GTD wheel offset

[Paul Hill]

Hello to all-
I am looking at replacing the wheels on my GTD- going to Vintage K.O.s.
I have a choice of offsets. The front wheels currently on the car have an slight negative offset, about -1/8th of an inch (4 7/8 inch backspace).

The new wheels would be about .725 negative (4.025 backspace) or they can be machined to a slight negative offset (4.775 backspace).

Anyone have experience with different offsets or know what GTD specs/preferences were?
Input appreciated.

Thanks,
Paul

 

[Geert-Jan]

I have been asking myself the same question.
I see alot of different offsets on pics of many members.

How about using spacers?

GJ

 

[Howard Jones]

The front of GTD's suffer from excessive kinkpin angle as a result of using the Granada upright. Moving the wheel centerline inboard would help with the kingpin angle problem BUT, and that's a big BUT, it is possible to run out of tire clearance pretty quickly if you move the tire inboard much at all.

The real solution is reducing the kingpin angle. This would require a redesign of the upright and then placing the wheel in the proper location by changing the offset to suit.

I hope I got that right..............anyway the problem with changing the wheel center is running out of inboard chassis/body clearance with the tire at full lock. Check this until you are certain that the tire won't rub before you try to move the wheel around.

 

[Mike Pass]

I don't think that the GTD (using Ford Granada Mk2 uprights) king pin inclination angle is too much. If anything it is too small. The problem is that the wheel offset from is too great. In an ideal world the line through the two swivels about which the wheel pivots would pass through the exact centre of the tyre contact patch. On the GTD the king pin axis is almost at the inner edge of the tyre. Thus rather than the tyre rotating about the centre of the contact patch the tyre moves in an arc about the king pin axis. An increase in KPI would move the point outwards towards the contact patch centre which would help to reduce the kickback through the wheel.
A further issue with the front suspension is that there is no Ackerman effect. There is in fact an anti Ackerman effect. Ackerman angle is the set up of the steering arms so that the inner wheel in a corner is turned more than the outer so that they can both follow the correct turn radius. This is normally achieved by "aiming" the steering arms at the centre of the rear axle. This usually gives pretty much the desired effect. This is much easier with rearward facing steering arns. The standard GTD at slow speed with full lock will produce a lot of tyre scrub as the wheels try to follow the wrong arcs. In most other circumstances there is no real issue with the way the car drives as most steering angles on the move are very small so geometry issues aren't a problem.
The situation can be resolved by using another upright. These can be custom made or the Granada MK3 uprights can be used with a "mushroom" or strut balljoint adapter in the socket normally occupied by the Mcpherson strut. New wishbones would need to be made up in both cases.
Cheers
Mike

 

[Mike Pass]

Pic attached of the Granada/ Sierra type upright with a (blue coloured) "mushroom" fitted into the strut socket to take the top wishbone swivel. The pic is of the setup on a seven type kitcar so is probably a Sierra upright. The Granada Mk3 upright needs a larger diam mushroom than the Sierra one and the hub is 5 stud rather than 4 stud fitting.

The KPI of this setup is much better than the Granada Mk2 GTD type and should give a much better turn in as the rate of camber change as the wheel is turned will be much higher. As I am not at home I can't look up the KPI of the Mk2 and MK3 uprights.

Cheers
Mike

 

[Frank Catt]

Just for clarity, Howard has it all wrong and Mike is right, Frank

 

[Mackan]

Hi Mike!
I really could use the info about KPI measurements of the two uprights as i am working on my front suspension (scratch build)

/Markus

Pic attached of the Granada/ Sierra type upright with a (blue coloured) "mushroom" fitted into the strut socket to take the top wishbone swivel. The pic is of the setup on a seven type kitcar so is probably a Sierra upright. The Granada Mk3 upright needs a larger diam mushroom than the Sierra one and the hub is 5 stud rather than 4 stud fitting.

The KPI of this setup is much better than the Granada Mk2 GTD type and should give a much better turn in as the rate of camber change as the wheel is turned will be much higher. As I am not at home I can't look up the KPI of the Mk2 and MK3 uprights.

Cheers
Mike

 

[Howard Jones]

Ok, I get it now. Change excessive to inadeqate. Got it. Thanks

 

[David]

Old thread I know but for anyone else searching for this I found these figures.

Cortina has a KPI of 4.8
Granada has a KPI of 7.4

No guarantee of accuracy, use at your own risk etc.

IMO the KPI of the Granada is about right, just a shame about the offset, steering arms and bendy spindle!

 

[Robert May Eng.]

Its worth noting that the sierra/granada mcpherson type upright has negative trail built in which wipes out all the self centering effects of any caster you dial into the system. The 4x4 type strut is zero trail but comes with the big ugly driveshaft through the middle of it.

Bob

 

[Jim Cowden]

Just to add to this, Mike was explaining the ackerman.
My up rights I made so I can change steering arms.
I made a set of major anti ackerman arms ,F1 style toe in on turn and it turned in very very good.
Its a bitch to get in the garage though.


Jim