Front Top Wishbone Mounting Positions

[CarlW]

Hi Guys,

Design a new front suspension lay out with new front top wishbone and different upright.
To get top and bottom wishbones to be parallel, it looks like the top wishbone mounting points need to be closer to the top rail.
So rather than the foremost one being on the angle part of the front frame, it would be on the vertical part.
Any body have any thoughts on possible issue this could cause?

[flyerncle]

Not intending to pee on your bonfire so to speak,the car was designed to run Sierra uprights with the geometry as is and modifying it will give you all sorts of problems in my humble opinion.
Wish I had the CAD and knowledge to it !

[voucht]

Why do you want your bottom and top wishbones to be parallel ? There is no reason for that (I think!), and they are not parallel in the ordinal design.
So I think you could keep the original vertical position of the book's mounting points.

[garyt]

hi the reason that the top ones are angled is to do with tyre contact with cornering loads
in a car with parrelel bones and therefore the mounting points = to the hub top and bottom ball joints, when a car corners there is a lateral force which will try and compress that side of the suspension, now if we had a car that would corner flat the this would be acceptable but in reality this would need to be a solid axle with no suspension.
with the angled top bone this allows the top of the wheel to prescribe an arc as the suspension moves in relation to the cars lateral cornering forces.
in the former situation the tyre will lift reducing the contact patch available thus less grip and by association less cornering ability. in the second situation the suspension movement allows the tyre contact patch to remain constant ( to a point) as the body rolls thus allowing the grip and cornering ability that we love.
and we haven't even considered tyre pressures and sidewall flex :)
Now this is a very simplistic explanation of a very complex issue that many more learned people than me have studied and fought with for years and probably will continue to do so
there are many tomes on the subject but staniforths race and rally car source book explains it well
sorry to be so long winded and I hope it makes some sense
all the best Gary

[CarlW]

Hi Guys,

Going off the above, which is gold by the way, what is the best wishbone design?
I mean there is the book design then others?

I am aiming at Castor of 6 degrees, camber of -2 degrees? From thi I can CAD the correc wishobnes etc.

The bottom is wishbone is a simple design, the top is the optitune one?


Cheers!

[garyt]

Just go with the book design simple and strong parts readily available just set your preferences when making the jig
As an aside I used inner end rose joints on the lower wishbone and then shimmed to get the caster that I needed to help with the centering also combined with the upper drag link adjustment I have a fully adjustable front end now :)
From my trials extra caster and to much toe in made the front really nervous now I have almost zero toe and it feels great with centering to boot
Gary

[CarlW]

I will model up the book one and see what the angles are like.
I wanted to start with 0 - 2 deg camber and 6 deg castor 0 deg toe.

The top has to be right or will need tweaking to get right so I like to try right first time lol

[CarlW]

Hi all,

I have modled up the top and bottom wishbones to the book and I measure a castor of 8.4 degrees. Isnt this a bit to much for the Sierra upright set up?
Camber is measuring 0.7 degrees but this is obviously adjustable.

[garyt]

It shows as a lot ( don't know what the optimum should be) as our 7's are notoriously hard to get the self centering required for iva many have failed on this point so I guess the book works out that way to ensure there is some compliance
As I mentioned earlier I am able to shim mine and ended up with 1 x misalignment washer at the rear of the mounting bracket then the rose joint then the shims in front this gives me the trail required and I am happy with her now
Oh should have also said I am using mx5 uprights so can't be sierra specific sorry
Gary

[garyt]

Just out of interest I just googled optimum castor angle.and it seems over 7 rev is common ...... mmmm

[CarlW]

Hi ya,

I would rather have say 7 degrees and the shim if needed, 8.4 just sounds tomuch, over kill?

[garyt]

Yes that sounds reasonable I also got an old skool tester who said " mm rear wheel drive I will give it some throttle, that will help" and off he went
The problem with iva is that it says int book of rules "must show a degree of self centering " now if that isn't open to individual testers interpretation then I don't know what is
Good luck