Steering Geometry

[sanzomat]

I'm very likely to ask a few daft questions here so please forgive me if I'm being thick. There seem to be some very clever people on here so I thought I'd check a couple of things out.

Since I first got my GTM Spyder I've felt that the steering is a bit "slow".

The set up is a bit unusual in that the steering rack is forward of the front axle so the steering arms project forward from the hub carriers and a left hand drive steering rack is used upside down so the car steers the way you turn the wheel rather than the opposite way as it would with a RHD rack the right way up in this location. The rack on the car is a LHD Metro rack with 3.6 turns lock to lock. I've read that an easy way to quicken the steering is to replace this with a LHD MG TF rack as it is a direct swap and is geared at 2.7 turns lock to lock with the same throw. I've managed to find a reasonably priced LHD MG TF rack that I've now purchased but not fitted yet.

First question - with the steering arms projecting foward, to get some Ackerman effect should the arms project outwards from the centre line of the car? All the diagrams I've ever seen of the Ackerman principle show the arms going backwards and inwards along a line from the steering axis to the centre of the rear axle - should this line be projected forwards beyond the axis and the arms still line up with it? I ask because the steering arms appear to be angled inwards and at first glance this doesn't seem right although this may just be that due to the inclination of the steering axis and the offset of where the arms fix to the hubs the actual "virtual" line of the arm is facing outwards and the physical line of the arm is just correcting this from being too much. I'm not proposing to change this, just interested. It would be posisble to make a change easily as the arms are bolted to the hub carrier with one of the two holes offset so it would be easy to simply swap the arms either side of the car and then they would point outwards.

Second question - Can the steering ratio (if thats the correct term) i.e. the amount the road wheels steer for how much you move the steering wheel, be quickened by having shorter steering arms? It seems to me that it should, albeit the steering effort would increase as for any given movement of the rack the the steering arm would go through a greater angle. I ask because it seems that GTM supplied two different lengths of steering arms in the kit and the longer ones are fitted to my car and I found the shorter ones in one of the many boxes of spare bits that came with the car. If so then I could quicken the steering by just swapping the arms over. I guess there is a risk that the wheels could turn too far for the same rack throw so stops might be required. Clearly the rack ratio wouldn't change but the number of turns for any given steering effect would reduce?

Third Question - Having read a few threads on GTM forums (mostly over ten years old when most of these cars were being built) there seems to be the suggestion that if you change the rack for an MG TF one you should also change to the shorter arms at the same time "to preserve the geometry". I'm struggling with this as surely just changing the rack without changing the arms would do nothing to the geometry, just make the rack throw more for any given steering wheel movement and nothing else should change? It seems to me that if both are changed the steering will get much quicker due to the combined effect of both changes (and much heavier too).

As I've now bought a new rack my thinking is that I'll swap the rack but not the arms as in my mind this will keep the geometry as it is (as long as I get the track rod ends the same distance apart as they are now) - have I missed something???

Sorry for a long post. Any wisdom would be gratefully received.

[SPAXIMUS]

What I would do is give Dave Gallop a call at Track Developments and speak with him. He will have all the answers if you have accurate information.

The design of the car will give decent geometry otherwise they would not handle at all. The way the arms point is irrelevant in a well designed system as many cars have front arms, all the Westfield have arms at the front for example

[Camber Dave]

Here we go. Sit up straight - pay attention

Since I first got my GTM Spyder I've felt that the steering is a bit "slow". Many, many cars have steering systems that deliver 20 Deg of inner wheel movement for 1 turn of the wheel. The outer wheel turns 16 to 18 Deg. A quick rack produces 20Deg for just ¾ turn.

The set up is a bit unusual... I've managed to find a reasonably priced LHD MG TF rack that I've now purchased but not fitted yet.

First question - with the steering arms projecting foward, to get some Ackerman effect should the arms project outwards from the centre line of the car? Yes All the diagrams I've ever seen of the Ackerman principle show the arms going backwards and inwards along a line from the steering axis to the centre of the rear axle This gives 100% Ackerman – Far more than normally needed See above. should this line be projected forwards beyond the axis and the arms still line up with it? Yes
I ask because the steering arms appear to be angled inwards and at first glance this doesn't seem right Subject to drawing the layout or checking the car, this would appear to cause Anti Ackerman with the outer wheel tuning more than the inner causing tyre scrub! although ... I'm not proposing to change this, just interested. It would be possible to make a change easily as the arms are bolted to the hub carrier with one of the two holes offset so it would be easy to simply swap the arms either side of the car and then they would point outwards. If you turn the arms over the Track rod ends will be on the opposite side of the arm (too high or low) and cause huge Bump Steer.
With conventional double wishbone suspension as the wheel bumps it also increases NEGATIVE camber. This is to offset the POSITIVE Camber caused when the same car rolls in a corner. Bump steer is caused when the steering track rod (a different length than either wishbone) follows an arc that pulls or pushes the road wheel as if steering the car ie Bump > Steer!

Second question - Can the steering ratio (if thats the correct term) i.e. the amount the road wheels steer for how much you move the steering wheel, be quickened by having shorter steering arms? Yes as long as there clearance for track rods etc.  It seems to me that it should, albeit the steering effort would increase as for any given movement of the rack the the steering arm would go through a greater angle. I ask because it seems that GTM supplied two different lengths of steering arms in the kit and the longer ones are fitted to my car and I found the shorter ones in one of the many boxes of spare bits that came with the car. If so then I could quicken the steering by just swapping the arms over. I guess there is a risk that the wheels could turn too far for the same rack throw so stops might be required. Clearly the rack ratio wouldn't change but the number of turns for any given steering effect would reduce? Yes See above

Third Question - Having read a few threads on GTM forums (mostly over ten years old when most of these cars were being built) there seems to be the suggestion that if you change the rack for an MG TF one you should also change to the shorter arms at the same time "to preserve the geometry".  I'm struggling with this as surely just changing the rack without changing the arms would do nothing to the geometry, just make the rack throw more for any given steering wheel movement and nothing else should change?
It may be that the MG rack has a different mounting height so revised arms may be needed to avoid bump steer. It seems to me that if both are changed the steering will get much quicker due to the combined effect of both changes (and much heavier too).

As I've now bought a new rack my thinking is that I'll swap the rack but not the arms as in my mind this will keep the geometry as it is (as long as I get the track rod ends the same distance apart as they are now) - have I missed something???Check the height of each rack centre to mounting height: re bumpsteer as above

Sorry for a long post. And Much longer reply...

[Brandy Barrel]

My westie is so totally the opposite, sensitive and oversteer I have never come across before.

[Iancider]

A visit to Track Developments is well worthwhile - Dave produces transformations in handling.

Give him a call on 07977 948883 - his workshop is just North of Castle Combe. 

[Swampy]

So to bring back a post from last year. Now my car is nearing completion and has had a couple of outings on the road its clear that i can now start to play around with parts on the car and the steering geometry is certainly one area that needs attention. Whilst building the car the need to have components on the chassis and the budget sort of dominated the build . I took into consideration so elements of future proofing but was going to look into them when the car was finished. Much like your GTM i have a poor steering ratio with a lot more effort being put into the steering wheel then is generated at the hubs.
I also was aware from the build that my ackerman angle was not perfect to say the least, however i knew it could be modified at a later date.
I have my rack mounted behind the front wheel axle and as such my steering arms point inwards towards the chassis. I have yet to find a ideal set up for the ackerman angle but am aware that the angle is currently decreased leading to the outside wheel (in a turn) is toeing in more than the inside wheel, this is leading to a lot of scrub.

Ive measured this (crudely but cheaply to give some idea of the situation) by sticking some tape on the floor parallel with the wheels using a piece of wood to keep a straight edge.

Then turning the wheels to full lock i have stuck another piece of tape on the floor (again with a piece of wood as a straight edge) and using these two pieces of tape you can see the angle achieved by each wheel on a full lock turn.
Now you could do this with string or chalk but the genius part about using the tape was that you could then peel it off of the floor and lay it on top of one another to measure the difference in angle between the LH and RH side of the car.

In my case the inside wheel had rotated 22 degrees left and the outside wheel had rotated 26 degrees.

This steering angle on the outside wheel is currently causing the wheel to scrub heavily and when driving i believe it is adding to an over centre steering feeling when approaching full lock. Steering has an amount of self centring which suggests that the caster in the upright is apparent however the akerman is overpowering the sensation in tight turns.
With regards to the lack of response in from steering wheel input at the same time as adding more angle to the upright toe arm i intend on shortening the length also. Un fortunately unlike your GTM arm my uprights are cast and need cutting and welding. I am happy for this to be done as have access to a good welder and with correct heating of the parent cast material and cooling i am confident that the welds will not fail.

My problem now is how to work out what ackerman angle will be a good or ideal base and how much to reduce the arm for the steering to feel quick but not twitchy.
I have been reading the speed pro series "THE SPORTS CAR AND KIT CAR SUSPENSION" book throughout the build and will be looking to this along side outside knowledge for input into these decisions. I have a spare set of uprights that i can modify but as welding cast isn't ideal die rather only do it once.



How have you got on with making any changes to your GTM?

[sanzomat]

As others have said, it is worth talking to/visiting Camber Dave. I had a few quick quick chars with him that were reassuring about the approach I was taking. He isn't using this forum anymore but you can get his contact details direct from the Track Developments site.

I went ahead and swapped over to the TF rack with new track rod ends but didn't change anything else other than alter the toe in slightly.

My ackerman feels about right but I haven't measured it. I checked the relative angles and there certainly is some so not "anti ackerman".

The weighting feels great now whilst driving and is more positive without being over sensitive. It is a bit heavier at parking speeds but not unmanageable.

Good luck sorting yours.