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Old 30th July 2003, 01:11 PM   #41 (permalink)
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Simon,

I for one am interested in this subject but to be honest, the technicalities and especially the bickering (pointing no fingers) can become tedious and almost irrelevant. I suppose what I'm saying is that whilst it's good to appreciate how these 'things' work and affect the car, it would be great if we could take something from the subject which could be used practically to benefit the handling characteristics of our cars.

I'm sure that will not always be possible as one would then be looking at parts which may or not be available but, for my part, having adjustable suspension, it would be good to know that perhaps increasing the rebound rate on one axle compared with the other would for example improve handling on a circuit...
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Old 30th July 2003, 01:33 PM   #42 (permalink)
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i am especially interested in something mycroft said early on about weight distribution. a 50/50 weight distribution changing due to acceleration and braking makes itan even weight distribution not so important as all the manufactueres lead us to believe?
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Old 30th July 2003, 01:39 PM   #43 (permalink)
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I am following the thread with interest, depsite the oppsoing views sometimes posted

Just to clarify on the point of tyre loading versus grip generated, my very simplistic view here is that the tyre generates friction with the road surface, hence our grip. This means that the tyre has a coefficient of friction, and hence the 'grip' it can generate is basically a product of the vertical force upon it and the coef. of friction? I understood this to be a straightforward linear equation (F x u), but your posts would suggest this is not the case - load versus grip relationship is non linear? Are we talking in a practical situation here, where the wheel is not always vertical to the road surface (the car rolling may make the wheel more or less perpendicular to the road surface depending on the suspension design), or would this still apply in a theoretical, 'assume the wheel is perpendicular to the road at all times' situation? If so, how? Just trying to gain a better understanding in my own mind here!

I apologise if I don't use the correct terms to describe things, I am a car enthusiast, and not a chassis engineer I just enjoy driving and improving my own car. Mycroft - I'll buy the book/s you have recommened, it will clearly be a good starting point .

Thanks

Kes
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Old 30th July 2003, 02:15 PM   #44 (permalink)
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Hi All

That's GREAT that you're interested, thank you for the excellent questions...

Quote:
it would be good to know that perhaps increasing the rebound rate on one axle compared with the other would for example improve handling on a circuit...
The following assumes that you have independant adjustment of the rebound rate available to you.

The important thing to remember is that dampers only have a large effect on the handling of a car WHILST the body of the car is rolling / pitching... once it has settled to it's an approximate steady state, the majority of effects are generated by springs and bars.

So dampers are most useful during the transient states for instance...

Whilst just getting on the brakes,
Just turning in to the corner,
starting to apply power on exit,

etc.

The further you get away from these specific situations, (eg after turn-in but before apex) the dampers have gradually less effect, but still DO have an affect.

OK...

If you increase the rebound rate of the rear in relation to the front (effectively increasing the rebound resistance in the suspension), it means that the tyres cannot be pressed in to the ground as quickly when the suspension is extending.

So as you hit the brakes, more load would be transfered forward during this transition.

When you turn in, the inside rear wheel will again transfer more load away from it - mainly to the outside rear.

So.. Due to the fact that weight transfer (Across contact patches) loses you grip as a sum of those two tyres, you will effectively be reducing your grip during braking and turn-in at the rear.

A bonus with this, is that during exit, the whole thing works in reverse (sort of - if you follow my logic) in that more load is transfered back again across the rear axle due to the outside rear having increased rebound resistance. So you end up with more grip.

The above follows the same principle on the front end, but obviously the outcome for handling is oposite.

One thing to remember about rebound damping however is that it's primary function is to "damp" the rebound action of the spring. This means that there is a narrow window of adjustment available in rebound damping whilst keeping it matched to your spring rate.

Quote:
a 50/50 weight distribution changing due to acceleration and braking makes itan even weight distribution not so important as all the manufactueres lead us to believe?
the 50/50 weight distribution does not change at all. What DOES change is the vertical load on the contact patches. The mass of the car remains the same.

50/50 weight distribution is incredibly important - if that is what you're after. As it affects the polar moment of innertia characteristics as well as vertical load and a host of other things.

323GT-R

That's a really cool question. The actual coefficient of friction is only a part of the grip produced by the tyre. The grip is produced using two major mechanisms... mechanical locking (the actual rubber molding itself into the tarmac at a micrscopic level) and chemical locking which is the actual interaction between the surfaces at a chemical level.

Now.. the thing we call grip, is not just the coefficient of friction of the rubber with the road. It is the way in which the tyre resists the forces of the car.

When cornering, the tyre is not just being squished sideways, it is also being twisted. The twist is created by the way the tyre reaches the ground and what happens to the leading edge once that has happened.

This is REALLY difficult to explain and picture, but I'll try.

Imagine that the tyre is *travelling* straight ahead, and *pointing* at 5 degrees to the left. That means the tyre is currently producing a 5 degree slip angle. Now, fix it in your mind in that position.

Let it move now whilst stuck to the ground, but don't let the wheel actually roll. The contact patch will be pulled backwards (at a 5 [edited to change typo from 4 - 5] degree angle to the wheel). Now hold it in place again.

Now roll the tyre very slightly in the direction it is pointing. The bit of tyre that is JUST about to touch the ground is very slightly to the left of the direction of travel. Once it hits the ground, the bit of tyre ahead of it, has enough stretch still in it to allow it to be stretched backwards like the rest of the tyre, and so the process continues.

Not sure if that is clear, or even relevant to your question. Hope so

Cheers

Simon
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Old 30th July 2003, 02:21 PM   #45 (permalink)
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ducking back to the previous promise I made...





All the best

Simon
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Old 30th July 2003, 02:27 PM   #46 (permalink)
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Simon,

It makes sense, and at least now I can see why it's not a straight forward linear relationship.
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Old 30th July 2003, 04:28 PM   #47 (permalink)
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Quote:
Originally posted by SDB
One thing to remember about rebound damping however is that it's primary function is to "damp" the rebound action of the spring. This means that there is a narrow window of adjustment available in rebound damping whilst keeping it matched to your spring rate.
Showing my ignorance here but I'm assuming that the adjuster on the top of adjustable shocks is for rebound rate? Would you expect an aftermarket manufacturer to measure this narrow window of adjustment when producing shocks? That is, would the 0 -12 settings I have on my HKS Hiper D's all be within this window?

Out of interest, the weigh distribution on my car is approx 57%/43% (F/R) with me in the car....
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Old 30th July 2003, 04:41 PM   #48 (permalink)
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Hi Peter

It's difficult to say for sure. Some may produce the damper so it is adjustable enough to allow you to tune it for many different spring rates (should you wish to change them), others may do it purely for tweaking.

Most competition dampers I've used have independ bump and rebound adjustment, and have a range allowing them to be tuned to a range of springs.

One thing that is also important to be aware (although not worry about) is that an adjustable damper adjusted to a setting, will rarely be as good as a quality damper valved specifically for that resistance. Usually they will be valved for a specific small range and going outside that will make them slightly less ideal. As I said, it's not something to worry about, but just to be aware of.

Cheers

Simon

PS. Sorry that's not a definitive answer. There are some good easy tricks you can use to get your rebound damping in the right ball park before you start tweaking. If you're interested I'll explain.
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Old 30th July 2003, 04:59 PM   #49 (permalink)
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Quote:
Originally posted by SDB
There are some good easy tricks you can use to get your rebound damping in the right ball park before you start tweaking. If you're interested I'll explain.
Simon,

No, not interested at all!

Well, actually I am....
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Old 30th July 2003, 05:04 PM   #50 (permalink)
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LOL

OK.. a really simple way of getting roughly in the right ball park is to do this...

Wind the rear rebound to full stiff and the front to full soft.

Get a mate to drive the car at about 15 MPH along a road with slight low frequency bumps on it and some smooth parts.

As the car goes over a bump, watch the front of the car in relation to the wheel.

With the rebound being extra soft, the body is likely to oscilate slowly up and down once or twice. Now wind the front damping up a bit at a time until the oscilation stops and the car just oscilates once. In other words, the suspension is compressed as it goes over the bump, then extends quickly to roughly the right position without going past it.

Then repeat the same with the rear.

This really is a very rough simplistic idea, but it will get you roughly in the right region.

Cheers

Simon
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Old 30th July 2003, 05:07 PM   #51 (permalink)
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Thinking about it...
and easier way for the front would be to watch it if the driver sharply brakes and then let's go as it could be confusing watching it over bumps. A speed bump would probably be ideal though.
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Old 30th July 2003, 05:14 PM   #52 (permalink)
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Simon,

So do I have to let somebody else drive *my* car....?

I'll have a go tonight then although speed bumps are a thing of the past for me...

Thanks.
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Old 30th July 2003, 05:29 PM   #53 (permalink)
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Quote:
Originally posted by SDB
---Your reply to my post of 11.17hrs---
1/. You are confusing Mass with weight [again]

ie... The rear of the car does weigh less, but its mass remains the same.

2/. Stretching , squeezing or distorting graphs is not the way to answer anything Simon. There will always be an ideal size of tyre for weight giving max grip, your failure to see this is not really worth the bandwidth refuting. We see this whenever it rains or whenever we 'upgrade'.

Your denial of the blindingly obvious does you no credit at all, perhaps we should all start seeing 500kg 35hpn trabants running on 285/30s'

3/. ''It is unquestionable that doubling the load on a tyre less than doubles the grip it produces (we've all agreed this above).''
WRONG, I question this and I am yet to be orovided with any proof of such.

4/. I will E-mail Doug Millikenn with your comments and a link to here, he might have some issues with what you say!

5/. A Mass Distrubution Line is just that ''A Mass Distribution Line'' it is not and never will be the COG.

6/. My apologies you said twisting force , in the light of the fact you are rather vague with your nomenclature, I was just trying to help you out.
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Old 30th July 2003, 05:39 PM   #54 (permalink)
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If anyone believes Mycroft has a point, please let me know and I'll explain. Otherwise (as can be seen over the previous pages) it will be pointless.

Cheers

Simon
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Old 30th July 2003, 05:57 PM   #55 (permalink)
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Quote:
Originally posted by SDB
So dampers are most useful during the transient states for instance...

Whilst just getting on the brakes,
Just turning in to the corner,
starting to apply power on exit,
etc.

The further you get away from these specific situations, (eg after turn-in but before apex) the dampers have gradually less effect, but still DO have an affect.
At last, a nugget of useful information, succinct too! [nearly worth the wait]

Quote:
Originally posted by SDB
If you increase the rebound rate of the rear in relation to the front (effectively increasing the rebound resistance in the suspension), it means that the tyres cannot be pressed in to the ground as quickly when the suspension is extending.

So as you hit the brakes, more load would be transfered forward during this transition.

When you turn in, the inside rear wheel will again transfer more load away from it - mainly to the outside rear.

So.. Due to the fact that weight transfer (Across contact patches) loses you grip as a sum of those two tyres, you will effectively be reducing your grip during braking and turn-in at the rear.

A bonus with this, is that during exit, the whole thing works in reverse (sort of - if you follow my logic) in that more load is transfered back again across the rear axle due to the outside rear having increased rebound resistance. So you end up with more grip.

The above follows the same principle on the front end, but obviously the outcome for handling is oposite.

One thing to remember about rebound damping however is that it's primary function is to "damp" the rebound action of the spring. This means that there is a narrow window of adjustment available in rebound damping whilst keeping it matched to your spring rate.
Then you go and spoil it by posting a rambling thing like that!

Peter, did you gain any insight at all from that?

Buggered if I did!

Why can't you just say:- If you increase the 'rebound rate' of rear shocks you will get less understeer for gawds sake!

Will add to this after another cuppa... BRB
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Old 30th July 2003, 05:59 PM   #56 (permalink)
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Quote:
Why can't you just say:- If you increase the 'rebound rate' of rear shocks you will get less understeer for gawds sake!
Because it's not always true. As can be seen from my post if you actually read it.
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Old 30th July 2003, 06:11 PM   #57 (permalink)
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But it is damned sight more useful that that waffle you produced to say [not quite] the same thing.

Please reply to this.

3/. ''It is unquestionable that doubling the load on a tyre less than doubles the grip it produces (we've all agreed this above).''
WRONG, I question this and I am yet to be orovided with any proof of such.
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Old 30th July 2003, 06:22 PM   #58 (permalink)
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Mycroft

Do you have anything to add to this discussion?

If you disagree with my statements, please provide ANY proof or insight that contradicts it and I will be happy to see it. Until then, I suggest you just soak it all up as you will be able to quote it as your own work later on somewhere else.

Do you HONESTLY think that ANYONE reading this cannot see what a fool you're making of yourself? Do you HONESTLY think that ANYONE believes the stuff you're posting?

You could be so valuable if you were just honest and said "hey guys, not sure if this is the full story, but I've read x, y, z".

You could add so much more to the conversation if you said "hang on simon.. I see what you're saying, but doesn't that mean x??", and then when that is answered, do as others do and say "aha! I get it now".

I really genuinely do not get you.

Please contact Doug (although I have no doubt this is just posturing), as it seems that you are going to need an insult of that level before you stop embarrasing yourself to such a degree.

Regards

Simon

PS. If it's the only way to calm you down, and you don't want to contact doug or bill, please let me know and I will do it myself. In-fact a friend / colleague is speaking to doug this evening.
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Old 30th July 2003, 06:40 PM   #59 (permalink)
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I've just noticed another "mycroft stealth edit". ...

Quote:
Please reply to this.

3/. ''It is unquestionable that doubling the load on a tyre less than doubles the grip it produces (we've all agreed this above).''
WRONG, I question this and I am yet to be orovided with any proof of such.
Again.. If anyone believes this needs answering, please let me know.

All the best

Simon
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Old 30th July 2003, 06:53 PM   #60 (permalink)
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Quote:
Originally posted by SDB
I've just noticed another "mycroft stealth edit". ...
There is no such thing as a 'srealth edit, there is just editing the time of the edit is displayed if you don't do it immediately.

Check the times.

Quote:
Originally posted by SDB
Again.. If anyone believes this needs answering, please let me know.
Me.

Post a single written proof... in your own time .
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