Suspension upright design

[johnny99]

I am currently designing a GT car and while searching for info and inspiration on here, I came across this, the first and second image from the top of the page.

http://www.f1technical.net/forum/viewto ... 14&t=13309

I see the wheel centre is not on the centreline of the upright, anyone know why and the advantages/disadvantages.

Thanks in advance

John

[Jersey Tom]

You can move the spindle axis fore and aft to introduce mechanical trail without having to do so with caster.

[Greg Locock]

Technical term is longitudinal hub offset, or sometimes hub lead. In an ideal world you'd have some great theory about how much castor you want, and you'd also like to be able to fiddle with the mech trail. In practice even when given the option most vehicle programs accept the spindle geometry as fixed and come up with some compromise for trail and castor.

My personal opinion is that production cars have generally settled on too much castor angle, in the 6-10 degree range, and they would be smarter to look at 4-5 degrees like a Corvette.

[johnny99]

Thanks Tom, I'm an avid reader of your posts. So the centreline of the wheel is behind the centreline of the upright, thus adding mech trail, adding feel to the steering, therefore needing less caster. It will not push down the inside wheel in a corner like positive caster angles will ?

John

[johnny99]

Thanks Greg. I see WTCC cars etc with massive caster angles, and I've run up to 16 degrees in the past with good results on FWD and 4WD racecars, but never played with trail. I am now making my own uprights and trying to get it as right as possible first time round, money is an object

John

[Jersey Tom]

Thanks Tom, I'm an avid reader of your posts. So the centreline of the wheel is behind the centreline of the upright, thus adding mech trail, adding feel to the steering, therefore needing less caster. It will not push down the inside wheel in a corner like positive caster angles will ?

John

Correct. Or another way of thinking about it, you could run a lot of caster and offset your spindle the opposite way to shrink the mechanical trail and not have very heavy steering. Though the specifics of how much weight jacking you'll have dynamically with high caster and real driving loads, camber, tire deflection etc. maybe isn't trivial to figure out.

To back up a few steps... on a performance car - especially if you have manual steering - my opinion is that really the first step is to just make sure your mechanical trail is reasonably sized so driving the car isn't like wrestling a bear. There's such a thing as too little steering feedback, and such a thing as too much. Whether you achieve that by caster or spindle offset... I'm going to stop short of saying, "doesn't matter," but there are much bigger performance gains to spend the time on elsewhere (tires, engine, differential [huge!!], static alignment, mass distribution, springs, bars, and dampers).''

Of course you can always build some adjustment range into the control arms and spindle when it comes to caster and longitudinal offset...

[johnny99]

Very true, but I've never tried changing trail before. The car is RWD, so I won't be running as much caster as I did on the FWD or 4WD cars, so I might add something like 10 to 20mm trail, and have adjustment for caster up to 6 degrees.

John

[bigpat]

Just my opinion, but I believe castor and trail are a function of the cars characteristics in roll, aero considerations, and importantly the tyres you are running....

If there is significant roll, but little camber gain in the geometry, then adding higher castor is a good way of ensuring the tyre stays flat to the track surface, especially in lower profile tyres. I would then reduce trail accordingly to ease steering effort. That as well as steering arm length off the upright.

The opposite would apply if the geometry if there was signification camber gain.....

I know castor is also a consideration on RWD cars, dependant on the type of differential. With spool diffs, castor is used to jack up in inside rear, to help reduce entry, and mid corner under steer. So the concept and mechanical layout of the car is important to know.

Also if aero is important, and running wide front tyres, I would think reduced castor would be better to reduce roll, and keeping a level platform,

At the end of the day, we are slaves to the tyre, as we need to give the tyre what it needs to perform its best. Depending whether the particular tyre needs camber thrust or not, sidewall stiffness, both vertically and laterally etc to perform is paramount to know. If possible, get as much useful data from the relevant tyre company, as well as talk to the tyre people at the track, I've found them to be a great help with info and feedback.

I feel that then you have the tools to help make a decision on what philosophy to apply to the geometry.

As I said, just my opinion......

[johnny99]

Yes Pat, I will have camber control in roll, and trying to keep caster low so I don't unload the inner rear, which I was doing on purpose on the FWD cars I have built. I could always build in trail adjustment into the upright, having two locations for the upper mounting, and having caster adjustment also, within the wishbone.

John