Excessive Understeer

[dick campbell]

Why does my car understeer???.....

it is a 9'6" wheelbase AW90 fitted with a 105 engine. Still running on 90, front springs.
If I look at the angle of the steering arms, they are around 12 deg which projects to an intersecting point at the center of the rear axle.....so you would think that the Ackerman geometry is ok.
Now the 105 engine is some 100lbs heavier than the 90, but surely this cannot be the sole reason. The tyres are standard Xply 19 x 5.5" at 35psi. and I have tried alternative tyres with no improvement.
If I increase the toe in to 3/4" it is better and presumably this helps the turn in. My inclination is to increase the Ackerman angle by making new steering arms but this is a lot of work if someone has a better solution.

In my ownership the car has always had the 105 engine so I have no comparison with the 90 engine. There are many 105 transplants......do they all understeer?

Regards,
Dick

[gordon higginbotham]

Dick,
I had an interesting correspondence on steering effects with David Adams who is an expert engineer on Sunbeams judging by his articles. He lives in New Zealand.

The nub of his advice was that tuning the caster angle is worth investigating. He had found that too much caster angle gave heavy steering on fast corners and at low speed a pothole on one wheel could cause violent wheel shimmy. It was made worse on the car when it was heavily loaded at the back which would have the effect of increasing caster angle.

On my Scout, 9’3” wheelbase, I estimated that rear springs settling by 2” relative to the front would increase the caster angle by ~ 1º. Since Talbots have more caster angle than Sunbeams there seems to be the possibility that reducing the caster angle by a degree or so might be worth trying. I am assuming that the toe-in is correct on your car.

Gordon

[dick campbell]

Thx Gordon.

I've sent him a note.
I'll nvestigate the castor angle.

It would be useful to know the distance between the trackrod ends centers on your car. Mine is 46". I am assuming that the axle beams, steering arms and track rods were common across all cars with the exception that the 95, 105 and 110s were drilled to accommodate wider springs.

knowing this dimension it is easy to work out the Ackerman angle.

Dick.

[gordon higginbotham]

Dick,

I have measured the distance between the track rod centres as well as I can on my Scout and agree with your 46".

Gordon.

[dick campbell]

Thx Gordon,

That rules a lot of things out. I'm going to take it to the local tyre place and see if they can measure all of the geometry and then go from there.

Does your car have neutral steering or is it prone to understeer?

Regards,

Dick

[gordon higginbotham]

Dick,
I am not sure that I am experienced enough to say whether my car understeers or not. What I do notice is that on a long corners the steering becomes heavier as you wind on lock. I did have a problem at low speed when hitting a pot hole. This produced violent wheel wobble. I believe the motorcyclists description of "tankslapper" is not unreasonable. I could not find anything wrong with any of the steering components. I cured the problem by adding a steering damper which was immediately 100% effective and had no effect on the feel of the steering. Its was much later that I was in contact with David Adams and he suggests that too much castor angle could produce this low speed wheel wobble.
With the new tyres I have on the car there is fractionally not enough clearance between tyre and mudguard so putting a spacer of 6mm thickness between spring and axle looks the thing to do and would be easy to have it as a 1 degree wedge.
Regards,
Gordon.

[Martin Lawrie]

I had awful shimmy on Fuggle (AG 14/45 with AD front axle) when I first picked it up. Despite having next to no performance from the engine, I could induce wheel shimmy by going over a white line (nearly!) and it made it virtually undriveable. I investigated castor angle and discovered a neat wedge in each spring pad but which was actually fitted the wrong way. Taking this out made some difference but didn't cure it. Eventually, I checked the toe-in and discovered that it was set toe-out. Once I'd managed to wrestle 1/8" toe-in, I didn't get the problem. What will happen when I actually have some performance, I don't know!

[gordon higginbotham]

Martin,
Michael Marshall's article in the Technical Manual, section 11 - Axles makes it clear that there is no adjustment for setting the amount of Toe-in. The track rod should be straight and is made to give the correct Toe-in. I am not aware of Roesch providing any packing piece between steering arm and hub. What did "wrestling" amount to?
Regards
Gordon

[Martin Lawrie]

Aha!

The track rod end is made up of a hardened cup on the inside which bears on the ball joint, followed by another cup, a spring and the crenellated bolt. I established that a very small amount of material ground off the back of the outer cup gave me toe-in of 1/8". I think it was 1mm each side but can't really remember.

[Guy Gregory]

As an aside, the TOC can now supply new track rod end springs available from Sue Wheildon

[gordon higginbotham]

Martin,
Interesting, but why was it wrong in the first place? Could it be that the axle was put together from pieces that Mr. Fuggle had available?
Gordon

[Martin Lawrie]

The brakes are AD items so I think the whole axle may also be AD. I'm not sure why this would make a difference, though, unless they used steering components from other models (not 14/45). Remember, this vehicle has been through two upheavals: The first when it was put together, alledgedly from parts lying around (I dispute that as the chassis was only three years old) and the second when it was restored in the 80's which may then have needed repalcements for stuff that was past its best having been stored for years.

[gordon higginbotham]

Martin,
I guess with all that history it will prove impossible to get a complete understanding. Did you have a chance to measure the distance between the centre of the track rod end centres?
Regards,
Gordon.

[dick campbell]

Well today I made some shims to go between the axle pads and the springs. I tipped the axle forwards using a 0.1600 shim.
This by calculation equates to removal of around 2 deg of caster angle.

What a transformation.....neutral to very slight understeer.

I cannot believe how sensitive caster angle is.

I will probably wreck the engine now that I can drive like Toad of Toadhall!!

[Alistair Robinson]

Hi Dick, the BA was understeering, I thought it just needed a bit more power through the corners and that certainly helped. I remember that from the AM90, it liked having the juice tap opened up when cornering . Meant you didn't have to put as much effort into turning the steering wheel. I'll try your solution of the shim when I get the car back on the road. I've got gearbox troubles though at present and with being in lockdown I can't get at the car to get the gearbox out to find out what is going on. I'll give you a call when I get it apart.

[gordon higginbotham]

Dick,
That is very encouraging. Have you measured what the new caster angle actually is or what it was to start with ?
Regards,
Gordon.