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.

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

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.

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]

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.

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]

quote:

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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.

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quote:

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Originally posted by Mr Whippy
I never liked De-dion as it looked half baked nonsence, like something Volvo would dream up. I

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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...)

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...

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?

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

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.

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)

This text is lifted verbatim from the 7faq.com website

quote:

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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.

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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

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

lotus 11 de dion 1957
Rescued attachment Maintenance2.JPG

quote:

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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.

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Ah of course.

hmmm, damn, i can't do a simple model of that

arrggh!

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

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?

quote:

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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).

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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

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.

Thought you lived in UK BT......... ;^)

Just got the sierra diff brackets from short track (excellent quality by the way - very impressed) weighed them and its another 2kg to the sprung weight.

(they are to the haynes roadster design so it needs a bit more steel welding into the chassis as well.)

I remember that caterham used a 'thin' tube. I also remember they had a problem with diff mounts bending / breaking / going through and had to revise the design. No idea what the revised design weighs (or if they just welded something thicker around the shock mount).

Bob - your weight, does that include the wheel weight? Mine includes a wheel at 14kg (which is what my steel escort 155x80x13 weighed). If it doesn't it makes it re-addresses the balance and makes the IRS look far less favourable.

quote:

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Originally posted by MikeR
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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all I can say Mike is are your scales right??

Firstly, 2.3kg of UNSPRUNG weight is definitely not to be sniffed at, BUT...........

How can the brakes be 7 kg, or if they are, then there's a good argument for discs!!!

The sierra rear std caliper is 3.1kg ish and the disc is approx 3 kg (before lightening it) so thats saved nearly a kilo for a kickoff.

then the driveshafts!!!....

14kg for those shoddy articles!!!

Lobro joints, again before lightening, are 1.5 kg each so 3 kg per shaft, and the shaft itself is around 1.6 kg in the case of my GB items, so that saves nearly 10kg a shaft, ok a bit less with grease and boots included but you get the idea

the scales could be wrong and bathroom scales are supposed to be less accurate at lower weights. At 80kg weights it gives an identical reading to an electronic set of scales i've got.

(i picked 80kg as i have an easily transportable 80kg mass known as my fat arse).

Drive shafts - yep, i was surprised. But they are physically heavy.

drums - thats back plate, drum + brake shoes and adjusters. That sierra stuff certainly isn't light weight, and compared to the drive shafts it seems to be about right.

As a 'check' i weighed the diff mounting brackets on kitchen scales and then on the bathroom scales. They did come out about the same weight.

Think i have decided to go de-dion. I'll just have to upgrade the crossflow engine to cope with the extra 30 or so kg mass the poor car is going to get to do it. Now i need to bite the bullet and start welding up the axle / modifying the rear end.

(at least if i change my mind again the rear end will cope with either axle!)


Oh, just realised, your disk argument - how much does the mounting bracket weigh for the calliper? (and will they fit in 13" wheels :p)

[Edited on 26/3/08 by MikeR]

Hi Mike,
yes wheels and tyres included in the IRS measurement, they were the 14" original mx5 wheels (slightly minilight look -e -like).
My rear & front unsprung weights are identical. I'm using mx5 uprights, wilwood dynalite single calipers & homebrew discs with alloy bells. The other chap who weighed his might well have been an indy with drums at the rear. Considering the above, a more "standard" rear setup would probably be +2kg for caliper and maybe another 1kg for the disc.
Bob
PS the www button below gives all the details......
PPS hazy memory has the wheel + tyre at around 14kg too......

[Edited on 26/3/08 by Bob C]

[Edited on 26/3/08 by Bob C]

Hopefully this link will work to old thread (plus details of measurement....)
http://www.locostbuilders.co.uk/viewthread.php?tid=33933&page=1

If i get the chance i'll re-weigh components tonight. I'll use the kitchen scales (the joy of not living with a woman) and update where necessary.

(i'm starting to think some of my weights must be wrong for things like the drum brakes / drive shafts etc).

Something mentioned in that thread that Bob linked to which hasn't been discussed for a while is inboard brakes. This is something that a de-dion (and IRS) make possible, and should result in a significant USW reduction.

I assume the Sierra driveshafts and CV's are up to the job for this when used on a Locost?

i've re-weighed the de-dion using the kitchen scales (ok and the bathroom scales for the de-dion beam, but with me holding it and then without it and the assumption its more correct at circa 90kg)

beam 5.3kg
cv tube 1.1 (but there are two)
ear 800g (but there are two)
trailing arm mount 550g (but there are two)
panhard rod mount 250g

giving total of 10.45kg (i did say 11kg before)

i'll be in the garage later weighing the brakes etc once the rain stops (assuming the footy hasn't started)

quote:

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Originally posted by MikeR
the scales could be wrong and bathroom scales are supposed to be less accurate at lower weights. At 80kg weights it gives an identical reading to an electronic set of scales i've got.

(i picked 80kg as i have an easily transportable 80kg mass known as my fat arse).

Drive shafts - yep, i was surprised. But they are physically heavy.

drums - thats back plate, drum + brake shoes and adjusters. That sierra stuff certainly isn't light weight, and compared to the drive shafts it seems to be about right.

As a 'check' i weighed the diff mounting brackets on kitchen scales and then on the bathroom scales. They did come out about the same weight.

Think i have decided to go de-dion. I'll just have to upgrade the crossflow engine to cope with the extra 30 or so kg mass the poor car is going to get to do it. Now i need to bite the bullet and start welding up the axle / modifying the rear end.

(at least if i change my mind again the rear end will cope with either axle!)


Oh, just realised, your disk argument - how much does the mounting bracket weigh for the calliper? (and will they fit in 13" wheels :p)

[Edited on 26/3/08 by MikeR]

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got into the garage and weighed a drum brake assembly in bits on the kitchen scales.

(Which i've just realised i've left in the garage with the bit of paper).

Basically, the initial weight of 7kg was close enough.

Only thing i need to re-weigh now is the drive shafts.

finally got round to re-weighing the drive shafts. Did it three times. once just on the scales and twice with and without me holding them on the scales.

Got a consistent set of results. 9kg (5kg lighter than before)

Makes De-dion far more interesting now (and with the option in future of doing lightened shafts from GB or just cut down shafts to fit escort track + rear disks perhaps)

I believe in one wheel bump de-dion is 9kg better off than an escort axle & 15kg better off in two wheel bump + with the total sprung de-dion weight (brackets / diff etc) being higher than the escort axle by just under 25kg that will also improve handling (more sprung weight for the unsprung to move).

Right, just need to get the drive shafts and trailing arm mounts made now