Formula ford rear suspension geometry

[quantic]

Hi all, i'm in the process of rebuilding an old FF2000 Royal RP30 - now that it's almost done i'm starting to think about setting it up and one thing that im not too sure about is one part of the rear suspension geometry, and that is the two almost parallel trailing arms from just in front of the engine to the rear upright. Both are adjustable for length, but i can't quite work out what the effect of adjusting them would be and why i would need to, and what i should be measuring to see if any adjustment is even required (which i'm sure it will as i've replaced some of the rose joints, and as much as ive tried to put them back in the same place thats not really satisfactory to my mind..)
My best guess is that they affect the arc which the upright will go through as the suspension moves up and down, perhaps something to do with squat or dive, but even if i'm right (probably not!) i still dont know what i should be looking for.
Many thanks

[phoenix]

Adjustment to the length of both trailing links affects the toe setting at the rear - shorter = more toe-in, longer = more toe-out as such adjustments either pull the upright forward or push the upright backwards. If the trailing links are parallel then the toe setting should remain constant as the suspension moves up and down and any camber changes will be determined by the wishbone length and angles alone.

With non-parallel links, each link will follow a slightly different arc at the wheel upright end, resulting in the top and bottom of the upright moving forwards/rearwards at slightly different rates. For example, if the top trailing link is horiztontal when the car is at ride height and the lower trailing link runs downwards towards the rear of the car, under compression (i.e. as the wheel moves up) the top of the upright will be pulled forwards and the bottom of the upright will be pushed backwards. This will cause an increase in negative camber, however slight, and also an increase in toe-out, however slight. So, adjusting the lengths of the trailing links can be used to adjust dynamic camber and toe settings.

edit: The angle of the trailing links, relative to each other and to the ground, is what affects dive/squat. Their length has no effect on this.

[ianpearson]

Originally Posted by quantic

Hi all, i'm in the process of rebuilding an old FF2000 Royal RP30 - now that it's almost done i'm starting to think about setting it up and one thing that im not too sure about is one part of the rear suspension geometry, and that is the two almost parallel trailing arms from just in front of the engine to the rear upright. Both are adjustable for length, but i can't quite work out what the effect of adjusting them would be and why i would need to, and what i should be measuring to see if any adjustment is even required (which i'm sure it will as i've replaced some of the rose joints, and as much as ive tried to put them back in the same place thats not really satisfactory to my mind..)
My best guess is that they affect the arc which the upright will go through as the suspension moves up and down, perhaps something to do with squat or dive, but even if i'm right (probably not!) i still dont know what i should be looking for.
Many thanks

We have an RP30 at home, the trailing arms will be for adjusting out your bump steer so that when you move the rear wheel through its suspension travel it does not change the tow settings. Camber is adjusted with the top link running from the chassis brace to the upright & tow in or out is adjusted with the two rod ends on the bottom wishbone.

What are you looking at racing the car in? check out the classic formual ford 2000 page on facebook if you are on there

http://www.facebook.com/pages/Univer...27436007366707

[quantic]

Thanks phoenix, that's very helpful-it does raise one more question for me though and that is that there is a rose joint on the lower rear wishbone to upright joint which I had assumed was for adjusting the rear toe. If I am to adjust the rear toe with trailing arms, what does this one do?!

[phoenix]

Originally Posted by quantic

Thanks phoenix, that's very helpful-it does raise one more question for me though and that is that there is a rose joint on the lower rear wishbone to upright joint which I had assumed was for adjusting the rear toe. If I am to adjust the rear toe with trailing arms, what does this one do?!

Your assumption is correct. The rose joint mentioned is the one you should use to set the toe. In my post I was trying to explain the effect of adjusting the trailing link lengths. The effect is on toe (and potentially camber and caster). You shouldn't use the trailing links to adjust toe - just to correct toe changes in bump/rebound once static toe has been set. However, changing the length will have the side effect of affecting static toe - even if it is by only a small amount. Hence static toe should be re-checked after adjusting the trailing links.

[quantic]

Ok, thanks both-thats pretty much cleared up my query.
Ian, i bought the car with the intention of doing the Historic FF2000 races, but i have some date clashes with other commitments that will stop me doing some of those (plus the fact i havent finished putting it together yet!) and so may well do one or two URS rounds too