De Dion rear end design..

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Ratu
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Re: De Dion rear end design..

Post by Ratu » Fri Aug 31, 2018 4:21 am

Brian

There is a Honda HRV not far from here. A neighbour's wife (I think) has it. I ought to go over & ask if I can have a look.

Was Honda's objective with the 4wd in that car to deliver benefits in low traction conditions? Was that the idea or was it for reducing understeer or addressing some other dynamic vice?

Brian P
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Re: De Dion rear end design..

Post by Brian P » Fri Aug 31, 2018 7:36 pm

I'm sure it helps a bit when driving through snow, perhaps you don't have to clear the driveway as well, and maybe if you have to go down the street before the plow comes, perhaps you have a better chance of making it to the end. And, the marketing department gets to say "available all wheel drive" and the manufacturer gets to charge a bit more for the option.

It's not for rock crawling or serious off-roading, that's for sure. The rear drivetrain on most of these basically-front-drive vehicles isn't heavy-duty at all, and the ground clearance of a Honda HR-V is not really any better than the Honda Fit/Jazz that it started out as.

The Fit/Jazz, and the front-drive HR-V which is the same thing underneath, has a twist-beam rear suspension, just like many other front-drive vehicles. I'm sure when the HR-V was designed, they were stuck with keeping the same suspension attachment points, so they just made a new twist-beam that has a bend in it for drive shaft clearance and redesigned the hubs so that they could connect to the half-shafts.

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Re: De Dion rear end design..

Post by Ratu » Fri Sep 07, 2018 4:44 pm

Brian P

Yes, I got a look. I didn't realise that this had been used by Honda. Interesting set up. Ah, "the Honda way"!

There is another system I came across which is kinda de Dion. The Alfa Romeo 8C had a solid beam axle rear suspension. There was a twist* to it though. The differential was directly attached to the rear of the gearbox. There were two fixed length driveshafts heading back from there, each to a bevel gear, one on the right side of the beam axle and one on the left side. These bevels each drove a rear wheel. So it was a dead axle and the differential was part of the sprung mass.

There is not that much available about the system in common circulation. I was able to discover that the two drive shafts rotated in the same direction and so drive torques acting on the axle would not have cancelled out. On accleration, one wheel would have experienced more traction and one would have experienced reduced traction, just as with a live rear axle. That was a wasted opportunity...

The justification for the design was reduction of unsprung weight, which it certainly did achieve. Aside from that there were other potentials. The previously mentioned possibility of evening out rear wheel traction existed but went un-exploited (hard to fathom that decision, as they certainly were aware of the opportunity). There was also the possibility to seat the driver lower in the chassis. While it is correct he was sitting a little lower than in the regular live axle designs, there does not seem to have been a concerted effort to take much advantage in this direction. The reasons have been difficult for me to find, although my suspicion is that the drivers preferred the visibility a higher position provided them (perhaps they were used to how things were and did not warm to too many innovations in one car!).

The design was not repeated elsewhere so far as I have been able to discover. It is interesting nonetheless. Perhaps it could have worked for Detroit at one time. I wonder if they even knew about it!

*meant as a figure of speach. The axle was intended as a rigid beam and not intended to twist.

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