Suspension mounts in Tony P's book...

[RaceMag]

In the good Tony's book there is a picture of wishbone inner end fittings (page 54 fig 9.11 top middle of the main block to be precise) that show the A arm suspension bearing fitted into a machined housing that is bolted to the chassis. This is instead of the bearing being part of the A arm. The A arm appears to have a top and bottom plate to the bearing and a bolt passes through.
What is the downside to this arrangement vs the housing on the A arm? Does it get used on any other cars?

[Nick Skidmore]

Other cars inc.

PCD Saxon
DJ
Tatuus
Reynick 893

No drawbacks also I have the parts to do this if you need them

[RaceMag]

Thanks Nick
Actually nick I would like to see your current list of bits - can you PM me your website link?

[SteveSlowboy]

I would think there's a tiny reduction in un-sprung weight. The wishbones would be slightly simpler (and hence cheaper to make?) and the improved strength of the joint by using a spherical instead of a rod-end (you get rid of the threaded part which is the weak link). And sphericals are (sometimes) cheaper than rod-ends!
Only drawback is you need to devise another method of adjusting for length of the wishbone legs.

Steve

[The DOA]

Just make sure the wishbones fail before the chassis or the mounting block the spherical sits in. Pretty common practice and saves the inevitable failure of rod ends due to bad wishbone end design that can put them into bending and overstresses the thread roots.

[Ledon Racing]

Originally Posted by SteveSlowboy

Only drawback is you need to devise another method of adjusting for length of the wishbone legs.

Relatively easily achieved with a rod end at the outboard end or shims under the bearing housing.

[The DOA]

Originally Posted by Ledon Racing

Relatively easily achieved with a rod end at the outboard end or shims under the bearing housing.

The most effective way IMO is by using shims at the outboard end of the upper wishbone as is very commonly seen. Of course, if you can make the shimmed part the steering arm then there is also no need to adjust toe for each adjustment of the camber. I have seen shims being used at the inboard end of the wishbones and its painfull to change double the number of shims and then have to adjust the toe to compensate, not to mention the potential to introduce unwanted bump steer while you are at it....

The problem with a rod end at the outboard end of the wishbone is that this induces bending in the rod end/ends under braking conditions (and acceleration at the rear). Similarly, if using a pull or pushrod, using a rod end at the outboard end of the actuating wishbone induces bending in the rod end again from the reaction force of the pushrod/pullrod. Sure you can counter this with a bigger rod end but if your reason to start with is lower unsprung weight, you lose part of your advantage and as widely used as this method is, it is not very good engineering practice to bend a threaded component IMO. Of course, these are only my thoughts on the subject but I beleive Carol Smith used to have pretty similar views and he certainly had a lot more knowledge on the subject than me .

Just by the way though, if you did a cost analysis I think you would come to the conclusion that using rod ends at any end of a wishbone is a lot cheaper/less labour intensive than using sphericals even if the rod ends cost more in the first place. As for any potential weight loss by using sphericals, I think that sphericals would work out heavier though there would be very little in it.

[RaceMag]

thanks for the replies.
I can clear up some aspects: we are using shims on the upright to alter camber and the shims are between the upper mount/steering arm and the upright. Front on left, rear on right:
I was wondering on this type of mount for two reasons: to make the wishbone a bit easier to make (to be honest given the purpose of this car, weight is less of a concern than ruggedness, serviceability and reliability). Second reason was adjustability. I wondered if I used a steel mount to hold the bearing (with a enlarged flat section as a footplate where it butts up against the chassis tube), that this could have a single 10mm threaded bolt that runs through a machined slot (2.5mm walls) welded into the tube (35mm x 1.6mm SHS). This would allow us to quickly change front RCH and anti-dive without having to cut brackets off the chassis etc. This would only be on the top arms BTW.

[The DOA]

For what my opinion is worth, the optimum solution in terms of weight, simplicity and good engineering practice for your scheme would be to use a spherical at the outboard end within the steering arm and rod ends at the inboard end. The only hazard then is that you might pre-bend the wishbone tubes/rod end threads by adjusting the inboard rod ends too far in or out which might increase the likelihood of buckling of the wishbone tubes if not using anti-intrusion bars or of bending the rod end threads leading to potential failure. Whatever you do, try and maintain the load paths by keeping the rod ends in line with the long sides of the triangle formed by the wishbones three (or four at the rear) pivot points.

I have seen this layout used on Juno's racecars and its an elegant compromise IMO that gives a good failure mechanism in the wishbone (the inner rod end shanks should fail first) that will minimise damage to other components should the worst happen and you take a corner off.

Of course, I say all this and yet rod ends have been used at both ends of wishbones for a number of years without too many failures .

[RaceMag]

Thanks for the thought. I have a dislike of rod ends in bending, though I acknowledge that failures are rare. I have already purchased spherical bearings to use, which are probably larger than required. However let me clarify that I am interested in the possibility of vertical adjustment in the top wishbone mount to allow for suspension tuning.