jcduroc
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| posted on 1/9/03 at 08:23 PM |
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quote:
Originally posted by Noodle
quote:
In normal (smooth) circunstances a dD axle keeps the wheels perpendicular to the ground
A de-dion axle keeps the wheels at a fixed camber to the ground under normal circumstances. You can set your axle to be anything you like at
normal ride-height.
Isn't that what I said? OK. You can make it with some camber angle; I referred to 0º camber.
This doesn't change my statement which is: "a dD axle doesn't change camber (relative to the tube being paralell to the ground, if
it isn't that's another story)"
João
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pbura
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| posted on 1/9/03 at 09:14 PM |
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quote:
In normal (smooth) circunstances a dD axle keeps the wheels perpendicular to the ground but that is not allways (seldom?) the case. Any
irregularity in the road (bump) afects both axle wheels equally as will a very stiff front axle compared to the rear one, especially if the chassis is
not very very torsionally rigid.
With an IRS you have more freedom to preview all those situations and hopefully care for them; the problem is, as you stated, the complexity involved.
An IRS also gives less unsprung weight than dD which, I agree, has less unsprung weight than a pure live axle as at least it sprungs the diff.
As to being simpler or not I cann't make a statement as I've never built a dD but I did IRS's.
As in almost everything it is open to discussion: technically or just a matter of taste. Otherwise all the Locosts would be yellow!...
Well,
yes, João, for bumpy road conditions, IRS would be preferable.
Agreed with your last paragraph, too!
 Pete
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craig1410
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| posted on 1/9/03 at 09:42 PM |
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My view on all this, for what it is worth, is that I am preparing my car to perform optimally on the track for the occasional track day and hence
bumps should be less of a problem. Even on main roads, where the maximum potential of the car should not really be explored, bumps should not be a
major problem. In any event, a car with only 2 inches of suspension travel and 4 or 5 inches of ground clearance should be considered a car for smooth
conditions and therefore should be designed to handle smooth conditions, not bumpy conditions.
Cheers,
Craig.
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jcduroc
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| posted on 1/9/03 at 10:03 PM |
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quote:
Originally posted by craig1410
My view on all this, for what it is worth, is that I am preparing my car to perform optimally on the track for the occasional track day and hence
bumps should be less of a problem. Even on main roads, where the maximum potential of the car should not really be explored, bumps should not be a
major problem. In any event, a car with only 2 inches of suspension travel and 4 or 5 inches of ground clearance should be considered a car for smooth
conditions and therefore should be designed to handle smooth conditions, not bumpy conditions.
Cheers,
Craig.
Craig, I can only agree with you.
The fact is that I have to keep in mind the framework of my project (as in any project) and, believe me, we have (in Portugal) some very
"bumpy" national roads and my Locost's susp has to be prepared to deal with this situation.
I guess that if/when I go to a track day I may adjust my susps stiffness (springs, arb's and shocks) accordingly.
Cheers
João
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Tudor (Ted) Miron
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| posted on 2/9/03 at 08:26 AM |
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Hi Craig and All,
I must say that when tolking about bumps I didn't mean public roads - I meant race tracks. Guys who have race track expireance will agree on
how bumpey race track could be at speed.
Main disadvantage - IMHO- is dedion tends to snap camber changes. Imagine going through fast chicane and attaking FIA curb - you hit it with uladen
wheel which is OK with IRS - it's not doing much anyway. Very different story with live axsel/dedion - you get instantenious camber change on
laden wheel. Just what racing tire likes list - rapid changes. Assuming you're on the very limit this leads to very unpleasent events. In
reality you'll just have to take that chikane slower, attaking that curb less and losing ground.
Just my 2 roubles
Ted
PS: sorry for my English
[Edited on 2/9/03 by Tudor (Ted) Miron]
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craig1410
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| posted on 2/9/03 at 11:54 AM |
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Ted,
Yes that's a fair point but again I would say that unless the kerbs were very low then you shouldn't be hitting them anyway on a car with
so little suspension travel, and in this case the camber angle won't change much anyway. The beauty of de-dion compared with Live axle is that
you can build the system with negative camber and a bit of toe-in so that you can tolerate a bit of positive camber change without getting
unrecoverable breakaway of the backend mid corner.
As for tracks, yes I am aware (from experience) that these can be bumpy but in my experience the sort of bumps that I have seen tend to be across the
entire track surface and thus would be handled by de-dion fine.
Remember my original argument was that most people who are building a Locost car won't have the equipment or experience to set up a home built
IRS system properly and would be better with live or de-dion. You may well find that a typical de-dion equiped car would corner faster than the poorly
setup IRS car WITHOUT even touching the kerbs...
Anyway, I appreciate your comments and respect your opinion. I should find out in 12 months or so whether you are correct... 
Cheers,
Craig.
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stephen_gusterson
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| posted on 2/9/03 at 01:12 PM |
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I can agree with the IRS is hard to set up argument.
I will likely have my car set up professionally at a 4 wheel alignment centre once its passed sva.
Trying to improvise at home, setting up camber, castor and toe isnt at all accurate or easy!
atb
steve
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craig1410
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| posted on 2/9/03 at 07:36 PM |
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Steve,
Yes a professional alignment is fully justified but what settings do you go for? Do you set it up as per a westfield or a caterham or just guess? To
me the infinite variables created by the process of building a Locost mean that each car will require different settings to get it close to optimum.
My car will have a V8 and thus be a bit nose heavy compared to a 1.3 x-flow car so I will need stiffer springs at the front for example. I'm
sure that optimum geometry will also be affected by things like engine choice.
Don't get me wrong, you are much much better to get a professional alignment done and set it to sensible settings such as rear toe-in intead of
toe-out and a bit of negative camber, not positive etc etc
You can then experiment from a known stable configuration to get the handling you want.
It'll be interesting when we all get our cars on the track to see what ACTUAL differences all these variables and different suspension systems
make...
Cheers,
Craig.
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jcduroc
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| posted on 17/9/03 at 10:22 PM |
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quote:
Originally posted by craig1410
My car will have a V8 and thus be a bit nose heavy compared to a 1.3 x-flow car so I will need stiffer springs at the front for example. I'm
sure that optimum geometry will also be affected by things like engine choice.
Cheers,
Craig.
Craig
I think you're talking of two different issues here:
1. Weight (let's rather say Mass, which is more correct) and it's distribution dictate your springs & shocks (and anti-roll bars) and,
of course, weight transfer and, therefore, charge on each wheel (dynamics);
2. Suspension geometry is merely kinetics, ie, dictate how your wheels will move in bump & rebound (hence in roll).
What I mean is that you can design your susp geo to gain -3º camber in 100mm bump but if the anti-roll stiffness rates will only allow a maximum of 40
mm outer wheel bump in full roll (let's say 1º30' that's what srings/shocks/a-rollbars have to cope for.
Of course susp geom dictates RC height and this is a factor in weight transfer.
Someone correct me if I'm wrong.
JCM
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pbura
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| posted on 18/9/03 at 04:43 AM |
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quote:
Originally posted by jcduroc
Suspension geometry is merely kinetics, ie, dictate how your wheels will move in bump & rebound (hence in roll).
What I mean is that you can design your susp geo to gain -3º camber in 100mm bump but if the anti-roll stiffness rates will only allow a maximum of 40
mm outer wheel bump in full roll (let's say 1º30' that's what srings/shocks/a-rollbars have to cope for.
Of course susp geom dictates RC height and this is a factor in weight transfer.
Someone correct me if I'm wrong.
Weight transfer, etc., is a fascinating area, of which I hope to have a fuller understanding before I get to the point of specifying springs and
anti-roll bar.
It's possible to predict how much your car will roll in response to lateral Gs, and how much weight will be tranferred via the front and rear
suspensions, respectively. Smithees Race Car Technologies in Oz has a weight transfer worksheet on their website that is truly mind-expanding:
http://www.smithees-racetech.com.au/wttransworksheet.html
This site has great information about weight transfer, roll resistance, and roll couple distribution:
http://members.aol.com/sccacuda/cars/1SmthTa.html
One last thought: If you can get your front and rear suspensions to want to roll at the same rates, in terms of degrees of roll per G of lateral
acceleration, the chassis does not have to be as stiff!
One thing that would help in dealing with weight transfer issues would be tire charts, showing tractive force at different levels of vertical load.
The big-time racers get these.
Track testing is the only way to tune a car's handling. However, it sure would be nice to have 90% of the job done, and to avoid built-in
handicaps, with a good initial setup.
If I can be self-disciplined enough, I'd like to weigh all the major pieces going into the car. That way, I'll know how much unsprung
weight there is at each end of the car, and have some idea where the mass centroid axis lies (for selecting roll centers).
The other data for these calculations, chiefly weight distribution and Cg location, can be gathered when the car is built up.
Food for thought!
Pete
Pete
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Stu16v
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| posted on 18/9/03 at 09:29 AM |
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quote:
One last thought: If you can get your front and rear suspensions to want to roll at the same rates, in terms of degrees of roll per G of lateral
acceleration, the chassis does not have to be as stiff!
But most of the info that I have took on board over the years warns against this. A slight difference in RC heights will discourage the car from
rolling in the first place, and it gives the designer another opportunity to load axles as required mid corner.
Even more food for thought....
And I still think Dedion is the way forward
Dont just build it.....make it!
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pbura
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| posted on 18/9/03 at 02:14 PM |
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quote:
Originally posted by Stu16v
quote:
One last thought: If you can get your front and rear suspensions to want to roll at the same rates, in terms of degrees of roll per G of lateral
acceleration, the chassis does not have to be as stiff!
But most of the info that I have took on board over the years warns against this. A slight difference in RC heights will discourage the car from
rolling in the first place, and it gives the designer another opportunity to load axles as required mid corner.
Even more food for thought....
And I still think Dedion is the way forward
No argument here! What I was getting at is that if both ends of the car want to roll say, 2 degrees, then the chassis would not be subject to any
torsion.
This would be a function of Cg height, weight, roll center height, and roll stiffness at each axle (independently). In other words, you could have
low roll stiffness at the rear, but if the RC is high, the rear end will still only want to roll 2 degrees for a 1G turn, the same as the front
This really helps make the case for a higher rear RC.  I think this may have been part of what Carroll Smith was getting at when he said the
front and rear RCs should be "happy with each other".
I haven't yet worked out how to do these roll calculations, btw, but hope to by the time I get to the rear axle linkage. I'm thinking of
doing a Mumford link, like the Superperformance (http://www.superformance.com/s1_more.asp), because it allows you to put the rear RC where
you want it. A Panhard rod may be good enough unless there's a special need to lower the RC some more.
Also, de Dion rules! 
Pete
Pete
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craig1410
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| posted on 20/9/03 at 12:21 PM |
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quote:
Originally posted by jcduroc
Craig
I think you're talking of two different issues here:
1. Weight (let's rather say Mass, which is more correct) and it's distribution dictate your springs & shocks (and anti-roll bars) and,
of course, weight transfer and, therefore, charge on each wheel (dynamics);
2. Suspension geometry is merely kinetics, ie, dictate how your wheels will move in bump & rebound (hence in roll).
What I mean is that you can design your susp geo to gain -3º camber in 100mm bump but if the anti-roll stiffness rates will only allow a maximum of 40
mm outer wheel bump in full roll (let's say 1º30' that's what srings/shocks/a-rollbars have to cope for.
Of course susp geom dictates RC height and this is a factor in weight transfer.
Someone correct me if I'm wrong.
My view is that you can't really look at anything in isolation as everything is part of the same system. You must of course try to break the
system down into smaller and simpler sub-systems to enable incremental improvements to be made and measured. My point in my previous posting was that
there are no "correct" suspension settings available for our individual cars unless of course we build accurately to a tried and tested
design.
After all there is no point designing your geometry to give you the -3deg camber during suspension bump in your example when your choice of engine has
made the car heavier at the nose, raised the front CoG, increased roll and now needs -4 degrees.
All I want to try to achieve is a good solid foundation from which to build and I am confident that using the de-dion system gets me closer, more
quickly to that goal than if I had gone the IRS route.
Oh and by the way, in case you are worried, I do know the difference between Weight and Mass
Cheers,
Craig.
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jcduroc
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| posted on 22/9/03 at 11:24 PM |
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quote:
Originally posted by craig1410
Oh and by the way, in case you are worried, I do know the difference between Weight and Mass
Cheers,
Craig.
Craig please do not misunderstand me I did not mean to correct you in any way. It was just a precision for when we are talking about kilos: kg f or
Newtons or kg (mass) to avoid confusions.
Excuse me anyway.
Cheers
JCM
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craig1410
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| posted on 23/9/03 at 08:23 PM |
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No worries, I could see that you were only being precise and not pedantic, I was just kidding hence the two cheeky faces at the end.
It'll take a lot more than that to offend me I'm happy to say
[No that's not a challenge guys!!!]
Cheers,
Craig.
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