded tyre, but not enough to break both at once.

Either way, I suspect not having enough droop travel, and the car being too rigid causes the problem to be magnified. I'll know soon when I race at Barbagallo next weekend. I get the diff slip on the left hander heading up the hill every time. If the new suspension stops it then it might be the extra travel I now have on the rear. Of course the high rate springs I'm running could make it all worse.

[The American]

4x the torque (assuming 80/20 spilt) being transmitted by an unloaded wheel scrabbling for grip, is going to be dictated by the grip available and not the torque available. More wheel spin from additional HP might translate to less grip/torque transmitted, in a given moment of low traction.

[greenMachine]

Don't forget that playing with shock settings can affect balance, and stiffening the front while softening the rear will improve the rear grip, and maybe that would help.

[Magpie]

What I have learnt:

1. Rear roll stiffness needs to be reduced to avoid lifting the inner rear to the point that the torsen fails to work.
a. Roll resistance is determined by a combination of springs AND sway bars.
b. With respect to roll stiffness sway bar and spring rates and interchangeable.
b. The balance of the car can remain unchanged provided that if the sway bar is reduced the spring rate at that end is increased

2. If wheel droop travel is not sufficient AND there is too much chassis roll then the inside wheel will become unloaded.
a. Disconnect the rear sway as this will allow more droop.
b. No rear sway will promote understeer.
c. No rear sway bar maybe OK for low speed corners but could be dangerous on high speed.

3. Shock length (coilovers)
a. Most appropriate method of setting shock length should be used. That is as per MCA method.
b. If spring becomes uncaptive at full droop then a helper spring maybe sufficient to 'push' the inner wheel back in contact.
Note: King springs make 2 x helper springs at 1.25/4.0 (kgs/mm) which compress from 70mm to 16/20mm.
c. Correct spring length should be used in preference to helper springs.

4. Making changes to car setup
a. Keep it too yourself and don't post your thoughts on a forum
b. Minimise suspension changes until the driver becomes consistent as this removes an unstable variable.
c. Making changes may fix one problem but create another.
d. Consider changes in driving style as a possible alternative.
e. Other people have similar problems and their input/advice is valuable.

I now have a plan on what I will be doing, but will not post it However I think this discussion is valuable just to help myself (and others) understand suspension.

[The American]

Well, I like it when you post your thoughts on the forum.

There's always someone with a different view of how to skin the cat. But really matters is... and then perhaps the cat metaphor is at its limits.

I've heard people say that engineering is always compromise. Maybe you WANT greater roll stiffness and are prepared the sacrifice some drive out of tighter corners to get it. Maybe you want a bit of slip in slower tighter corners to help keep the revs in the power band. Etc etc.

I'm guilty of not changing a suspension related settings other than tyre pressures in at least a year.

I'm not sure how well that bodes for the afore mentioned cat.

[project.r.racing]

b. If spring becomes uncaptive at full droop then a helper spring maybe sufficient to 'push' the inner wheel back in contact. Note: King springs make 2 x helper springs at 1.25/4.0 (kgs/mm) which compress from 70mm to 16/20mm.

I know the whole post was a comic stitch up. But this line had me falling out of my chair with laughter.

[Magpie]

Point noted Project.r.racing....

This is data from the previous setup with a larger front sway bar (purple) and OEM front (red). From 1,040m RPM for the red line was increasing but speed was not. Further it also shows that for this part of the track I was a little faster until 1,040m part.

unloading_01 by Eipeip, on Flickr

The track position for both cars at 1,040 (cross)
unloading_02 by Eipeip, on Flickr