Anti Dive Roll Bar Spacers (Revised castor angle)

[busheytrader]

Page 26 of the latest TR Action shows modified anti dive blocks from Hamilton Motorsport that push the a/r bar forward increasing the castor angle with associated benefits. Anyone had a look at this? Any thoughts?

www.hamilton-Motorsport.co.uk

[saabfast]

Would that not 'twist' the TCA in the bush where fixed to the chassis? I can't even get poly bushes into the connection of the ARB to the TCA, must be difficult if you are also trying to pull the TCA forward at the same time.

[busheytrader]

I've had to take the arb off the wedge a number of times over the years and don't use ratchet straps to pull the ends into the track control arms.

I use the weight of the car on its wheels and tyres to align the TCA's and ARB, so long as the engine and gearbox are in place. With the TCA's close to horizontal, the holes for the ARB and its bushes are in the right place to just slot the arb's ends in and position the bolts for the ARB anti dive blocks. Yes, it does mean you're on your back with little or no room under the wedge and then jacking up the front to fully tighten up the bolts.

[Hasbeen]

I'd be very careful putting extra castor on a 7. They already have pretty high castor & increasing it is likely to lead to a new wheel wobble under braking, particularly if the toe gets out very much as well.

The crew accidently put too much castor on the F1 Brabham Repco before a Bathurst meeting. The thing was wobbling so hard at the bottom of Conrod Straight, braking from 180 MPH it was almost tearing the wheel out of my hands. Fortunately the wobble only reached it's worst at under about 80 MPH.

At the time we had no idea it was caused by high castor. We only found why the car was doing it, when we found the high castor at the next wheel alignment. Correcting the castor fixed the wobble.

Castor only really does much at slow cornering speeds, when you have a lot of lock on, so unless your hard driving is all around hairpins it won't be much use.

If you have, or develop a new wheel wobble at a lower speed than our usual balance wobble, at around 45 to 52 MPH, look for excessive toe or castor. It is a different type of wobble, being high magnitude, but considerably lower frequency than the balance wobble around 60 MPH.

Hasbeen

[Stag76]

I built something similar when I fitted PAS to my last TR7.
I used washers on the ends of the bar where it fitted into the lower control arm
to adjust it.
It improved the steering and tracking.

[Rich K]

I think I am correct in thinking that increased castor angle should improve the self centring of the steering. I have always found that the 7 steering does not self centre very well in comparison to some cars.

[trsforever]

Hi, I have seen these before but I think its possibly too much and as Hasbeen said "you can have too much of a good thing", but I agree
with Rich that the self centering is not good, I want to get a stiffer roll bar made and will have the shoulder/step/threaded end made to
allow the wheel to move forward more, then it can be set up both sides the same using washers as shims, similar to Stag 76's adjustment
method.

Regards Scott

[Stag76]

The stresses from braking and turning over the last 40 years are all contained within
the sub-frame, and, as it fatigues, the caster angle is eventually reduced.
The self centering/tracking effect is reduced first, followed by the wheel wobble.
Restoring or increasing the caster angle will fix this, but will also increase the
steering load by a considerable amount.
This is probably one of the reasons that forum members have different steering
loads, some heavy and some light. Other reasons are due to lubrication.

[Rich K]

So if I understand it correctly, if you increase the castor angle the steering will get heavier. I guess that is not such a problem if you have power assisted steering like most modern cars but not such a good idea with non power assisted TR7s.
Rich.

[saabfast]

Simple illustration of castor is the supermarket trolley. The wheel is mounted at an angle back from the vertical pivot line. The more castor angle (ie the further back the wheel is from the vertical line) the stronger the castor or self centering action and therefore the more effort required to overcome it and turn the wheel.

[FI Spyder]

That's true but you only need a little I'm thinking. As it approaches zero everything must be perfect and perfect doesn't last for long.

[sonscar]

I would find it interesting to measure the strut tube angle on several cars and see what the castor is.Steve

[Stag76]

You can measure it using an inclinometer.
Attach it to the axle, wheel or brake disc. (Most are magnetic on all 4 sides and the back)
Turn the wheels 20 degrees to the left.
Zero the inclinometer.
Turn the wheels 20 degrees to the right, and the caster angle will be shown.
Camber angle is measured by zeroing on a flat surface under the wheel, then attaching to the wheel
and reading the angle.

[Hasbeen]

Triumph always used a lot of castor. You need it to achieve the very tight turning circles they liked. High castor gives you a lot of negative camber on the outside wheel, at high road wheel angles, helping with tight turns.

The self centring force only increases as your speed increases, & you don't need much lock to turn the curves you can take at higher speeds, so the effect on steering weight diminishes. Triumph did this pretty well, with the castor set quite high, but not too high. Much more than stock is going to combine with any slop in the steering gear, or any excessive toe, in or out, to make driving the thing quite unpleasant.

Hasbeen

[prackers]

If I am understanding this correctly the standard 7 caster is 3 1/2 degrees + or - 1 degree? I checked mine and it was virtually neutral, which probably explains why it didn't really feel "planted" in a straight line. Since reading this I made a pair of modified (offset) ARB lowering blocks, which pull the ARB forward 20mm giving approx. 4 degrees of caster, this has made a massive improvement to the straight line stability but it still doesn't feel quite right into the corners, insomuch as it tends to "dart" into the corner as you start to turn the wheel rather than a smooth transition. I am assuming that moving the ARB forward has had some effect on the camber? My question is should I play some more with caster, I can move it backwards or forwards 10mm either way or begin playing with the camber? I have some adjustable TCA's but not fitted yet.