Castor

[THR]

how do you check it?

single seater racing car
suspension travel of about 20mm
8 and 10 inch wheels.
we know wot the castor should be.. from the manufacturer. somethink like 7deg i think
and we guess.. well the only way you can change it is by the rod ends.
The hubs have a flat top and square sides.

how do you pysically check it tho?

[bobdar]

There are instruments that measure castor, the most common is the Dunlop gage. It compares the change in angle as the steering wheel is turned, and reads out the castor angle. There are also electronic instruments. Alternatively, you may be able to build a fixture that can be held against the top and lower connections on the upright, then use an electronic level. I have made one for my rear uprights, and it is simply a triangle of aluminum with two "studs" (one is adjustable) protruding out that index against the upright casting at the top and bottom. One my vintage FF, the front is a bit more of a challenge, so I simply borrow a friend's Dunlop gage.

[Moving Chicane]

C/C gauges are readily available from any number of sources (Intercomp, RebCo, Longacre, etc.....}. Smart tool (digital), will work as well. Most all will range between $90 and $130, your preferance. The trick in using this method is being able to reference your steers against the rear axle aim. The need for the reference is to make the required turn sweep of the wheels {i.e. 20 degrees one way and 20 degrees the other way}, most of the bubble types are calibrated for 20 degree sweeps. Obviously anything short will result in an error. I prefer to go digital; as those bubbles change in size with temp, and then use a formula that allows me to use any degree of sweep.

Caster = 180/pi(T2-T1) x camber change

Check your toe after a caster change has been made, it will be affected.

[Tony Harman]

Hi Paul
Good ol' Castor again eh...
I've played around with various measuring devices. The thing to remember is castor is a fore aft measurement but difficult to take directly without some kind of adaptor to fit parallel to the upright accurately so all the guages on the market use a change in camber angle measurement to calculate castor. If you look at any camber / castor guage you will see that the camber scale and castor scale are different, I am sure now that this is because they do a calculation to extrapolate the camber measurement at 20 degrees, which is what they all recommend, to give castor. I have yet ot be able to find out what this calculation is as it could be quite useful, (especially if you don't have 20 deg of lock available...). I have yet to find that castor radically effects handling, in the way camber or ackerman or tracking do, I think if it is in the ballpark and equal on both sides then you shouldn't have a problem.
I have just bought a Smart Level from the States for $60 ~ £40 - heck of a lot cheaper than over here, and it has already proved really useful.

[bobdar]

Rear castor affects bump steer. Obviously, measuring rear castor requires a fixture of some sort. I also use a Smart Level, it's really a very handy device.

[Tony Harman]

Good point Bobdar, castor on the rear effects bumpr steer setup. On my car I have twin bottom arms out to the upright so I simply put the level across those to measure castor.

[Griff]

Hi Tony
just a thought about your lower arm system for
checking Castor, The lower wishbone
inner pick up points,the rake of the chassis and the top wishbone position all have a bearing on the Castor angle,
All readings are best taken directly off the upright.
I have allways found though that Castor being a steering angle used mainly for stability and feedback has no real
effect on the stability or handling of the chassis when used
on the rear upright.
Hope this help's but if anybody has had any different feedback from testing I'd like to know. Allways willing to learn, enough of this rabiting hope your season goes
well Tony.
GRIFF.