De Dion rear end design..

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Re: De Dion rear end design..

Post by Truckedup » Mon Jun 11, 2018 12:15 pm

englertracing wrote:
Fri Sep 01, 2017 12:13 am
The testing was done in the 60s I believe.
Also regarding the full independent....
Usually the trophy trucks and truggys finish the Baja 1k before the irs buggys :D
High power off roaders prefer a long travel solid rear axle
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire

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Re: De Dion rear end design..

Post by Nwguy » Wed Jun 13, 2018 7:11 pm

Live axles and anti squat... Baja racers need forward drive. Back to De Dion axles, Chrysler AWD minivans. Shocked me to see that set up in a grocery getter.

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Re: De Dion rear end design..

Post by BillyShope » Wed Jun 27, 2018 8:49 am

Brian P wrote:
Fri Jan 27, 2017 10:18 pm

A traditional "live" axle can be set up to have a lot of anti-squat on acceleration. Neither IRS nor de Dion can do this, because the differential torque reaction goes right into the chassis without going through the suspension links, so there's no advantage to de Dion in that respect, either.


Fortunately, this is a very common...but very false...statement. Consider the IRS. The 100% antisquat line (or nosquat/norise line or whatever you want to call it) has the same CG height/wheelbase slope as the live axle. The difference is that it passes through the ring gear center instead of the rear tire patch. With the sort of link geometry associated with 4links, it is next to impossible to get the car to do anything but squat on launch. This is when you go back to something taught you in high school: Parallel lines meet at infinity. With this knowledge, all that is needed are parallel link lines with a slope greater than CG height/wheelbase for rise and less than the same ratio for squat. So, you can have almost any amount of squat...OR RISE...that you want.
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Re: De Dion rear end design..

Post by Brian P » Wed Jun 27, 2018 12:27 pm

Yes, I'm aware of that. The downside, if you do that with a de Dion axle, is that if you want appreciable antisquat (via trailing links that slope down towards the rear or leaf springs that are substantially sloped), you will get lots of roll oversteer because the end of the axle on the outside of the corner gets pushed back as it compresses and the end of the axle on the inside of the corner gets pulled forward as it unloads. Bump-steer on one-wheel bumps will probably be pretty noticeable.

Multi-link IRS can have such sloped-downward trailing links or inclined A-arm pivots without necessarily introducing toe or bump-steer effects. They can even have a little roll understeer together with such geometry.

The Alfasud rear suspension, and for that matter the smart fortwo rear suspension, has the central pivot at approximately the same height as the lateral locating linkage (and for that matter, little different from the wheel centerline height) in order to have benign bump-steer and roll-steer characteristics. The Alfasud was known for having pretty good handling characteristics in its day. The smart, not so much, but that's for other reasons. No appreciable antisquat on either one ...

Multilink IRS allows the bump / roll steer characteristics to be separated from the anti-squat effects and the compliance-steer effects (rearward sharp-edge bump impact can be compliantly absorbed with little "steering" effect from changes in toe). You just can't do that as well with beam axle or twist-beam designs.

For what it's worth, most modern IRS designs in rear drive cars with IRS, don't have much anti-squat. (It's possible to build it into the design, but it just isn't done.) The trailing arm pivot is usually quite close to the same height as the wheel center-line height when it is at nominal ride height.

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Re: De Dion rear end design..

Post by BillyShope » Wed Jun 27, 2018 4:50 pm

First, note that my correction was addressed to your comments regarding the IRS. With that in mind, I was merely pointing out that you were incorrect. I have helped dragracers with their problems with an IRS. They have accomplished the rise they desired with parallel links. And that, after all, is all that's important.
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Re: De Dion rear end design..

Post by BillyShope » Wed Jun 27, 2018 4:51 pm

My correct URL is now shopeshop.org
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Re: De Dion rear end design..

Post by Brian P » Wed Jun 27, 2018 5:31 pm

Ah, gotcha, I was thinking mostly about de Dion applications because that's what this thread is about. Although possible to have antisquat with IRS as described, in practice I haven't seen it done at an OEM level to any appreciable degree ... probably because the chassis end of the trailing links would end up above the floorpan and intruding on the interior. Perhaps with enough digging, someone could find an OEM example of IRS with significant anti-squat. I can't think of one.

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Re: De Dion rear end design..

Post by BillyShope » Thu Jun 28, 2018 6:20 am

Sorry, Brian. It's been many years since I worked in Detroit and I couldn't care less about what they're doing to limit squat with a De Dion or IRS. Apparently, they also couldn't care less.
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Re: De Dion rear end design..

Post by Ratu » Mon Aug 27, 2018 5:43 am

The challenge, "Perhaps with enough digging, someone could find an OEM example of IRS with significant anti-squat."

Took a bit of time to find anything, but what about Jaguar E-type, XJ Series 1,2 & 3 and also XJS? They appear to have anti-squat and they are IRS. Jaguar certainly claimed they had anti-squat in their promotional literature. Seems likely the later XJ40, XK8 and later cars (up to X308) had anti-squat as well. All IRS designs with fixed length drive shafts acting as top transverse link.

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Re: De Dion rear end design..

Post by BillyShope » Mon Aug 27, 2018 7:10 am

Appreciate the information. No magic here. All they have to do is get that instant center on the elevated 100% antisquat line. The easiest way is parallel links, but there are probably many other ways.
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Re: De Dion rear end design..

Post by Ratu » Tue Aug 28, 2018 3:44 am

Hi Billy

Looks to me that what Jaguar (was it their design engineer Harry Mundy?) did in the XJ series 1, 2, 3, XJS and XK8 (looks like some of the Astons used these components as well) was to arrange the axis of the lower wishbone pivots to slope upwards. The trailing arm seems not to be a critical element in terms of anti-squat. The half shaft, in terms of geometric restraint, likely isn't either. The later cars do something quite similar also. Their IRS appears to have been designed by Bob Randall.

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Re: De Dion rear end design..

Post by Ratu » Tue Aug 28, 2018 3:51 am

Speaking of de Dion rear suspension, I found some interesting correspondence in the last few issues of Racecar Engineering magazine. It is between Mark Ortiz (who writes "The Consultant" columns) and a reader. They are discussing means of revising the rear suspension of the Lagonda. A twist beam de Dion is discussed in the July 2018 issue. Interesting read.

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Re: De Dion rear end design..

Post by BillyShope » Tue Aug 28, 2018 7:44 am

Funny, but not so funny. That magazine still owes me $650 for an article. I misplaced the check and didn't find it until years later. I think the magazine had been sold 2 or 3 times in the interval. The article was in the June 2007 issue, if you collect them. Moral: I should cash checks immediately.
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Re: De Dion rear end design..

Post by clshore » Tue Aug 28, 2018 9:21 am

Ratu wrote:
Tue Aug 28, 2018 3:51 am
Speaking of de Dion rear suspension, I found some interesting correspondence in the last few issues of Racecar Engineering magazine. It is between Mark Ortiz (who writes "The Consultant" columns) and a reader. They are discussing means of revising the rear suspension of the Lagonda. A twist beam de Dion is discussed in the July 2018 issue. Interesting read.
I'll have to track that article down.
I had investigated a twist beam DeDion, where the beam was torsionally compliant, but stiffer in bending to handle camber and toe.

Thanks for the heads up.

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Re: De Dion rear end design..

Post by Brian P » Tue Aug 28, 2018 6:14 pm

Twist-beam deDion implies a twist-beam rear suspension design together with a differential fixed to the chassis and driving the wheels through CV-jointed shafts.

Have a look under the rear of a current Honda HR-V with all wheel drive ...

I know, it's hardly a sports car, and I know, it's basically a front wheel drive vehicle with a cosmetic rear-drive system that does just enough so that they can call it all wheel drive, but still ... it's a twist-beam rear suspension design together with a differential fixed to the chassis and driving the wheels through CV-jointed shafts ...

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