chassis build 101 question

This might rate as a dumb question but can anyone tell me the best way to set up a repro A front c'member. Ive made a pair of tapered rails from RHS 4X2 (100x50mm) but how do I make sure that the cmember is dead straight iside the rails as they taper forward.

Cheers


Steve P

 

I always establish a center line and build off of that. Run a string line down the center. Measure from the center point of the rear crossmember location to the ends of the front crossmember. When they match the front crossmember is square with the centerline. Do the same for the rear cross member. The taper of the rails can then be matched by measuring off the string line at different locations along that line. When it's all set up the diagonal measurements from corner to corner should be the same.

There are lots of way to do it but establish a center line and measure off of it.

 

Tommy makes some good points and I'd like to add - when you cut the repro crossmember for the frame, it's a double angle cut.
One angle due to the frames taper and the other due to the negative caster of the crossmember proper.

Aside from being very careful in the measuring and cutting, I suggest you cut the crossmember 1/16 - 3/32" oversize on both sides and use a small (4") grinder to bring it to the finished size.

My 31 on 32 rails roadster has the crossmember set at 8 degrees negative. (Kingpin tops to the rear. I wrote positive last time and I was wrong. Hope I'm not wrong again. )

The 32 frame rail has a 2 degree kickup near the cowl front and once it's up on wheels you'll end up with a bit of a downhill rake on the stance.
About 2 degrees or a little less depending - tire size, drop front and rear etc.
That allows for a 6 degree negative caster on the axle and the front transverse spring operating straight up and down within the crossmember. The up and down bit measured against the crossmember and not the ground.

 

Thanks for the help guys. So reading your comments C9
Are you saying I should add a couple of degrees to compensate for the extra rake made by the rear wheels?
The weight of the engine shouldnt affect the caster as it just pushes straight down - is that right?

Thanks again fellas

Steve

 

If its leaned back a little much, it will go down the road good, maybe a little harder to park. If its too much the other way, hard to keep it in yer lane, but turns easy while not moving.........OLDBEET