Sumo Caster

[bluetoon]

Hi Guys,
I need to make some adustments to the front suspension to improve self centering.

Does anyone know how to measure the castor angle?.

I would like to know what it is now then does some changes and see the effects.

Regards
Jim

[chedz]

I measured mine as best I could but it wasn't exact.

To do it I jacked the car up level, then set plumb lines down from the centre of the hub, then using a vast array of clamps I attached straight edges to the hub in the same angle as the axis of the rotation (you end up with a triangle). Using a bit of pythagoras was able to work out the caster angle.

another interesting thing to do it draw a line across where the plumb line hits the garage floor perpendicular to the direction of travel, then turn the wheel and draw marks at regular intervals on the floor where the caster angle line hits the floor. You should then get an arc on the floor. As you approach full lock you can see the caster angle reduces until eventually the arc crosses the straight line and so you go from self centring to steering tightening. I suspect that the change in direction of the car as you move forward when driving would complicate this a bit and make it a bit better than it appears (more complex than I could calculate) but it shows how with insufficient caster you can end up running out of angle before full lock is achieved.

Hopefully this is clearish.

I still have a load of arcs drawn on my garage floor to remind me of happy times in the past....NOT!!!!

Cheers

Graham

[bluetoon]

Hi Graham,
Thanks for that, I think i am getting a better idea now.

Can you have a look at the photo and check my understanding is right about where to get the lines from.

How much castor angle is about 'right' for the sumo. Or is it a case of "it self centres now" so that will do?

Cheers
jim

[chedz]

Spot on.

I think its a case of more is better and don't think you can have too much really. There isn't a lot you can do other than move the top wishbone back by trimming the chassis tube down at the front so too much is unlikely. (or if like me you get really really desparate get some of Petes office furniture, cut it up and make new wishbones).

One thing to watch out for, make sure there is still slack on the bottom ball joints as the more you lean them over the closer you get to the stop at the back, I ended up putting washers at the front of the lower ball joint plate to angle them back slightly.

Cheers

G

[sumoboy]

5 to 6 degrees is the aim

[gareth08]

Originally Posted by chedz

Spot on.

I think its a case of more is better and don't think you can have too much really. There isn't a lot you can do other than move the top wishbone back by trimming the chassis tube down at the front so too much is unlikely. (or if like me you get really really desparate get some of Petes office furniture, cut it up and make new wishbones).

One thing to watch out for, make sure there is still slack on the bottom ball joints as the more you lean them over the closer you get to the stop at the back, I ended up putting washers at the front of the lower ball joint plate to angle them back slightly.

Cheers

G

Theres loads you can do just depends on how much time or money you have and how far you want to go

[chedz]

Ah yes, I forgot about that, you can always redesign the entire front suspension like Gareth has

My comments were refering to mortals like myself who only have an angle grinder and a bit of string available.

[Revets]

Hi,

An alternative way of measuring the caster that looks easier is described here - How to: Measuring Caster and Camber - How To - Circle Track Magazine.
They use a bubble gauge to measure the angle the wheel tilts, but with a plumb line and some trig you should be able to work it out OK.

Good luck,

Steve

[bluetoon]

Thanks Graham & Steve

Jim