kreb
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| posted on 24/10/05 at 03:48 PM |
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Dedion vs. live axle unsprung weights
How much unsprung weight do you really save going Dedion over a live axle? My racing guru calls Dedion "the answer to the question that nobody
asked".
Specifically, I'm comparing the merits of converting a Miata IRS (I think it's called MX-5 in europe) to Dedion, versus using a 1st
generation RX7 live axle. I expect the car to get a lot of track time, so I want a relatively sure thing handling wise versus the difficulty of
sorting out IRS.
https://www.supercars.net/blog/wp-content/uploads/2016/04/1966_FiatAbarth_1000SP1.jpg
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JonBowden
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| posted on 24/10/05 at 04:09 PM |
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A DeDion is basically a live axle without the mass of the diff in the middle. This isn't quite right, since the propshaft willnot contribute to
the unsprung mass and the drive shafts contribution to the unsprung mass will be different.
Jon
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gutball
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| posted on 24/10/05 at 04:24 PM |
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Quite unscientific, but I imagine the numbers will add up somewhat like this...
Live axle:
whole axle (inc diff, tube, and both drive shafts)
+
half propshaft
+
half links (trailing, panhard, etc depending on setup)
De-dion:
de-dion tube
+
half drive shafts
+
half links
That leaves a difference of the diff, one drive shaft and half the prop.
[Edited on 24/10/05 by gutball]
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gazza285
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| posted on 24/10/05 at 05:02 PM |
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quote:
Originally posted by gutball
Quite unscientific, but I imagine the numbers will add up somewhat like this...
Live axle:
whole axle (inc diff, tube, and both drive shafts)
+
half propshaft
+
half links (trailing, panhard, etc depending on setup)
De-dion:
de-dion tube
+
half drive shafts
+
half links
That leaves a difference of the diff, one drive shaft and half the prop.
Don't forget to add on the weight of a CV joint to the Dedion setup, and the weight of a much larger wheel bearing.
DO NOT PUT ON KNOB OR BOLLOCKS!
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MikeR
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| posted on 24/10/05 at 05:33 PM |
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i've often wondered this ..........
it always looks to me that the carrier wheel bearings / brakes for the de-dion is huge. Add to this that if you're going over a single axle bump
only half the live axle moves ......
love to see some proper figures.....
could someone with a de-dion weigh all the unsprung parts? (ignore trailing links / panhard rod / drive shaft)
if we get someone with a live axle to do the same (ignore the same bits as well) it would be an interesting exercise.
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britishtrident
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| posted on 24/10/05 at 06:04 PM |
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It no just the large saving in unsprung weight that give the de Dion an advantage other considerations come into it --- torque reactions, coupling
effects on single wheel bumps.
A live axle can give very good road holding on very smooth surfaces but as soon as single wheel bumps are encountered IRS or de Dion will keep better
contact with the road.
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gazza285
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| posted on 24/10/05 at 06:26 PM |
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One of the troubles with Dedion is that while unsprung weight is lower than a live axle the unsprung weight is mainly concentrated on the outside of
the axle, which can lead to the rear end being a little unsettled by larger bumps. With the extra overall weight and complication that comes with a
Dedion I'd rather have a live axle.
DO NOT PUT ON KNOB OR BOLLOCKS!
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britishtrident
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| posted on 24/10/05 at 07:09 PM |
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The weight being concentrated at the ends is actually major advantage of the de Dion --- it reduces the transmision of upward motion from a single
wheel bump to the body/chassis and hence disturbance to the wheel that dosen't hit a bump --- all due to coupling effects the late George
Bishop wrote a very good article on it in "Car Design" magazine circa 1988/89 in which he covered the physics of a number of obscure
dynamic effects on suspension.
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mangogrooveworkshop
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| posted on 24/10/05 at 09:03 PM |
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It works and thats all I care. Every system has its drawbacks. Unless you have a F1 type budget I would stick with what has gone before....KISS
[img][/img]
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MikeR
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| posted on 24/10/05 at 10:20 PM |
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don't suppose you could weight that big yellow bit and teh drive shafts (separately) could you?
means when i take my english axle to bits i can do the same and we'll get some comparisons.
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NS Dev
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| posted on 25/10/05 at 12:17 AM |
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quote:
Originally posted by britishtrident
It no just the large saving in unsprung weight that give the de Dion an advantage other considerations come into it --- torque reactions, coupling
effects on single wheel bumps.
A live axle can give very good road holding on very smooth surfaces but as soon as single wheel bumps are encountered IRS or de Dion will keep better
contact with the road.
PERFECTLY PUT!!!
Ultimately you can probably build a bmc live axled car lighter than any other 7, and it will handle extremely well on a track, but for rough surface
use the de-dion is much better.
IRS is all well and good but live axles (or de-dion) have a number of advantages over it which I can't be bothered to get into!
Ultimately, the BIG problem for live axled sevens is getting the rear dampers to work. This depends on giving them a fighting chance by making the
ratio of sprung to unsprung weight over the rear of the car significant, which is not easy on something as rear end light as a 7.
Fact is that most 7's have such excessive spring and damper rates that you'll never notice the difference anyway!!
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kango
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| posted on 25/10/05 at 04:24 AM |
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Won't a trailing arm system be even lighter. You eliminate the pipe and panhard rod and can do a bit of tweeking.
Always have been conserned about the accuracy of a De Dion built at home. If a flange is welded on ever so slightly scew it is like that for ever.
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Fred W B
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| posted on 25/10/05 at 06:03 AM |
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quote:
Always have been conserned about the accuracy of a De Dion built at home. If a flange is welded on ever so slightly scew it is like that for ever.
I would have thought you could correct/adjust the toe and camber by using shims behind the hub plate when you bolt it on?
cheers
Fred WB
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CGILL
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| posted on 25/10/05 at 06:18 AM |
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A big advantage is that your trans tunnel can be smaller as the prop shaft & diff will no be moving, you can also run uneven axle lengths and have
the pinion center-line of the chassis to give more butt room. To make the most you'd use inboard brakes, this will make the axle lighter and
will also remove torque reaction on the axle when braking.
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NS Dev
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| posted on 25/10/05 at 10:25 AM |
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quote:
Originally posted by kango
Won't a trailing arm system be even lighter. You eliminate the pipe and panhard rod and can do a bit of tweeking.
Always have been conserned about the accuracy of a De Dion built at home. If a flange is welded on ever so slightly scew it is like that for ever.
Trailing arms are a compromise in a lot of ways.
Roll centre control is quite tricky, camber control in roll is very tricky if you want any anti-roll built in at all.
Arms also need to be rather heavily engineered because they concentrate the suspension loadings into a fairly confined area of the car.
My grasser uses trailing arms because they give massive wheel travel with very little camber change for huge startline grip, but roll control is not
good and the arms are heavy in order to be strong enough.
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Rorty
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| posted on 26/10/05 at 04:25 AM |
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quote:
Originally posted by kango
Won't a trailing arm system be even lighter. You eliminate the pipe and panhard rod and can do a bit of tweeking.
Always have been conserned about the accuracy of a De Dion built at home. If a flange is welded on ever so slightly scew it is like that for ever.
You could say the same for any part of the car.
If it's done correctly and carefully, then misalignment can be minimised. As Fred WB says, if you discover a large discrepancy, you can always
resort to shims to correct the alignment.
If you saw how much out of alignment some production cars are, you would be amazed!
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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kango
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| posted on 27/10/05 at 04:21 AM |
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How would you set up the jigging to build an accurate de Dion (at home)
With the main carrier at an angle all the way to the flanges, it also needs to be cut very accurately. Then welding on the flanges... then the short
little tubes...
Lost of places to build up mis allignment.
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Rorty
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| posted on 28/10/05 at 05:32 AM |
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quote:
Originally posted by kango
How would you set up the jigging to build an accurate de Dion (at home)
With the main carrier at an angle all the way to the flanges, it also needs to be cut very accurately. Then welding on the flanges... then the short
little tubes...
Lost of places to build up mis allignment.
Re: jigging a De Dion axle, see the 29th post on this thread.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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