Tudor (Ted) Miron
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| posted on 23/8/03 at 09:49 AM |
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IRS vs Dedion
Hello All,
I'd like to ask what is difference between
Dedion and proper IRS on track (bumpy track?)? Did anyone make any sort of real comparition? Head to head track test? (assuming all rest is the same)
Thank you
Ted
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craig1410
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| posted on 23/8/03 at 11:02 PM |
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Ah, I replied to the post on the chassis forum... I guess this is the correct place though so I'll go and get it and post it here. I suggest you
delete the duplicate post on the other forum before ChrisW(Webmaster) spots it... 
Cheers,
Craig.
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craig1410
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| posted on 23/8/03 at 11:03 PM |
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(Moved from Chassis forum)
I'm a de-dion advocate but I would expect (theoretically only) that IRS would be preferable to de-dion on a bumpy circuit.
With de-dion the bumps would affect both wheel's geometry even if only one wheel hit the bump. This would be particularly damaging to lateral
grip if the inside wheel hit a bump mid-corner and caused the outside wheel to gain positive camber and hence lose grip. With IRS this shouldn't
happen unless you had a very tight anti-roll bar in place.
On a smooth circuit and good quality main roads I would expect de-dion to give better traction on acceleration (both off the line and out of corners)
as wheel geometry is not affected by squat when the power is turned on as it is with IRS.
I guess it comes down to how bumpy your road or track is likely to be but if it's that bumpy then maybe you need an off-road vehicle
instead...
Hope this helps,
Craig.
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Stu16v
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| posted on 24/8/03 at 01:36 AM |
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I've posted in Chassis forum. How the hell did you move yours Craig?
P.s. It was before I read this. How spooky........
:edit:
It would take a very experienced driver to make any significant differences to lap times with a well set up dedion car versus a well set up IRS.
I personally favour dedion (but I have a car with it, so I would...... ), but it suits my car and what I want from it. I personally think dedion is
the best way to get A LOT of power to the road. With a good, well designed beam, traction is truly amazing. But IRS is very good on bumpy surfaces,
especially around corners (it's surprising how bumpy some race tracks are at high speed....), not that there is a problem with dedion, but a
tidy IRS probably has the edge. As for weight, live axle is your best bet. Good handling can be got from these as well.....  
Hurrah, I did it!
Thanks Craig....
[Edited on 25/8/03 by Stu16v]
Dont just build it.....make it!
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Metal Hippy
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| posted on 24/8/03 at 01:37 AM |
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Cut and paste in this topic, delete the old one?
Cock off or cock on. You choose.
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pbura
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| posted on 24/8/03 at 02:05 AM |
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Some interesting remarks from Clive Roberts of Caterham on why they went with DD:
http://7faq.com/owbase/ow.asp?deDionOrIRS
This backs up remarks by Craig and Stu (assuming Stu's post survives the move ).
Pete
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Tudor (Ted) Miron
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| posted on 24/8/03 at 08:30 AM |
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Thanks for your opinions! Special thanks to Pbura - great link.
Ted
PS: Sorry for posting it in many forums
just wanted more replys/opinions
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Stu16v
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| posted on 24/8/03 at 11:30 PM |
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quote:
Cut and paste in this topic, delete the old one?
Tryed that, but either my (steam powered) computer, or the 'system' wouldnt let me do it. I guess it's my computer then....
Dont just build it.....make it!
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craig1410
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| posted on 25/8/03 at 11:09 AM |
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Stu,
Go to your old post, press the edit post button and then click in the box with the post in it. Press Ctrl-A to select all and then Ctrl-C for copy.
Then just tick the delete post button at the bottom and click the edit post button at the bottom of the form.
Now go straight to the other forum and create a new reply, click in the text box and press Ctrl-P to paste the text in. Now just post the reply.
Simple...
Cheers,
Craig.
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Tudor (Ted) Miron
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| posted on 29/8/03 at 02:05 PM |
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Hi All,
Just found a good answer to this little discussion:
that was quoting the racing
driver Phil Hill. Phil was driving a factory GTO. He was asked about
the "old fashioned" 5-link, solid axle rear suspension under the very
successful GTO, including some less than wonderfully smooth courses
like Mille Mille (I know that is not the correct spelling). I remember
his answer was something like: "A truly excellent independent
suspension can be better than the best a solid axle can be. But the
solid axle 5-link suspension is much easier to tune to a very high
state of excellence, and a very good solid axle is better than a
mediocre independent any day."
Cheers
Ted
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craig1410
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| posted on 29/8/03 at 07:02 PM |
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Yep, I agree with that 100% and is one of the deciding factors why I went with De-dion. Maybe for future builds (gulp!) when possibly I know what
I'm doing, I may build an IRS chassis but more likely I'll just buy a good IRS design and build from there. Now that I've (almost)
built a chassis from the ground up I don't see the point in doing it again.
Cheers,
Craig.
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pbura
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| posted on 30/8/03 at 02:48 AM |
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I started looking at the de Dion because my donor car is IRS and I wanted to (A) use my parts, and (B) keep the design work simple.
If I had a donor with a solid axle, I would use that, but I'll be happy to have a de Dion because of the low unsprung weight and good camber
control.
A couple of good links for seeing how a de Dion is constructed:
http://au.geocities.com/phil1rowe4/rear.html
http://www.newtier.com/graber/mid-engine/Images/Rear_Drive_Cradle/index.html
Best of luck,
Pete
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jcduroc
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| posted on 30/8/03 at 09:32 PM |
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quote:
Originally posted by pbura
Some interesting remarks from Clive Roberts of Caterham on why they went with DD:
http://7faq.com/owbase/ow.asp?deDionOrIRS
quote:
from the link above
Neither Reg Price nor I felt that we had the knowledge or resources to design and develop a really good IRS system, considering the stages of concept,
design, development and validation (including durability testing). In the early 80's, without today's PCs and modelling software, it
needed either a vast amount of experience to lay down the right geometry from scratch, or a lot of time (and money) to get there by trial and error
testing. ....
We didn't feel confident of getting it all right with little time or money, so went for the system that gave us a head start.
...
Pete
Astonishing!...
So how did some of us designed IRS's back in the 70ies without any PC's at all? (by hand and sliding rule of course).
A DeDion axle is either a well thought option or a mistake; a live axle is much simpler an IRS is much better (the thesis of dD wheel perpendicularity
is crap...).
I still can not see the complexity of IRS; it's a mere 2 wishbone setup with a different upright/hub carrier. Is this much more complex than
making a huge tube that holds hub carriers together?
I guess I'm being provocative, sorry.
Cheers
Joćo Matoso
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Stu16v
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| posted on 30/8/03 at 10:34 PM |
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Nothing wrong with being provocative...it promotes discussion.
The design/building and subsequent setting up of IRS is more compicated than a live axle system.
With a live axle, all you have to do is make/fit parallel trailing arms, and a panhard rod that is parallel when the car is at the desired ride
height. Subject to the right spring/shocker choice, you will have something that is pretty much guarenteed to work, and work quite well too.
The dedion is merely exploiting the benefits of the live axle, whilst losing some of the downfalls, i.e. unsprung weight (the biggie) and the ability
to play with the wheel geometry (even if it is only in the manufacturing stage).
With double wishbone IRS there is so many variables that it is possible to end up with a strange handling car if the wrong choices are made. All of a
sudden the designer has an real influence on how the suspension is going to perform, which can be good or bad.
I was originally planning to convert my Westy from live axle to IRS, but I am so impressed with the dedion on my Locost that it looks like my other
toy will be going the same way.
Dont just build it.....make it!
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JoelP
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| posted on 30/8/03 at 10:52 PM |
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If i were to start again i'd do it dD, does seem easier to put together than IRS.
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jcduroc
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| posted on 30/8/03 at 10:54 PM |
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Sure it is
quote:
Originally posted by Stu16v
The design/building and subsequent setting up of IRS is more complicated than a live axle system.
With a live axle, all you have to do is make/fit parallel trailing arms, and a panhard rod that is parallel when the car is at the desired ride
height. Subject to the right spring/shocker choice, you will have something that is pretty much guarenteed to work, and work quite well too.
The dedion is merely exploiting the benefits of the live axle, whilst losing some of the downfalls, i.e. unsprung weight (the biggie) and the ability
to play with the wheel geometry (even if it is only in the manufacturing stage).
I completely agree with you. But that is (at leasa as far as I am concerned) the beauty of the DIY car building...
quote:
Originally posted by Stu16v
With double wishbone IRS there is so many variables that it is possible to end up with a strange handling car if the wrong choices are made. All of a
sudden the designer has an real influence on how the suspension is going to perform, which can be good or bad.
Cheers
Joćo Matoso
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pbura
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| posted on 31/8/03 at 12:11 AM |
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quote:
the thesis of dD wheel perpendicularity is crap...
Not sure I understand your meaning here, Joćo. Do you mean that the wheels are not perpendicular to the road? Or that it doesn't matter?
Would appreciate your explaining your reasoning.
With computers and CAD, designing an IRS is not as difficult as it used to be, even for the home builder. Whether to do so or not boils down to what
your priories are, how much you care to spend, and what you're willing to screw around with. You pay your money and you take your chances
Thoroughly agreed with your remark about the beauty of DIY car building.
I would also agree that a new car manufacturer would have to be nuts to design a de Dion for mass production. Expensive parts being used for a humble
purpose...a little more Locost magic!
Pete
[Edited on 31/8/03 by pbura]
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leto
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| posted on 31/8/03 at 06:19 AM |
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quote:
Originally posted by pbura
.............
I would also agree that a new car manufacturer would have to be nuts to design a de Dion for mass production. Expensive parts being used for a humble
purpose...a little more Locost magic there!
Pete
Speking of humble purpose...
At least some Volvo 340/360 have a deDion. Unfortunately the gearbox is built into the diff and to big to fit in a locos.
Cheers Leif
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craig1410
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| posted on 31/8/03 at 10:33 AM |
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Yes IRS is the mass produced way to go but in Locost building we have a different set of problems and the lightness of the chassis makes unsprung
weight much more of an issue than it would normally be on a 1.5 ton family saloon. You should look at unsprung weight in terms of the percentage of
total vehicle weight to get a good comparison. 100% unsprung = no suspension which would be very skittish indeed. 0% unsprung is impossible obviously
but would be nice if it could be achieved. Basically you need minimum unsprung weight without sacrificing strength in the wheels/hubs/brakes etc.
Live axle is generally lighter overall but has higher "unsprung" weight than de-dion. IRS is generally the heaviest overall but should
have slightly lower unsprung weight again than de-dion.
Probably the biggest reason I can see for going the de-dion route is the lack of live axle donor's available today compared with 5 or 10 years
ago and the fact that ratios and LSD options are more limited. In another 10 years the Sierra will be thin on the ground too as a donor so another
"Locost" solution will probably be required. For today though, you can use a Sierra diff, shafts and hubs with a de-dion axle and still
enjoy the simplicity, quick build time and ease of setup of a 5 link suspension system.
The chances of building an IRS system from scratch and have it perform as good as a bog standard live or de-dion system is virtually nil in my opinion
as there are so many parameters which can fundamentally affect handling. This is exactly what you want if you have the experience, knowledge and
equipment to set IRS up correctly but for most of us on this forum this is simply not the case.
Anyway, 'nuff said from me.
Cheers,
Craig.
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sgraber
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| posted on 31/8/03 at 03:28 PM |
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I've been reading this thread with interest as my deDion equipped middy is within weeks of driving under it's own power.
I made the decision to go with deDion over IRS almost 2 years ago because of the 'idiot-proof-ness' (is that a word?!) in designing one.
I read Phil Hills quote at that time and I was sold.
It's important for first timers and newbies to note that vehicle suspensions have to be designed as a'System'. The front is not
seperate from the rear as far as vehicle dynamics is concerned. If you make a fantastic front design but screw up the rear with too low of an RC and
something that squats terribly under acceleration, then your car as a whole will be an ill handling pig and you will hate it.,.
That said - If you decide that you have the skills and determination to build an IRS then an IRS design that I think looks very simple to adapt to a
Lowcost chassis' is Alan B's Meerkat IRS. Standard trailing arms (using rose joints) and 3 more links on each side. For you front engine
builders look at the attached image, remove the engine from your minds put a differential in it's place and just imagine the rear of the diff
structure being beefed up to attach the brackets. and the inboard links getting located further forward, where the rear of your differential would
be.
(edited later...) Alan - I hope you don't mind that I posted this image. I realize that I should have asked for your permission, since it is
your intellectual property. My apologies if this upset you...
[Edited on 8/31/03 by sgraber]
Steve Graber
http://www.grabercars.com/
"Quickness through lightness"
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craig1410
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| posted on 31/8/03 at 07:23 PM |
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Neat design, certainly very different from anything else I've seen. I guess the unconventional arrangement of the "wishbones" is to
increase the space available in the engine bay. I wonder whether the design is as effective as a conventional IRS design with double wishbones? I
expect the geometry will take a bit of studying to figure out what to adjust to get the desired results.
Cheers,
Craig.
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Alan B
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| posted on 1/9/03 at 01:19 AM |
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Craig:
Thanks.
I'll let everyone know how it works in due course..
Steve:
No problem. It's already on my site
anyway.
More finished picture.....
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Alan B
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| posted on 1/9/03 at 01:23 AM |
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another view:
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jcduroc
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| posted on 1/9/03 at 03:53 PM |
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quote:
Originally posted by pbura
quote:
the thesis of dD wheel perpendicularity is crap...
Not sure I understand your meaning here, Joćo. Do you mean that the wheels are not perpendicular to the road? Or that it doesn't matter?
Would appreciate your explaining your reasoning.
[Edited on 31/8/03 by pbura]
Hi Pete
Sorry for the delay in answering but yesterday was a very busy Sunday being my eldest grandaughter's 4th anniversary.
I guess I must clarify my statement. 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!...
Cheers
Joćo Matoso
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Noodle
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| posted on 1/9/03 at 07:05 PM |
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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.
Cheers,
Mr. Pedant.
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