Discussion
Hi
I have a long term suspicion that my precat Griffith front suspension has a basic geometry issue, and have fallen towards thinking that the wheel offset is not as it should be. (The symptom affects all Griffs to some degree, and is bult in from new - not that most think this is true!). I have measured the scrub radius, and get around 20 to 25mm positive. I would expect 0mm ideally.
Two questions, does anyone have a method for measuring scrub radius to allow me to compare how I did it, secondly is 20mm too much? Anyone know for sure?
Thanks
Nic
I have a long term suspicion that my precat Griffith front suspension has a basic geometry issue, and have fallen towards thinking that the wheel offset is not as it should be. (The symptom affects all Griffs to some degree, and is bult in from new - not that most think this is true!). I have measured the scrub radius, and get around 20 to 25mm positive. I would expect 0mm ideally.
Two questions, does anyone have a method for measuring scrub radius to allow me to compare how I did it, secondly is 20mm too much? Anyone know for sure?
Thanks
Nic
As far as I know, a scrub radius of 0 is the ideal (centre point steering), but sometimes quite hard to achieve.
Reading various books on suspension indicates that, when viewed from the front, a line drawn through the top and bottom ball joints should intersect the ground somewhere between the centre line of the tyre and the inside edge of the tyre.
The way I worked out my cars details was to take plenty of measurements from a suitable datum and then draw it out half scale.
Not deadly accurate, but accurate enough for my purposes.
Reading various books on suspension indicates that, when viewed from the front, a line drawn through the top and bottom ball joints should intersect the ground somewhere between the centre line of the tyre and the inside edge of the tyre.
The way I worked out my cars details was to take plenty of measurements from a suitable datum and then draw it out half scale.
Not deadly accurate, but accurate enough for my purposes.
There's no 'perfect' figure for scrub radius (steering axis offset)- like everything else with suspension design, it's a matter of juggling all the different elements to come up with an overall balance that you're happy with. Zero offset seems like a nice, theoretically clean, target to aim for, but in practice SAO is one of the things that gives 'feel' to the steering and centrepoint steering can tend to feel a little vague. The other most significant factors in terms of steering feel are castor angle/trail and the self aligning torque of the tyes, but with the Griffith I wouldn't be surprised if the issue isn't further muddled by a certain amount of bumpsteer.
20mm isn't an extreme figure, by any means, but I agree - Griffith steering tends to kick back and tramline more than is comfortable.
You can check SAO either by very careful measurements of the upright geometry, as previously suggested (and bearing in mind that castor means that the whole thing is working in 3 dimensions, not two!), or by supporting the car so that the suspension is loaded, then using a plumb line or jig fixed to the hub to actually plot the arc as you swing the steering.
In practice, I wouldn't bother - if you suspect that excessive offset is a problem, the only way to find out is by testing alternatives. Using shims or spacers to increase the offset further (to see if the problem is exacerbated) is cheaper than buying a set of wheels with less offset to see if it improves, though, as a starting point!
20mm isn't an extreme figure, by any means, but I agree - Griffith steering tends to kick back and tramline more than is comfortable.
You can check SAO either by very careful measurements of the upright geometry, as previously suggested (and bearing in mind that castor means that the whole thing is working in 3 dimensions, not two!), or by supporting the car so that the suspension is loaded, then using a plumb line or jig fixed to the hub to actually plot the arc as you swing the steering.
In practice, I wouldn't bother - if you suspect that excessive offset is a problem, the only way to find out is by testing alternatives. Using shims or spacers to increase the offset further (to see if the problem is exacerbated) is cheaper than buying a set of wheels with less offset to see if it improves, though, as a starting point!
Thanks both. I measured the scrub raduis by placing two sheets of 6mm hardboard under the front wheel, about 2 foot square. The upper sheet grips the tyre, the lower the ground, but they slide easily on each other. Turning the steering turns the upper board on the lower. Turn the steering whilst holding a biro still on the upper board gives an arc, do this several times and mark the tyre postion. You can then plot the tangents to the arc to get the centre of rotation, and compare to the tyre centre.
I would love to try alternative wheels and tyres, unfortunately as you say, not so cheap to play about with. I will try some positive spacers to see if it gets worse.
Just for interest, I have checked bump steer, and it is pretty good, only a mm or so at wheel rim over full travel (shocking!).
It is theoretically possible to increase kpi by making a custom upper for the sierra upright. I may give this a shot.
Nic
I would love to try alternative wheels and tyres, unfortunately as you say, not so cheap to play about with. I will try some positive spacers to see if it gets worse.
Just for interest, I have checked bump steer, and it is pretty good, only a mm or so at wheel rim over full travel (shocking!).
It is theoretically possible to increase kpi by making a custom upper for the sierra upright. I may give this a shot.
Nic
Edited by nicbowman on Tuesday 24th April 18:06
Nic, I did similar to you whilst 'tinkering', but I just drew a line using the top board as a rule and marked off the centre. Later I could then use a set square to plot back the several points I made (to a reasonable estimate of swivel point). I used different coloured pens whilst adjusting the spring platforms, to evaluate the affects of ride height. Not sure how accurate my positioning of the tyre centre was, but it seemed that an increased ride height got close to zero. I tried it & liked it 
Dave
Dave
Can't remember exactly what the Griffith is but 20-25mm sounds about right. It was originally designed without power steering and I think the idea was to try and lighten the steering a bit at parking speeds by letting the tyres "roll" rather than twist on the spot. Interestingly, it was always thought at the factory that this had a lot to do with the tramlining but I vaguely remember they tried a special pair of top wishbones to make the scrub radius anything from about -25 to +75 and it didn't seem to make any difference to the tramlining!
Thanks Ian
You seem to have some knowledge of the factory, were you involved in some way?
They got somthing right, the steering is lighter than the S.
After all this thinking, I am tempted to just try different tyres? Toyo's seem popular, maybe the Bridgestones sidewalls are stiffer? Lots of complex stuff, all costs too much time and money to muck with! I like it when you can unbolt and jig around.
Nic
You seem to have some knowledge of the factory, were you involved in some way?
They got somthing right, the steering is lighter than the S.
After all this thinking, I am tempted to just try different tyres? Toyo's seem popular, maybe the Bridgestones sidewalls are stiffer? Lots of complex stuff, all costs too much time and money to muck with! I like it when you can unbolt and jig around.
Nic
Yes. Worked there as a development engineer from 1989-95. Had some good times! Unfortunately I wasn't directly involved with chassis & suspension although job descriptions at TVR were always pretty "fluid"! I'll try to draw this thread to the attention of a colleague who was much more directly involved. None of us ever really got to the bottom of the tramlining though. One of the big problems (we thought) was that the car was very light - there weren't many tyres of that size which were working around the middle of their carrying capacity on the front of a Griffith! I've an idea they weren't too bad on Toyos or, indeed, Avons but we had a tie-up with Bridgestone at the time.
Avocet said:
...but I vaguely remember they tried a special pair of top wishbones to make the scrub radius anything from about -25 to +75 and it didn't seem to make any difference to the tramlining!
Forgive my ignorance - but surely the top wishbone can only set camber & the degree of camber change on deflection? The scrub radius is reasonably fixed by the hub/upright arrangement and you can only 'tweak' it with subtle camber changes and tyre deflection. Not sniping - just interested + I get lost in some of these 3d visualisations very often
Dave
spend said:
...The scrub radius is reasonably fixed by the hub/upright arrangement and you can only 'tweak' it with subtle camber changes and tyre deflection.
Steady on, Dave... bit subtle that. If they were that clever at TVR they'd have been able to design cars that handled right in the first place!
You're quite right, though... unless you have a separate 'carrier' for the top ball joint and steering arm(as the Lotus Elise and many race cars do, to enable camber adjustment by shims without upsetting anything else), the only sensible way to make quick changes to scrub radius is by changing the wheel offset.
Edited by Sam_68 on Sunday 29th April 16:53
Hi
There is a machined extension bolted to the top of the Ford upright, which provides the mount for the ball joint. I have been assuming that I could machine a new part, with the ball joint off axis, thus providing more KPI and no more camber. Has a risk of additional torque loading on the hub to machined joint.
May be easier to make new wishbones 20mm longer!
Nic
There is a machined extension bolted to the top of the Ford upright, which provides the mount for the ball joint. I have been assuming that I could machine a new part, with the ball joint off axis, thus providing more KPI and no more camber. Has a risk of additional torque loading on the hub to machined joint.
May be easier to make new wishbones 20mm longer!
Nic
nicbowman said:
Hi
There is a machined extension bolted to the top of the Ford upright, which provides the mount for the ball joint. I have been assuming that I could machine a new part, with the ball joint off axis, thus providing more KPI and no more camber. Has a risk of additional torque loading on the hub to machined joint.
May be easier to make new wishbones 20mm longer!
Nic
There is a machined extension bolted to the top of the Ford upright, which provides the mount for the ball joint. I have been assuming that I could machine a new part, with the ball joint off axis, thus providing more KPI and no more camber. Has a risk of additional torque loading on the hub to machined joint.
May be easier to make new wishbones 20mm longer!
Nic
Errm... surely the hub & wheel will just end up pointing downwards? The hub axis to the ball joint axis is the critical angle (which is fixed by the casting) so you would need new uprights to achieve what you want I would have thought...
Dave
Edited by spend on Sunday 29th April 22:21
spend said:
nicbowman said:
Hi
There is a machined extension bolted to the top of the Ford upright, which provides the mount for the ball joint. I have been assuming that I could machine a new part, with the ball joint off axis, thus providing more KPI and no more camber. Has a risk of additional torque loading on the hub to machined joint.
May be easier to make new wishbones 20mm longer!
Nic
There is a machined extension bolted to the top of the Ford upright, which provides the mount for the ball joint. I have been assuming that I could machine a new part, with the ball joint off axis, thus providing more KPI and no more camber. Has a risk of additional torque loading on the hub to machined joint.
May be easier to make new wishbones 20mm longer!
Nic
Errm... surely the hub & wheel will just end up pointing downwards? The hub axis to the ball joint axis is the critical angle (which is fixed by the casting) so you would need new uprights to achieve what you want I would have thought...
Dave
Edited by spend on Sunday 29th April 22:21
The upright is effectively in two bits - the Ford casting, and the TVR bespoke bit that they shove in the top to take the top balljoint. Changing the bespoke bit for one with a different geometry would enable you to move the steering axis relative to the hub axis.
Sorry, I was thing of the "May be easier to make new wishbones 20mm longer! " bit, didnt quote it very well..
I thought the upright casting was just bog standard Ford - ie the bit that the saab bj nestles into. I guess the upright was designed with jellymould tyres in mind - wonder if any of the kitcaretc. manufacturers that use similar running gear have an upright design that is more suitable for what you want? It would more than likely be lighter as well I do occasionally keep my eye out for alternative uprights after carrying one of the rears 
Dave
.. I thought the upright casting was just bog standard Ford - ie the bit that the saab bj nestles into. I guess the upright was designed with jellymould tyres in mind - wonder if any of the kitcaretc. manufacturers that use similar running gear have an upright design that is more suitable for what you want? It would more than likely be lighter as well
I do occasionally keep my eye out for alternative uprights after carrying one of the rears Dave
GreenV8S said:
The upright is effectively in two bits - the Ford casting, and the TVR bespoke bit that they shove in the top to take the top balljoint. Changing the bespoke bit for one with a different geometry would enable you to move the steering axis relative to the hub axis.
...but would presumably fcuk up other elements of the geometry, including bump steer, so not an ideal (straightforward} way of trying to tune out the Griff/Chimp tramlining problems? You might end up improving one issue, only to create several more! Oooops! Sorry, yes, of course! Hey, cut me some slack here chaps - it was over 10 years ago! Ayway, yes, from what I remember it was a bodged insert that went in the top of the Sierra upright (AND a new pair of top wishbones!) You're quite right though, just making new top wishbones would simply have changed the camber. It is also true that this arrangement cocked up all sorts of other things geometrically (and wasn't strong enough to drive hard structurally) but as anyone who has suffered the tramlining will know only too well, it can be very noticeable at very low speeds too. As I recall, the eventual move to power steering wasn't so much to reduce the steering effort as to try and introduce a dirty great steering damper into the system. I thought the PAS cars were a little better in that respect.
Hi, it would be just as good a test to run wheels of a different offset, ET55 or similar, thus putting the scrub radius to zero - as negative spacers are hard to machine! Buying wheels just to test seems a bit rash though.
Anyone any ideas ref wheels that might be suitable for testing purpose, on 4 spoke Ford pattern, and available second hand?
If this worked it would give hope for the 20mm longer wishbone idea (if you extend the steering arms it shouldn't affect bump steer), with different wheels.
Nic
Anyone any ideas ref wheels that might be suitable for testing purpose, on 4 spoke Ford pattern, and available second hand?
If this worked it would give hope for the 20mm longer wishbone idea (if you extend the steering arms it shouldn't affect bump steer), with different wheels.
Nic
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