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Lower wishbone loads tubes in torsion
Soul-tez - 13/1/05 at 02:13 PM

I'm new to the forum but not to the idea of building a car. There is a ton of great information here but I can't seem to find any help with something that bothers me about the book chassis. The lower rear wishbone mounts load tubes F2,F1,G1,G2 in torsion under braking and cornering. The Costin, Phipps book says in a number of places that the suspension should load the tubes in compresion or tension. Part of me thinks that the book mounts are stiff enough since they are so close to tubes FU1,FU2, and E. Another part of me thinks that I'm starting from scratch so why not make it as perfect as possible? The panhard bar mount in the rear causes similar problems. Any other opions would be appreciated.

blueshift - 13/1/05 at 02:16 PM

IMHO, don't worry about it. there are plenty of other areas to improve on the book rather than worrying about the wishbone brackets loading tubes in torsion. A few to get you started:

- beef up the rear trailing arm brackets

- improve the design of the bottom wishbone

- fix the castor angle problem

- add cymtriks' chassis mods for stiffness

I know how easy it is to worry about everything, but after a loooong time considering these issues, let me put a vote in for not worrying about the front brackets.

britishtrident - 13/1/05 at 06:32 PM

quote:
Originally posted by Soul-tez
I'm new to the forum but not to the idea of building a car. There is a ton of great information here but I can't seem to find any help with something that bothers me about the book chassis. The lower rear wishbone mounts load tubes F2,F1,G1,G2 in torsion under braking and cornering. The Costin, Phipps book says in a number of places that the suspension should load the tubes in compresion or tension. Part of me thinks that the book mounts are stiff enough since they are so close to tubes FU1,FU2, and E. Another part of me thinks that I'm starting from scratch so why not make it as perfect as possible? The panhard bar mount in the rear causes similar problems. Any other opions would be appreciated.


Only very very trivial torsion loads at wishbone and panhard rod unles you use solid bushes on the panhard rod ---- the bending loads on the F2,F1,G1,G2 members are much moresignificant but not due to forces from the wishbone but the loads from the engine mounting, it is almost a classic classic text book beam with 2 suppoprts carrying an almost central load in bending
.
The one place in the chassis you will find substantial torsional loads is the rear spring mounting plate.


[Edited on 13/1/05 by britishtrident]

[Edited on 13/1/05 by britishtrident]

MikeRJ - 13/1/05 at 06:54 PM

quote:
Originally posted by britishtrident
The one place in the chassis you will find substantial torsional loads is the rear spring mounting plate.


And that IS something that has worried me ever since I first saw the chassis. There is, as far as I know no significant history of failure here, but the 3mm plate is essenatialy unsupported on the rear outside corner. The tube connecting to this point is proabably doing more work supporting the rear frame than vice versa.

If it were just the suspension mount I wouldn't be worried, but the anti-roll bar bolts up here, so it needs to be as strong as possible. Thoughts?

[Edited on 13/1/05 by MikeRJ]

timmy - 13/1/05 at 09:01 PM

I agree that the rear spring/shocker mounts are a bit on the light side.
On my chassis I replaced those 3mm plates and the lateral member behind the driver's head with a single piece of 100 x 50 x 3 RHS. This gives a great mounting place for the springs and also the rollbar. I can post pic's if you'd like a bit more detail.

Tim

Marcus - 14/1/05 at 08:25 PM

I thought this, but welding a good strong rollbar on, effectively joining the two plates together went some way to allaying my fear.

Marcus