Mounting a rack above the top wishbones??

[MrNoo]

Hi, just a quick question, is it possible to get away with mounting the rack above (1 inch) the top wishbone centre pivots and get an acceptable amount to no bump steer?? I cannot move the rack lower due to body parts being in the way. The rack ball joints are centered on a line that dissects both the top and bottom inner pivot points, just above the top pivot.
In an ideal world I would have mounted it in line with the top pivots but can't.
The rack is behind the CL of the wheels.
With regards the steering arms, we originally planned to run ackerman but am thinking the bump steer issue may have us run steering arms that line up with a dissecting line between the outside top and bottom pivots.
I have jury rigged a dial gauge and can get it somewhere handy around ride height but anymore than 1 1/2" bump and it goes into toe in and anything more than 1" droop and again goes into toe in. I had planned on only having 3" total travel but would just like to ask those with more knowledge if it is possible, if it is what should i be paying most attention to.
regards

[Tim.Wright]

What kind of car and tyres? Basically the lighter the car and the stickier the rubber, the more critical is your bumpsteer. For a race car (light weight with grippy tyre) the bumpsteer should be prioritised over the ackermann.

In theory you can put the rack anywhere you want if your outer toe link point is open to modification. If your outer point is fixed then you are stuffed. The rack will only work in one small area without bumpsteer.

Anyway none of this stuff can be generalised. i.e. noone can say what effect having the rack higher than the upper wishbone will have on the bumpsteer because it's such a hyper sensitive parameter and depends on your wishbone geometry down to the millimetre level. There are a few software applications such as wishbone.bas (free) or dynatune (cheap) which will answer your question.

Either way you will need to design in some adjustability into the rack height because manufacturing tolerances are usually wider than whats acceptable to keep the bumpsteer "in-spec"

[MrNoo]

Thanks Tim for your reply, it is a 917 Porsche replica that I have been building from scratch for the last 4 years. https://www.flickr.com/photos/47153142@ ... 3861979360
It will be fairly light but not race car light and will running road spec tyres.
My outer point can be changed as the arm/top wishbone outer link is bolted to the upright (for camber change without affecting steering) so will enable new arms to be machined. The rack height I can play with a bit maybe half an inch up or down max, so it looks like i should be able to get something satisfactory out of it.
You state the rack will only work in one small area without bump steer, I guess I just need to find a way of making this small area as large as possible.
many thanks for your input, much appreciated.

[Greg Locock]

Wot he said.

I can't remember seeing a rack completely above or below the wishbones, one reason for that is of course your steering arm (outer ball joint) needs to be in the same place as the wheel or tire. One possibility would be to go for a tall spindle wishbone,like this

https://en.wikipedia.org/wiki/Double_wi ... ension.jpg

which coincidentally is almost certainly one of mine! ...and then mount the steering arm above the tire, I think Audi have done something like that. You'll have problems with camber stiffness because the swan necked spindle is floppy but a bit of structural optimisation should be possible.

ah here we go

http://1.bp.blogspot.com/-wPHJMFJVTjw/V ... i%2BA4.JPG

It's a bit fugly but it keeps the bump steer nicely under control, the only critical relationship is with the upper arm. I can't be bothered to work out the effect on understeer (quantitaively), but a linear steel torsion spring (the spindle) is probably as good as you'll get feel wise.

[MrNoo]

Thanks all.

The spindle type upright is a no go as the bodywork would not allow such a feature. I will have a fiddle around today and see what I can come up with.

[Greg Locock]

Sorry about that for some reason despite several hours difference in posting time I didn't see your 9:10 am post.

[MrNoo]

Sorry about that for some reason despite several hours difference in posting time I didn't see your 9:10 am post.

No worries Greg, my post had to go for moderation as I dont post much, just spectate from a distance!! Thanks for your input.

[MrNoo]

Well had a fiddle for a day and a half. The best I can come up with is, zero toe in/out at ride height, dial gauge motionless for around an inch of movement, i.e.. half inch droop and half inch compression but then it starts to toe in slightly, both in compression and droop. Over my wheel travel I get a total of .5mm toe in, both at full droop and full compression. So it goes from toe in at full compression down to zero just before and after ride height then back to toe in on full droop. I have tried dropping the rack, no change, I have moved the steering arm pick ups out, up, forward/backwards, down etc etc using brackets, lowered the rack but still no change, in fact a lot worse. The only thing I haven't done is raise the rack (due to mounts being in the way, but can move if it is an answer)
Any ideas?? I would prefer it to toe out in compression a tad if it's going to toe anywhere. Then is .5mm on the front of the rim (15") something to be worried about? Ideally I would like to get it right as detest understeer and reckon livening the rear up with a touch of toe out would not be the answer.

[Greg Locock]

"Over my wheel travel I get a total of .5mm toe in, both at full droop and full compression. So it goes from toe in at full compression down to zero just before and after ride height then back to toe in on full droop."

That implies the tie rod is the wrong length. Have you measured the geometry and dropped it into the wishbone program?

[MrNoo]

No, not yet Greg, my computer is a Mac, my GF has a windows based system so will try it at the weekend. The one thing I never tried was shortening the tie rod past it's original designed position, so stupid of me! Will have another look at it today and then try a wishbone program at weekend if no joy. I am more of a "Chalk on the floor" type as opposed to computer boffin!
Thanks for your input Greg