MikeR
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| posted on 20/3/08 at 11:56 AM |
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de-dion pointless - the proof ?
For a long time (2+ years) i've been looking at going de-dion. Well before i make the cuts to my chassis i decided to weigh things to prove to
myself its worth it. A final 'get out clause' if you like.
Can someone see what i've done wrong here cause according to these figures de-dion gives almost no advantage.
Escort axle, with wheels and brakes but not hand brake cable = total weight 75kg (no wonder its a bitch to move around)
Wheels (155x13 80 profile) 14kg
brakes (from sierra but look identical) 9" (thats drum, back plate, mechanism and shoes) 7kg
So weight of axle inc diff = 75 - (wheels X 2 + brakes X2) or .... 33kg
Going through a one wheel bump the 'mass' moved is .... 1 wheel + 1 brakes + guestimate 66% of axle (can't be half as the mass
between the other wheel and the centre must come into play somewhat) = 42.78kg. Or to put it another way "eeek".
Now de-dion.
Axle 11kg (unwelded).
drive shaft (drum brakes) 14kg
brakes (from before) 7kg
wheel (14kg)
so total weight is axle+ drive shaft *2 + brakes *2 + wheels *2 = 81kg
but i'll say unsprung means only 0.66 of the drive shafts or 73.86kg or 1.5kg lighter than the live axle.
Now more importantly one wheel bump which is what i'll encounter most of the time.
live axle, 1 wheel, 1 brakes and 66% of the axle weight = 42.78kg or yowsers, far higher than i expected.
De-dion 1 wheel bump = 1 wheel, 1 brake, 66% of drive shaft and 66% of de-dion beam or ... 39.88kg or 2.9 kg lighter.
(oh for reference the sierra push in shaft diff came to 20kg making the total de-dion package 26kg heavier without taking into account the chassis
mods for the rear axle or diff mounting. i'd guestimate maybe 30kg heavier in total.)
So, the million dollar question - what have i done wrong above? The advantage seems very small. For some reason i was expecting something like half to
two thirds the weight in 1 wheel bump.
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Mr Whippy
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| posted on 20/3/08 at 12:02 PM |
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I never liked De-dion as it looked half baked nonsence, like something Volvo would dream up. I would have been more impressed it they had replaced the
heavy driveshafts with lightweight hollow tube.
Oh yes that should offend plenty now 
Fame is when your old car is plastered all over the internet
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Paul TigerB6
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| posted on 20/3/08 at 12:11 PM |
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Best ring Caterham then to tell them they got it all wrong.
I really dont think you can simplify matters so much as to just reduce it right down to unsprung weight and so on. The way i see it, suspension design
is so much more complex than I (or no doubt 99.99% of us on here) will ever understand, so wouldnt state something like "pointless - the
proof" based on some simple measurements of weight and assumed ratios acting on one wheel.
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britishtrident
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| posted on 20/3/08 at 12:16 PM |
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The problems with a live axle that 95% of people miss are the massive weight located in the centre of the axle and the torque reactions.
Having weight (mass) located at the centre of the axle is a really bad thing when a wheel hits a single wheel bump, the kinematics to prove this are
quite complex but essentially a single wheel bump on a di Dion has much less effect on the opposite wheel and the movement of the body-chassis than
on a live axle.
Proper de Dion set ups have inboard brakes but even with outboard brakes the removal of torque reactions from the suspension is of great benefit.
Also for a de-dion single wheel bump only 50% of the drive shaft weight should be counted --- inner parts of drive shaft move much less than
outer.
Same goes for the de dion tube.
[Edited on 20/3/08 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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MikeR
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| posted on 20/3/08 at 12:17 PM |
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there was a question mark after the words "the proof" because i don't understand why it appears to have so little advantage and i
want to before i make chassis changes. I've already invested a couple of hundred quid on this axle / diff etc and want to make sure its worth
while.
I personally think caterham went de-dion because they didn't have the resources to go full IRS at the time and it was an easy way to a) show the
car was progressing and b) find a new source of axles as escort / itals where drying up. But that's another debate.
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MikeR
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| posted on 20/3/08 at 12:24 PM |
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Trident - i was aware of that, hence using the guestimated 66% effect of the axle weight. By using hte live axle this would take into account the diff
weight and on the de-dion, with no diff it would have in theory given it a huge advantage.
I've adjusted the de-dion axle and shafts rate to 50% and it now has a 5.7kg advantage or .... 15%. But it seems wrong to leave the live axle at
66% in my head as the mass along the axle is the same physics for both so i should use the same ratio. (if i do use .5 for the live axle its .5kg
heavier than de-dion).
[Edited on 20/3/08 by MikeR]
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britishtrident
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| posted on 20/3/08 at 12:28 PM |
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quote:
Originally posted by MikeR
there was a question mark after the words "the proof" because i don't understand why it appears to have so little advantage and i
want to before i make chassis changes. I've already invested a couple of hundred quid on this axle / diff etc and want to make sure its worth
while.
I personally think caterham went de-dion because they didn't have the resources to go full IRS at the time and it was an easy way to a) show the
car was progressing and b) find a new source of axles as escort / itals where drying up. But that's another debate.
Caterham went de dion because de dion system had a history in the Lotus Seven and 11.
The Lotus Seven A frome axle location was originally designed for de Dion suspension. In a de dion layout unlike a live axle the bush at apex
isn't exposed to drive torque.
The De Dion was in the Chapman Lotus traddion and required minimum changes to the chassis and no changes to the front roll centre.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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02GF74
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| posted on 20/3/08 at 01:37 PM |
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quote:
Originally posted by Mr Whippy
I never liked De-dion as it looked half baked nonsence, like something Volvo would dream up. I
wrong Mr Whippy, the inventor, Celine Dedion is French Canadian, not Swedish, that was Adda who invented IRS and wrote a song about it "Take a
chance on IRS".
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mcerd1
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| posted on 20/3/08 at 02:33 PM |
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I'm no expert on the suspension design, so I'm not going to comment on that....
but if your worried about the total weights of the whole setup, then why not design yourself a DeDion using a lighter diff than the sierra one
(freelander etc...)
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Lippoman
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| posted on 20/3/08 at 02:36 PM |
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In the live axle you also have to take part of the propshaft into the weight, as if the diff of the live axle moves so does the prop...
The deDion axle was used in racing by such greats as Mercedes, Alfa Romeo and Ferrari... (Alfa Romeo won races in the late 80's using
deDion...)
However, the Sierra parts may simply be too heavy to give the concept a fair chance...
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MikeR
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| posted on 20/3/08 at 02:44 PM |
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aha - good point. Of course it raises the question of how much weight would it add, 50% of it would be supported by the gearbox and it mates half way
down the axle. So add 50% of the weight to the 'diff' weight perhaps?
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Schrodinger
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| posted on 20/3/08 at 03:46 PM |
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From your total overall weights you seem to be missing the diff in the deDion package unless it is included in the "axle" weight in which
case the diff should be excluded from the bump equation.
I calculated from your deDion figures
wheel 14
brakes 7
drive shaft 14x.66 9.33
axle 11 x .66 7.26
37.59
not 39.88 giving a further saving of 2.3kg
Keith
Aviemore
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andyharding
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| posted on 20/3/08 at 04:05 PM |
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Of course your post assumes the advantage of DeDion is weight saving. In fact, I'd rather have a heavier DeDion setup than a live axle for the
other benefits.
Are you a Mac user or a retard?
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MikeR
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| posted on 20/3/08 at 05:44 PM |
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Ok. now i'm curious, what other benefits?
The only ones I can think of are ...
its easier to get an LSD. Something i'm not interested in.
Its easier to fit disk brakes. Something i'm also not interested in as the drums fit nicely on 13" wheels.
(although i suppose for a lot of people they are two damned good reasons)
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sgraber
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| posted on 20/3/08 at 06:36 PM |
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This text is lifted verbatim from the 7faq.com website
quote:
Many years ago, when it became apparent that the supply of new Ital axles would dry up, Caterham Cars were looking for alternatives. The
development of the de Dion rear end was undertaken by Clive Roberts and Reg Price. What follows is a response from Clive to a question asked on the
se7ens list back in 1998/9.
Clive goes on to explain his reasons for using the deDion system and provides a benefit analysis. You can read that entire article here:
http://7faq.com/owbase/ow.asp?deDionOrIRS
That said: I personally chose deDion because as a novice designer I knew there was no way that I could nail the geometry on my first try, and as a
budget builder, getting it right the first time was important to me. If you are using a tried and proven geometry that complements your front
suspension design then that of course changes the playing field.
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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Liam
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| posted on 20/3/08 at 06:48 PM |
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Less unsprung weight is less usnsprung weight ! Might not be much but it's still an advantage. Your figures are also hurt by the fact your
switching to heavier sierra stuff. Just comparing the two suspension layouts alone, with like-for-like diff and driveshaft weights the savings would
look better.
And as britishtrident says the main advantages are in dynamics, not overall weight saving - principally not having the axle torque reaction
interacting with the suspension.
Liam
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froggy
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| posted on 20/3/08 at 07:01 PM |
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plus you can design in different mounts for the front trailing arm links and the panhard rod so you can experiment with anti squat and roll centre
height
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britishtrident
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| posted on 20/3/08 at 07:28 PM |
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lotus 11 de dion 1957
 
Rescued attachment Maintenance2.JPG
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MikeRJ
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| posted on 21/3/08 at 05:39 PM |
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quote:
Originally posted by MikeR
Trident - i was aware of that, hence using the guestimated 66% effect of the axle weight. By using hte live axle this would take into account the diff
weight and on the de-dion, with no diff it would have in theory given it a huge advantage.
No, you have missed what BT has said. Having a big heavy lump in the middle doesn't just increase unsprung weight! That big heavy lump has a
lot of inertia; in one wheel bump it will want to try and stay still and in the process it will push the wheel on the other end of the axle downwards.
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MikeR
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| posted on 21/3/08 at 07:29 PM |
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Ah of course.
hmmm, damn, i can't do a simple model of that 
arrggh!
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Bob C
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| posted on 23/3/08 at 08:58 PM |
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Interesting - first time I've seen actual weights for live axle & dedion. Just for comparison I weighed my own IRS unsprung weight (using
bathroom scales) & it was 30 to 31kg, another IRS user with sierra drums on here did the same & got the same weight - so there is a clear
unsprung weight advantage for IRS, c.f. dedion, for the data given.
In fact I was surprised how little the escort axle weighed - i was expecting nearer 100kg!
Bob
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C10CoryM
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| posted on 23/3/08 at 11:22 PM |
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I've never really looked into De-Dion before. Seems to me you get the over all mass of the live axle, with the complexity and other drawbacks
of the IRS (mainly no antisquat). Other than the marginal less unsprung weight idea, are there any other benifits?
"Our watchword evermore shall be: The Maple Leaf Forever!"
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MikeRJ
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| posted on 24/3/08 at 12:01 AM |
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quote:
Originally posted by C10CoryM
I've never really looked into De-Dion before. Seems to me you get the over all mass of the live axle, with the complexity and other drawbacks
of the IRS (mainly no antisquat).
You could build anti-squat into a de-dion design if you thought it was worthwhile.
Complexity in terms of geometry is significantly lower than IRS. Provided you make the De-dion tube correctly you don't have any problems with
gaining or losing excessive camber in bump. Even if you don't make the tube correctly, the geometry problems are largely static and can be
corrected by shimming the hubs etc. Compare that to an IRS that has been badly designed!
I would always expect a De-dion to be slightly heavier than a double wishbone IRS, the big tube connecting the two hubs is going to weigh more than
the small tubes used for wishbones, though I suspect many of the designs used for locosts are sub-optimal in this respect.
Without a doubt, if properly implemented, it gives good benefits over a live axle.
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Bob C
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| posted on 24/3/08 at 04:09 PM |
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Back to MikeR's original measurement..........
single rear wheel "unsprung weight"
live axle 42.8kg
dedion 39.9kg
IRS 31kg
The evidence looks to me like de dion is 1.07x better than live axle for unsprung weight - plus BT's inertial benefits, but IRS is 1.38x
better!!!
Highly instructive to see real measurements applied to this!
Bob
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britishtrident
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| posted on 24/3/08 at 05:20 PM |
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Most Locost De Dions are relatively heavy gauge steel, the Caterham De Dion is made entirely of light alloy.
Comparing a Sierra based De Dion to an english axle is also a bit unfair, the Cortina 2 litre axle is a closer comparison and the Cortina axle is much
heavier than the english axle.
With irs camber in any given situation has to be a comprimise, if you design in enough (negative) camber gain for cornering the camber under braking
and acceleration is up the creek.
If given the choice between an english axle and De Dion or IRS I would go English axle.
If given the choice between a Cortina axle and De Dion or IRS I would go De Dion.
If I lived in an area where the roads were rough I would go IRS.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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