strugrat
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| posted on 23/6/04 at 10:27 AM |
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De Dion Vs IRS
What are the advantages/ disadvantages to a de dion based car such as the GTS compared with say a IRS car such as the MK?
Had no luck with searching previous posts but this must have been discussed before?
Cant make up my mind and have had no solid answers.
Car will mainly be used on the road with occasional trackdays and hillclimbs/sprints and will be bike engined.
Any help great
Cheers
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ned
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| posted on 23/6/04 at 10:34 AM |
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This has been a long standing debate.
To try and give a brief definition:
Dedion will be lighter than IRS
IRS if set up properly should ultimately giv better handling on a circuit
Dedion should give better startline/straightline traction
I *think irs has less unsprung weight
Dedion is a simple mod to a book chassis
IRS should give better ride
Dedion is cheaper and easier to obtain, install and setup, less variables invovled.
If you are sure the geometry is accurate and yo uare willing to spend time setting it up go for irs. for ease, speed and cost go for dedion.
i personally think the differences in performance will be unoticable on the road and very negligable on the track.
it's all down to personal taste/opinion.
Ned.
[Edited on 23/6/04 by ned]
beware, I've got yellow skin
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stressy
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| posted on 23/6/04 at 10:47 AM |
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ifyou gonna hill climb and track, and stay below national speed limits on the road then go for minimum weight, Live axle!!!
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ned
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| posted on 23/6/04 at 11:02 AM |
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i would suggest dedion over a live axle for numerous reasons.. but that's a different arguement!
Ned.
beware, I've got yellow skin
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strugrat
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| posted on 23/6/04 at 11:08 AM |
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Its all too much!!! Too many choices!!!!
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ned
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| posted on 23/6/04 at 11:14 AM |
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live axle will be the lightest, but the worst in handling terms.
beware, I've got yellow skin
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craig1410
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| posted on 23/6/04 at 12:06 PM |
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For me the crucial thing will be whether or not you have the knowledge and/or inclination to set up your IRS correctly. I've gone with De-dion
because it gives 99% of the potential performance of IRS straight out of the box whereas IRS will require considerable fettling on a 4-wheel alignment
jig to reach even 90% of its potential.
The other crucial thing is that IRS will take you much longer to build than an equivalent de-dion chassis if you are planning a DIY build. If
you're going to buy from MK or GTS then this is clearly not a concern.
I went for de-dion myself and am more than happy with my choice. There are many good debates on the forum on this topic if you do a few searches.
Cheers,
Craig.
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Bob C
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| posted on 23/6/04 at 12:22 PM |
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Interesting, I was happier making an diy IRS than I would have been making a diy de-dion axle - maybe it's because I used a donor with wishbone
IRS (mx5) so the uprights are all ready made. I agree the book locost design lends itself more towards de-dion, though I reckon (hope) the proper IRS
will prove a worthwhile improvement on the rough roads round here!
Bob C
PS why should de-dion have better traction? 'cos of the fixed camber?
PPS I don't forsee setting up being a problem, adjust camber & toe then drive off into the sunset!!
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stephen_gusterson
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| posted on 23/6/04 at 12:38 PM |
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if you are not gonna race it, track day it, or are gonna risk life and limb by cornering at di^&head speeds on public roads, then a live axle is
all you need......
find someone that says a live axle locost handles like crap...
atb
steve
[Edited on 23/6/04 by stephen_gusterson]
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craig1410
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| posted on 23/6/04 at 12:44 PM |
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Steve,
Agreed in principle but as we all know, donor axles are getting very hard to come by these days and LSD options are limited (pardon the pun). Using
Sierra donor parts is very attractive hence the need for either de-dion or IRS.
Cheers,
Craig.
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ned
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| posted on 23/6/04 at 12:58 PM |
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quote:
Originally posted by stephen_gusterson
find someone that says a live axle locost handles like crap...
Steve,
Didn't you build your own IRS?
Bob,
Yes I believe its due to the fact that there is no camber change under rear suspension loading that you might get under acceleration.
IRS gives camber change under roll and loading of the suspension, the way round this, yet another complication in the setup of IRS is to add an ARB.
Yet another variable to try and sort out!
I would agree that in buying a gts or mk these arguements are of little importance as the cars should be jigged when built, but are more important for
home builders as craig and bob suggest.
Ned.
beware, I've got yellow skin
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PeterGT4
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| posted on 23/6/04 at 06:26 PM |
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A live axle or a DeDion would handle pretty much the same since they both locate the rear wheels in the same place (perpendicularly to the road).
DeDion would be a bit better since slight camber and toe-in can be manufactured into it to help handling further (almost like IRS). And of course,
DeDion is lighter since the diff is located on the car itself - faster response in suspension movement.
DeDion is the system I'll use for my Locost.
As for IRS, my experience with it is my Ferrari 308 and all I'll say, is it can be very, very tricky at the limit. At least with live
axle/DeDion, the limit window is so wide open, you'd have to be blind not to see it coming. With the Ferrari, it handles incredibly, but any
little bit over the limit and its unrecoverable... I've spun the car once and it scared the s--- out of me.
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greggors84
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| posted on 23/6/04 at 06:33 PM |
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Wouldnt IRS be lighter than DeDion, as you only have the diff and wishbones and not the big de dion tube?
Chris
The Magnificent 7!
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PeterGT4
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| posted on 23/6/04 at 06:41 PM |
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In order of lightest to heaviest (both in respect to unsprung and overall weight):
-IRS
-DeDion
-Live axle
Keep in mind, that although IRS is a lighter system, there'll be extra tubes welded to the chassis to locate the suspension pick-up points. Yes,
lighter in unsprung weight, but only slightly lighter overall because of the extra chassis tubing.
DeDion can be adapted to the book chassis with really, no extra pieces/tubing. Only brackets to carry the diff.
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pbura
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| posted on 23/6/04 at 07:27 PM |
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I like de Dion best overall, but am planning IRS in order to have inboard rear shocks (based on cheap bike shocks). Can't think of a inboard
layout for de Dion that doesn't involve interference with the bodywork.
This is a wingnut sort of thing, but just thought I'd throw it into the mix.
Pete
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stressy
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| posted on 23/6/04 at 07:45 PM |
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repeating a previous post......
live axle vs dedion vs irs
Having considered this in the past i came up with the following which you may find helpful. The numbers are from my own measurements and calculations
plus other peoples supplied info, so accuracy is not 100%.
live axle
axle casing inc brackets, standard diff, trailing and lateral links, coilovers, prop, brakes.
unsprung 56kg / sprung 7kg / total 63kg
dedion
std diff, trailing links, coilovers,lateral linkage, disc brakes and calipers,driveshafts and joints (narrow), propshaft, dedion tube an alloy ears
unsprung 37kg / sprung 37kg / total total 74kg
IRS
std diff, driveshafts, hubs/uprights(alloy), wisbones (std bushes), brakes, coilovers, propshaft, additional chassis work
unsprung 37kg / spung 46kg / total 83kg
Hopefully this is of some help.
The hanging masses such as shox etc are assumed 50% sprung and unsprung.
It will be worth noting that the live axle has drums not discs so could be deemed as not like for like but i didnt have the opportunity to get data
for drums from a seirra for comparison.
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PeterGT4
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| posted on 24/6/04 at 06:33 PM |
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quote:
Originally posted by stressy
live axle vs dedion vs irs
live axle.... total 63kg
dedion.... total 74kg
IRS.... total 83kg....
I'm very surprised by these numbers, in fact, I'm quite puzzled. How can IRS be the heaviest? Both live axle and DeDion use large steel
tubes and IRS use only a few small, short tubes and a few more brackets. What parts are you counting in your sprung weight in each example?
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crbrlfrost
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| posted on 24/6/04 at 08:05 PM |
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Actually, that is quite correct, the IRS will be the heaviest (absolutely), followed by the DeDion, and then the live axle. What is probably confusing
is the fact that the unsprung weight will be the opposite of that (ie live axle heaviest, then DeDion, then IRS). Personally I think its all a
compromise and the best set up will depend on what you do and where you do it. If the roads are a bugger around your area, than the live axle is going
to give the worst ride due to the high unsprung to sprung weight and the wheel on one side effecting the camber of the other. However, it has been
proven on smooth race tracks that a live axle can be every bit as good as an IRS. DeDion is a nice compromise with the simple live axle set-up and
less unsprung. If te roads are truly terrible though, it will suffer from the same camber issues. I think any set up will put grins on your face, but
its ultimate suitabilty will depend on your situation. Cheers!
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PeterGT4
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| posted on 25/6/04 at 07:20 PM |
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Fair enough, lets put aside the qualities as I understand the handling and feeling issues, but what I still do not grasp is the weight issue. I know
which parts of the suspension are unsprung, but what counts as sprung? I always thought everything above the suspension components (ie:
half of the shocks, mounting brackets and then the chassis, body, drivetrain, etc...) are sprung. What I'm reading here is that there are
specific suspension components that are separate of the overall, sprung weight of the car (that counts as its own sprung weight)? What components are
these?
Again, I'm not debating which is better, just trying to figure out why one system is lighter than another.
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britishtrident
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| posted on 25/6/04 at 08:12 PM |
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It isn't just a question of which is lighter in weight (sprung or unsprung) but how the mass\weight is distributed.
In overall weight sprung or unsprung a good de Dion and an good IRS will be be very close. The de Dion has the disadvantage that the tube not only
couples the wheels but puts some of the unsprung mass in the wrong place (ie the middle of the axle), both of these reduce the wheels ability to stay
in contact with the road without transmitting vertical motion to the body and disturbing the other wheel.
With a de Dion it is imperative that the actual de Dion tube be made as light as possible.
[Edited on 25/6/04 by britishtrident]
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craig1410
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| posted on 25/6/04 at 09:17 PM |
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PeterGT4,
If it helps, here are some weights from my de-dion design. Note that this probably isn't the lightest ever de-dion assembly but it does seem
very strong (ie. Over-engineered IMHO)
Bare De-dion axle (MK Engineering design) = 11Kg
Sierra rear driveshafts (Lobro) with hubs and carriers = 24Kg's
Sierra Differential (Lobro type) = 27.5Kg
GTS rear shocker unit = 2Kg each
The above weights were all taken by standing on bathroom scales holding onto the various components and subtracting my own weight. Accuracy is
probably around +/- 2 or 3 %.
My de-dion axle tube is made from 50mm diameter ERW with a wall thickness of 2.5mm IIRC. You can see quite detailed drawings of it on my website in
the de-dion section.
My own view on the weight differences between IRS and de-dion come from various quarters:
1. IRS requires extra chassis construction for the mountings for rear wishbone pickup points.
2. The uprights are probably heavier than the equivalent de-dion hub carriers and hubs.
3. The wishbones and associated rod-ends/bushes are probably heavier than a well designed de-dion axle although the de-dion does require trailing arms
and a panhard rod.
In any event, the majority of the weight (from my figures above) comes from the massively over-engineered (for Locost applications) differential.
Wouldn't it be nice if someone produced a nice cheap aftermarket diff specifically for lightweight RWD cars like the Locost? Also, a nice set of
lightweight driveshafts and hubs would round it off. You could probably lose 30Kg's without compromising reliability if it was good quality
stuff...
Cheers,
Craig.
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stressy
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| posted on 25/6/04 at 09:25 PM |
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Weight explanation
In the weights example i was looking at like for like installation of the suspension systems on a locost within a reasonable development budget.
For the purpose of the calcs i know you guys recognise sprung, 1/2 sprung and unsprung type idess so that is how the values are presented. i.e
suspension arms and shock are 1/2 sprung and 1/2 unsprung etc.
I actually consider unsprung as anything which is anything which moves with the suspension, i.e elments which support the sprung mass, i.e. susension
arms, axles, brakes, halfshafts, rocker links antiroll bars and propshft in the case of a live axle.
Hopefully this makes sense.
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stressy
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| posted on 25/6/04 at 09:32 PM |
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interesting craig
The figures you have add upto 64.5kg, add 3 kg for prop, 1.5 for nuts n bolts, 6kg for caliper and 6 for discs and you end up with 81kg. Heavier than
my value but that was using a caterham dedion tube which is around 7.5kg, clealy a well developed design over the years with alot of supporting test
data.
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JoelP
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| posted on 25/6/04 at 09:33 PM |
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i hate to split hairs, and i know is almost irrelevant (actually relevant but completely unimportant) but the shocker unit isnt exactly half sprung,
the spring is half sprung but the shock absorber in the middle can be divided into which parts move with the car and which with the wheel.
in fact thats such a crappy point i feel bad mentioning it. I apollo-guise in advance.
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craig1410
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| posted on 25/6/04 at 09:41 PM |
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Stressy,
Yes that's what I would expect. As I said, my de-dion kit is over engineered in my opinion.
Not that it would make a massive difference but my chassis is +4" wider than book and my driveshafts are full width which would make it 2 or 3
Kg's heavier than a standard width chassis with cut-down shafts.
So doesn't the Caterham use alloy calipers and custom driveshafts and perhaps even a custom diff? Surely it doesn't just use old Sierra
bits like me??
Cheers,
Craig.
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