craig1410
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| posted on 6/2/07 at 08:15 PM |
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quote:
Originally posted by kb58
Very true, Syd. Considering how much heavier these cars are ,makes the IRS solution that much more important. No matter how much work it takes, or how
much it costs, getting the unsprung weight down WILL make a big difference in ride quality. And, as said before, most cars will be driven on real
world, bumpy streets, where the sprung-to-unsprung weight ratio is a big deal.
Who cares about ride quality in a race car? Who cares about handling characteristics on a "bumpy street"? The weight of a de-dion axle
isn't much greater (if any) than an IRS setup, especially when you take account of the extra chassis work required to hang an IRS setup on. Oh
and one other thing from your earlier post. The front suspension tends to be independent because it is not usually (in Seven's) the driven axle
and needs to steer. Both requirements lend themselves to being independent.
If you don't mind me saying so, you have a very blinkered opinion which isn't going to help Oli make a balanced decision.
One more thing to favour de-dion in case you need more convincing - it has reputedly much better traction off the start line than an IRS setup. This
is because the camber doesn't change on squat like a standard IRS setup and the tyres stay flat to the road. Of course you could go the whole
hog and dial in anti-dive and anti-squat to your IRS setup but that is adding to the complexity and will make setup even more difficult.
If this was F1builders.co.uk and money and time were no object then IRS might be marginally better but for Locostbuilders.co.uk I think de-dion will
get your car on the road much quicker and it will be effective from day one, and won't tear up your first set of tyres in an afternoon like
badly setup IRS might.
HTH,
Craig.
[Edited on 6/2/2007 by craig1410]
[Edited on 6/2/2007 by craig1410]
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DIY Si
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| posted on 6/2/07 at 08:23 PM |
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He does have a point about ride quality though. Would you really drive a car that was flat and smooth on a track, but shook your teeth out on the way
home? I know I wouldn't. For most on here losing some all out handling won't matter at all, but the improvement in general
drivability/comfort will be do, as most of the use will be on the road.
PS, I agree with you on the De-Dion being quicker and easier to set up though.
[Edited on 6/2/07 by DIY Si]
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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britishtrident
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| posted on 6/2/07 at 08:27 PM |
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quote:
Originally posted by kb58
Very true, Syd. Considering how much heavier these cars are ,makes the IRS solution that much more important. No matter how much work it takes, or how
much it costs, getting the unsprung weight down WILL make a big difference in ride quality. And, as said before, most cars will be driven on real
world, bumpy streets, where the sprung-to-unsprung weight ratio is a big deal.
The unsprung weight issue doesn't actually hold much water.
How thick do you think the de Dion tube has to be ? try weighing an Escort English without the diff unit or shafts fitted the weight is almost
trivial. Ferrari used a cf composite for the test mule de Dion beam, it was an big ugly bit of kit aerodynamic as a barn door but very light and
stiff.
Most Locost de Dion axles are way over engineered, Locost IRS uprights are also tend to be over engineered in unsprung weight per wheel I doubt if
there is more than .5 to .75 kg difference.
Don't think of a de Dion axle as slightly improve live axle it is a different kettle of fish the saving in unsprung weight and wherew the mass
is located make a big difference. A diff unit is the third heaviest mechanical on a Locost and it is located slap bang in the middle of the axle --
I won't go into the newtonian mechanics involved but this is the worst place on the axle to locate a heavy mass as on a bump it will cause more
vertical wheel motion to be transmitted to the chassis.
[Edited on 6/2/07 by britishtrident]
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craig1410
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| posted on 6/2/07 at 08:39 PM |
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quote:
Originally posted by DIY Si
He does have a point about ride quality though. Would you really drive a car that was flat and smooth on a track, but shook your teeth out on the way
home? I know I wouldn't. For most on here losing some all out handling won't matter at all, but the improvement in general
drivability/comfort will be do, as most of the use will be on the road.
PS, I agree with you on the De-Dion being quicker and easier to set up though.
[Edited on 6/2/07 by DIY Si]
Well I'm using nylon suspension bushes on the front and rod-ends all over the rear so my fillings will come out anyway! A little bit of extra
vibration from the de-dion, if there is any, won't make much difference... 
Seriously though, I don't think IRS is any better for ride quality over de-dion. The only advantage of IRS over de-dion is that a bump on the
inside wheel going around a corner won't affect the outside wheel which of course is providing most of the grip. Having said that, many IRS cars
are fitted with a rear anti-roll bar which makes them less independent in order to reduce body roll. Conversely, on a smooth track the de-dion will
have equal grip in the corner and better traction out of the corner!
I think the main reason de-dion isn't used on an F1 car is because it would be difficult to maintain good aerodynamics with one fitted. It would
go right through the diffuser and would probably get in the way of the rear wing mountings.
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MikeRJ
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| posted on 6/2/07 at 09:04 PM |
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quote:
Originally posted by Syd Bridge
This is all more than a little amusing.
Let me ask this;
How many (modern, since 1960) racecars do you see with DeDion backends?
Ok, how about Caterham? They still use the De-dion, if it sucked that badly surely they'd have dropped it long ago?
How do Locosts in the 750MC series manage to get such respectable lap times with a paupers live axle in the back of their cars?
Lots of competition and even some performance road cars use a live axle in the USA.
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blueshift
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| posted on 6/2/07 at 09:06 PM |
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The most convincing arguments against IRS that I've seen are all about geometry and creating potentially lethally bad handling if you
don't really, really know what you're doing.
Syd's arguments that IRS is best for professional race teams are fair enough, but as far as I'm aware there is no generally available
highly regarded set of plans like a "mcsorley IRS".. so the problem is being capable of designing your own IRS that's a) safe and b)
better than de-dion.
I have some idea what rear toe-out conditions can do for your life expectancy.
I judged myself not capable of designing a safe and effective IRS and not willing to put the effort in to become capable.
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JoelP
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| posted on 6/2/07 at 09:42 PM |
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i think its quite a simple one this. If you just want to make the car lighter and get back on the road asap, its a de dion. If you enjoy fiddling in
the garage (insert joke here) then irs is probably more for you. I can guarantee that if you blindfolded someone and stuck them in someones locost,
for a drive on a large airstrip, they wouldnt be able to tell the difference at all. And i bet a few bumps wouldnt give you any clue either.
Im putting IRS on my new car, because i can and i want to.
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andyps
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| posted on 6/2/07 at 10:32 PM |
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Mike RJ - the latest Caterham chassis has moved to IRS.
Surely, just as easily as it is possible to make a bad IRS, anything not aligned correctly on a de-dion would be pretty difficult to sort out and
would wear tyres out quickly.
Whilst there may be aerodynamic issues with de-dion on an F1 car, for all the enclosed bodywork racers this wouldn't apply, but very few of them
(particularly those designed specifically as race cars) deviate away from IRS, yet they run on the smoothest tarmac surfaces available. Says enough
for me to be convinced which ultimately works best.
Andy
An expert is someone who knows more and more about less and less
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craig1410
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| posted on 7/2/07 at 12:00 AM |
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I agree that IRS is probably better for out and out race cars because you want as many variables as possible to tweak to gain an edge but it is
exactly the large number of variables which makes it more difficult for an amateur builder to get IRS right.
Most amateurs don't have access to sophisticated computer modelling , can't afford to have multiple sets of slightly different components
(springs, dampers, anti-roll bars etc.) made to order and don't want to spend thousands of hours on a track trying to get it all to work. Most
amateur's won't have the engineering know-how to do it either even if they did have unlimited money and time.
Blueshift's point about rear toe-out strikes a chord with me too because I nearly had a bad accident in a mini when I was 17 because I
didn't realise that the inner radius arm pivot hole was slotted on a new rear subframe which I had fitted. The resulting rear toe-out made the
car oversteer badly when I went over a bump mid corner and I very very nearly crashed.
It's much easier to make a de-dion axle up in a jig to be either perfectly parallel (or to have very slight toe-in and negative camber as mine
has) than it is to construct an inherently safe IRS setup from scratch. You could of course use rod-ends on all the joints to allow you to adjust out
any gremlins but that will bring additional challenges.
I would confidently predict that an amateur built de-dion car would out perform an otherwise equivalent amateur built IRS car on any track in the
country.
Cheers,
Craig.
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kb58
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| posted on 7/2/07 at 01:52 AM |
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quote:
Originally posted by craig1410
Who cares about handling characteristics on a "bumpy street"?
Mayby Oli should answer - most normal people do care about ride.
quote:
If you don't mind me saying so, you have a very blinkered opinion which isn't going to help Oli make a balanced
decision.
"Balanced" means hearing from both sides. Your approach is to give a one-sided opinion, then be bothered by opposing points.
quote:
One more thing to favour de-dion in case you need more convincing - it has reputedly much better traction off the start line than an IRS setup.
This is because the camber doesn't change on squat like a standard IRS setup and the tyres stay flat to the road.
Why is it then that many race cars that use straight axle tubes, bend them to add camber? It's because when the car corners, the tires roll,
causing positive camber - a bad thing.
quote:
Of course you could go the whole hog and dial in anti-dive and anti-squat to your IRS setup but that is adding to the complexity and will make
setup even more difficult.
Or not - it's nice to have the choice. As I said before, I did say that what is "best" is a very personal thing. Argue all you like,
but it's not up to you, me, or anyone else, what he builds. However, he IS going to hear a two-sided discussion on this. Oh, and I too will
close with a  to make it all better.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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kb58
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| posted on 7/2/07 at 02:26 AM |
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quote:
Originally posted by craig1410
I agree that IRS is probably better for out and out race cars because you want as many variables as possible to tweak to gain an edge but it is
exactly the large number of variables which makes it more difficult for an amateur builder to get IRS right.
Most amateurs don't have access to sophisticated computer modelling , can't afford to have multiple sets of slightly different components
(springs, dampers, anti-roll bars etc.) made to order and don't want to spend thousands of hours on a track trying to get it all to work. Most
amateur's won't have the engineering know-how to do it either even if they did have unlimited money and time.
Blueshift's point about rear toe-out strikes a chord with me too because I nearly had a bad accident in a mini when I was 17 because I
didn't realise that the inner radius arm pivot hole was slotted on a new rear subframe which I had fitted. The resulting rear toe-out made the
car oversteer badly when I went over a bump mid corner and I very very nearly crashed.
It's much easier to make a de-dion axle up in a jig to be either perfectly parallel (or to have very slight toe-in and negative camber as mine
has) than it is to construct an inherently safe IRS setup from scratch. You could of course use rod-ends on all the joints to allow you to adjust out
any gremlins but that will bring additional challenges.
I would confidently predict that an amateur built de-dion car would out perform an otherwise equivalent amateur built IRS car on any track in the
country.
Cheers,
Craig.
How is a de Dion properly built to ensure that there's never any toe out? It's measured carefully during design, during construction, and,
to make sure, after it's installed. And yet, depending where the trailing links are attached, it is still possible to get toe out. So, if you
don't know what your doing, the de Dion isn't going to save you.
About IRS having complicated toe-out issues, how hard is it to stretch a string along the side of the car and measure it? If this is too complicated,
how will this hapless builder ever be able to set front bumpsteer?
Sophisticated computer modelling? A string computer works, or there's a free suspension CAD sw program bouncing around this very forum.
Too advanced for the amateur builder... I designed one, and built it myself. It's just a matter of taking your time, researching, and
understanding what's going on.
It's unfortunate that once we venture off our tiny "The Book/McSorley Island," most builders think they're going to drown. Not
if you teach yourself to swim.
[Edited on 2/7/07 by kb58]
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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procomp
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| posted on 7/2/07 at 08:13 AM |
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Hi if you really want light go with live axle as this is the lightest option by far. The so called problems of unsprung weight is only a problem if
you dont sort out the dampers to match the setup these dampers can be had for £90 ea.
cheers matt
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Syd Bridge
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| posted on 7/2/07 at 09:20 AM |
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There's a lot of talk about 'ride' quality.
This is another of my hobby horses.
The quality of ride with any rear end system should differ little.
The biggest problem is educating you lot to put the SOFTEST springs in the car all round.
Far too many locosts are oversprung, overdamped, then complaints are made about dodgy handling and hard ride quality.
The rubbery chassis will not tolerate hard springs, and ends up acting as part of the spring/suspension medium itself.
Put whatever backend in you fancy, but please, spring it properly and then set the dampers as soft as is tolerable.
Cheers,
Syd.
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MikeRJ
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| posted on 7/2/07 at 09:26 AM |
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quote:
Originally posted by Syd Bridge
There's a lot of talk about 'ride' quality.
This is another of my hobby horses.
The quality of ride with any rear end system should differ little.
Even if the rear end choice has a very poor sprung/unsprung weight ratio?
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procomp
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| posted on 7/2/07 at 09:29 AM |
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Hi wise words from syd.
cheers matt
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MikeRJ
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| posted on 7/2/07 at 09:33 AM |
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quote:
Originally posted by craig1410
One more thing to favour de-dion in case you need more convincing - it has reputedly much better traction off the start line than an IRS setup. This
is because the camber doesn't change on squat like a standard IRS setup and the tyres stay flat to the road.
quote:
Originally posted by kb58
Why is it then that many race cars that use straight axle tubes, bend them to add camber? It's because when the car corners, the tires roll,
causing positive camber - a bad thing.
Some static negative camber is added to live axles/de-dions to help the tyre stay flat during cornering, it's not there to help with traction
when the suspension squats.
The point being made is that just like a live axle or De-dion, most IRS systems (double wishbone) are a compromise, each have their own strengths and
weaknesses.
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3GEComponents
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| posted on 7/2/07 at 09:38 AM |
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The reason Caterham went over to the De-dion axle was because of supply issues over live axles, the De-dion was a logical choice to keep the handling
pretty much the same as the live axle.
F1 cars are a bad example of IRS, as they're set so stiff the only travel in the set up is in the side walls of the tyres!
Alot of competion cars have to race in classes where the rules state that location points cannot vary from the production vehicles, so they have to
follow what the road car has, which in general is very biased to comfort!
Production cars are also designed to offer maximum interior space, so the suspension is often designed to fit into what space is left, not really an
ideal situation.
IRS has an advantage in the fact that it can be designed to be highly adjustable. Which is great if you're one of life's tinkerers, but in
something as light as a 7, you are hard pushed to beat a De-dion set up.
As MikeRj has said, they are all just a compromise.
[Edited on 7/2/07 by jroberts]
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Schrodinger
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| posted on 7/2/07 at 10:17 AM |
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oli
You're not going to ask if you should change to a BEC next are you?
FWIW
I'm not sure that the Cat has sufficient room to fit the trailing arms in place, also as the car was designed(in the widest terms) to take a
Sierra back end you may need to think carefully about the track width, you could also look, as you have already said, at grafting on an Avon back end
or using Rorty's drawings.
[Edited on 7/2/07 by Schrodinger]
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Fozzie
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| posted on 7/2/07 at 10:19 AM |
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quote:
Originally posted by Syd Bridge
There's a lot of talk about 'ride' quality.
This is another of my hobby horses.
The quality of ride with any rear end system should differ little.
The biggest problem is educating you lot to put the SOFTEST springs in the car all round.
Far too many locosts are oversprung, overdamped, then complaints are made about dodgy handling and hard ride quality.
The rubbery chassis will not tolerate hard springs, and ends up acting as part of the spring/suspension medium itself.
Put whatever backend in you fancy, but please, spring it properly and then set the dampers as soft as is tolerable.
Cheers,
Syd.
Oh so very, very true!........ I have had many conversations on that exact point.... 
Some listen, most do not.....
Sadly,you can lead a horse to water Syd, but you can't make it drink....
IMHO a very important part of the build, where not enough thought is given ...
Fozzie
'Racing is Life!...anything before or after is just waiting'....Steve McQueen
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flak monkey
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| posted on 7/2/07 at 10:30 AM |
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Theres designs for both de dion and irs on my website. What the geometry is like I dont know, but they might offer a starting point for designs.
Just know there are advantages and disadvantages to each solution. De dion is the quickest to retro fit to a locost as you dont need to do any chassis
mods. IRS is much more involved. Theres enough literature and knowledge out there to let you build a very good IRS set up, all depends how much time
and effort you are willing to put into the design.
Most people will never drive a locost to its true limit, and so the small differences between a good live axle/de dion/irs set up are irrelevant
IMO.
David
Sera
http://www.motosera.com
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iank
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| posted on 7/2/07 at 10:44 AM |
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quote:
Originally posted by Syd Bridge
...
The biggest problem is educating you lot to put the SOFTEST springs in the car all round.
...
Put whatever backend in you fancy, but please, spring it properly and then set the dampers as soft as is tolerable.
Cheers,
Syd.
Agree completely, Colin Chapman, who knew a bit about suspension (allegedly ) was an advocate of soft springs allied to firm damping. Lotus models
have been accused of many things, but I can't remember poor handling being in the list.
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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britishtrident
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| posted on 7/2/07 at 10:57 AM |
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quote:
Originally posted by Fozzie
quote:
Originally posted by Syd Bridge
There's a lot of talk about 'ride' quality.
This is another of my hobby horses.
The quality of ride with any rear end system should differ little.
The biggest problem is educating you lot to put the SOFTEST springs in the car all round.
Far too many locosts are oversprung, overdamped, then complaints are made about dodgy handling and hard ride quality.
The rubbery chassis will not tolerate hard springs, and ends up acting as part of the spring/suspension medium itself.
Put whatever backend in you fancy, but please, spring it properly and then set the dampers as soft as is tolerable.
Cheers,
Syd.
Oh so very, very true!........ I have had many conversations on that exact point....
Some listen, most do not.....
Sadly,you can lead a horse to water Syd, but you can't make it drink....
IMHO a very important part of the build, where not enough thought is given ...
Fozzie
Actually a rubber chassis will only work with very hard springs, that was the way they built racing cars in the 1930s. As soon as space frames and
monocoques came in springs became softer -- look at pictures of 1960s F1 cars. Then massive aerodydamic downforce came in a suspension design
particulatry in F1 became a side issue, the main requirement of a F1 susension now is that it dosen't spoil the airflow.
From the F1 airflow point of view De Dion is a non-starter.
De Dion is difficult to apply because of packaging issues, aerodynamic in the case of formula cars or sports racers, while for road cars it just
won't fit in without major loss of boot space. Locost and Seven like cars are in a fairly unique position in that chassis allows a De Dion setup
easily.
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craig1410
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| posted on 7/2/07 at 01:12 PM |
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quote:
Originally posted by MikeRJ
Some static negative camber is added to live axles/de-dions to help the tyre stay flat during cornering, it's not there to help with traction
when the suspension squats.
Mike, I'm not sure if you are agreeing with me or not but I get the feeling that you have misunderstood me.
What I was saying is that:
1. Maximum rubber area on the road generally means better straight line traction.
2. Acceleration causes the back end of a car to squat down due to weight transfer to the back wheels.
3. On a de-dion car this squat will not change the camber (except for flex) so the tyres stay flat to the road.
4. On an IRS car without anti-squat geometry the rear wheels will normally increase negative camber which will reduce rubber contact area and reduce
traction.
The increase in camber on the IRS car is desirable when cornering but is undesirable for straight line traction. In other words it is a compromise.
De-dion axles often include a little bit of toe-in (for better stability) and a little bit of negative camber to compensate for chassis/axle flex and
tyre distortion.
Cheers,
Craig.
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Syd Bridge
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| posted on 7/2/07 at 01:16 PM |
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I've deleted a few posts. This was getting way off topic and heading towards juvenile.
And Flakmonkey, my knowledge is all I have to sell, I don't give it away. You will learn this one day. As for losing it with my passing, my sons
are well educated, and in my methods as well.
If someone wants to pay for my knowledge, then they get that benefit.
Syd.
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oliwb
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| posted on 7/2/07 at 01:31 PM |
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Can of worms or what! Firstly - this is a ROAD car not a track day weapon or racer. I'm really not that interested in how it performs on
track. If I was I'd have a striker with a bike engine and IRS rear setup (but I don't). Secondly - your all right I probably wont ever
get near the limits of the car but its always nice to know its there! Thirdly - I'm not planning on designing an IRS setup from scratch. There
are (as flakmonkey) says various designs already available. The original IRS plan was to graft an Avon rear end in place. Far from making the choice
clearer you are all confusing me more than ever. What I really want to know is - given the extra expense and hassle of IRS over de-dion (for a road
car) is it worth it??? Opinions gladly received but please don't go on about F1 cars again! I wish I could afford one but it aint gonna happen
so lets stick to 7's please.....Thanks in advance. Oli.
If your not living life on the edge you're taking up too much room!
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