i need dimensions!

[glen]

hello computer people, i need the measurement for the centre of the bolt hole for the front lower wishbone. no measurements on my plans!
something like "20mm up and 3 mm right". but i want to make sure. gtd chassis, cheers, glen

i need a hole here!

[glen]

bump!! please, i can't do much until i get this measurement, cheers

[Andy Green]

Hi Glen,
the position of these mounts will affect the bump steer, roll centre and and castor angle and will determinded by the design of stub axle and dimensions of the upper and lower wishbone. This is why people have probably been reluctant to give you specific dimensions

Have a read of Competition Car Suspension by Allan Staniford and it will help you determine where these points need to be

I know it's not the answer you were looking for, but you need to get it right to get the beast out of your car when its finished

Regards

Andy

[glen]

cheers andy, i suspected that was the case, but for now i just want the measurement of a gtd using standard nylon bushed gtd lower and upper arms, using standard granada front uprights.
i'm not scared of changing at a later date if it's that terrible, but by using gtd measurements with standard gtd stuff, i should be in the right ball park.
cheers, glen

[Paul W]

Glen your probably best trying to get to see for yourself a standard GTD and getting the measurements - I understand what you are after but haven't got the right chassis!. Why not pop over to Franks, he's usually got several cars there at a time.

[glen]

frank's done loads to help, as you have, so i didn't want to keep hastling him, especially for a measurement!! but if i have to, it's a small drive.
i can work it out from the pics i have, to within a couple of mm, which, going by the thickness of the shims i've seen on some chassis, should be ok.
just a small problem, it'll be ok in the end!!

[Mick Ridley]

hi Glen,
phone me,, i should be able to help you with the dimmension you are looking for

[glen]

cheers, pm me your no, glen

[Mike Pass]

Hi Glen,
In order to settle on the wishbone pivot points you need to start with the steering rack and the location of the track rod ball joints. In order to avoid bump steer a simple rule is that the track rod ball joints and the wishbone pivots should all be in the same plane. This will give a good starting point. The idea is that as the suspension moves up and down the movement of the wishbones and the track rod move through their arcs in such a way that the steering arm on the upright stays pointing in the same direction. If the arms are wrongly located the upright will turn as the suspension moves and you will have bump steer. Bumpsteer can be tuned out if not too severe by moving the steering rack around - usually up and down. On GTDs a common way the get rid of bump steer is to raise the rack on shims upwards by 7/16". Put a laser pointer on the hub and move the suspension up and down and mark the points as it moves on a wall or flat surface. Juggle the rack around until the pointer dot moves in the closest to a vertical straight line.
The top wishbone should be shorter so that the upright will become more negative cambered as the suspension goes in to bump and vice versa in droop. The idea of this is to keep the tyre contact patch flat to the road as the car rolls in cornering. The ratio of the wishbone lengths will control the rate of change of camber. How much roll you will actually get will depend on the spring rate and the anti roll bar stiffness.
As a very rough guide the bottom arm will angle downwards about 0.25 / 0.5" and the top wishbone will angle upwards about twice as much. Remember that only the pivot points matter so don't be confused by the angle of the wishbone bars themselves - all measurements are using the centre of ball joints and the centre of the wishbone pivots.
If you check out the Staniforth book you will find that it tells you almost everything you need to know but not the whole story. His idea of a "string computer" works OK. The idea is that you construct a model of your suspension out of thin sheet such as plywood or plastic sheet and then move the model up and down to see what moves where and what happens to the angle of the upright. A very small change in the suspension pick up points can have a very big effect.
The quick solution is to copy exactly a set up that works. However you will also copy any issues with that set up. GTDs have a very low KPI due to the Granada Mk2 uprights and also very poor Ackerman which leads to tyre scrub on full lock. However there are cars around which have had these problems fixed.
Hope this helps.
Cheers
Mike

[glen]

cheers mike, i like the "rough angle" of upper and lower arms, c to c, i should be able to work it out from there!