Live Axle vs. De Dion

[Mike Abell]

Can anyone explain the difference to me? My limited knowledge leads me to believe that De Dion is better for bumpy roads, whereas live axle is better for track use?

Thanks!

[Charles E-H]

Mike,

 

Although the de-dion design could be described as semi-independent, the main reason for the improvement in ride is that the weight of the differential is sprung - its solidly attached to the car. In a car as light as a seven, having significantly less mass interfering with the wheels ability to follow bumps has a beneficial effect on ride. On a smooth race track, ths becomes less important.

In theory, the geometry of the live axle and dedion should be the same; both will keep the wheels close to perpendicular to the road. However, the dedion chassis has had a lot more development over the past 15 years, and although heavier has the advantage of better tortional stiffness.

 

However, building a live axle car can be significantly cheaper(beware the weaker BMC/Ital axles though), and it is more in line with Chapman's original concept.

 

That why I'm building a live axle car.

 

regards,

 

Charles

 

 

 

 

[Mike Abell]

Thanks Charles

 

I'm looking to purchase a 7 2nd hand for mainly bumpy road use, so if I understand correctly, one with a live axle is cheaper but i'd probably arrive with a bigger grin in a dedion.

 

Good luck with the build, I'd like to build my own but have a serious lack of knowledge, time and garage space!

[Morgan C]

I might be talking rubbish here, but as I understand it some of the problem with taking in bumps with the live axle is partly due to it not being independent i.e if one wheel is lifted by a large bump, the other wheel may also be lifted across the axle, leading to loss of grip on both sides. However in addition the suspension travel on the live-axle has a large amount of bump-steer (if I remember correctly upto 4degrees alteration in toe in/out on full deflection. You can certainly feel this, for example crossing horizontal ridges on the curve of the Aston flyover in Birmingham. On the other hand the De Dion has only a minimal amount of bump-steer. Caterham have added an optional bit of jiggery-pokery called a Watts linkage, which eliminates bump-steer altogether. I am sure there was an article about this in Low Flying a few months back. Otherwise all of this came to me in a Kubla Khan type of dream....

[I.Mupferit]

The ride on a live axle car is greatly improved if you use progressive rate springs with Bilstein Dampers. Still not quite as good as de Dion but a reasonable compromise if cost is a factor in the equation.

[Mike Abell]

Found this in an excellent piece at http://www.se7ens.net/faq/7faq.htm#RevisionHistory

 

5.1. Live axle or De Dion?

This is a reply posted by Clive to a post by Brian Poulton :

 

>>Secondly, I have driven both live axle and De Dion cars and have the

following comments to make. On smooth roads, I wouldn't know which type of

car I was driving. On bumpy roads I would. I stopped using one of my

favourite routes to work as I damaged 2 sumps on a live axle car and I had

to slow down on the bumpy bits. Last week I tried the route again with the

De Dion car and it was a revelation. I was overtaking traffic where in the

live axle car I would not have dared travel at that speed at all.<<

 

Brian - thanks, I think you've described the essential difference. On smmoth

roads the differences will be minor - but when you get to the bumpy parts,

the de Dion will put its power down better and maintain its poise in a way

that gives the driver confidence to maintain a higher speed and pick his line

more accurately. A side effect, which some people apparently consider A Bad

Thing, is that tha car can also, if desired, be tuned to give a less

shattering ride, without giving up any handling (this last part should be in

italics !) - in fact, good handling on bumpy roads is so much a matter of

controlling wheel motion and dealing with the energy coming off the road,

that ride and handling go hand in hand. Personally, I think this is good news

- my idea of "being one with the car (and the road)" is that I should at

least be in contact with it (and the road) most of the time - easier to

achieve when it's not beating me up and throwing me around. Maybe I've missed

the point, but I thought the essence of the Seven was neat, precise, simple

and quick - if you want brutal for brutal's sake, don't you buy a TVR and a

chest wig, plus a copy of The Sun to read when you have to slow down in the

bumpy bits ?

Clive

 

[Mike Bees]

By the way Clive Roberts worked at Caterham and was heavily involved in the engineering of the De Dion setup. He left Caterham in the mid-80s and went to Lotus where he was the Program Manager for the M100. He was last heard of working for GM in ths US on their electric car program. Sadly he resigned from the sevens list in mysterious circumstances a few years ago.

 

Here are some links to related stuff he wrote on the sevens list:

 

http://www.se7ens.net/archive/sevens.w3archive/9701/msg00364.html

http://www.se7ens.net/archive/sevens.w3archive/9701/msg00409.html

http://www.se7ens.net/archive/sevens.w3archive/9701/msg00469.html

 

Mike