Upper Trailing Arm for Spridget

M

mg_john

Well-known member
Am trying to figure out how to mount the chassis end of an upper trailing arm for ther rear of a spridget. The axle end is mounted to a welded tab on the top of the rear end housing. The two lower arms are to be mounted at the original spring hanger location. All three arms are about 16.5" in length and have heim joints at each end. Will use a Watts linkage for side to side location.

Anybody have ideas, experience, pictures, diagrams?

Tie in to the upper part of the tunnel? Make cross bar to tie in with the original lever action shocks locations?

I really don't want to mount to sheet metal with self-tapping screws!

Thanks,

John
 
The 3 link is the way to go....no roll steer effects at all.

You might want to rethink the Watts linkage unless you know EXACTLY where the rear rollcenter needs to be. Panhard bar allows you to easily move the RC up and down during the tuning process. Tuning the watts link RC is FAR more difficult.

Just another dumb engineers opinion. Jay

PS- I built my H car with provisions for panhard bar, vertical watts, horizontal watts...all 3.....but never left the panhard because of it tunability ease.
 
Unsure of your specifics, but if memory serves me, it goes something like this:

To get equal length arms, the pivot will be forward, beyond the bulkhead;

To be equal distance in the vertical, the pivot will be on top of the tunnel.

The specifics may vary depending how much the chassis is lowered, and exactly where the pivot is located on top of the axle housing.

Try to get the axle housing squared to the block for u-joint phasing before securing the top, front pivot, or the arms will be unequal length by the time you're all done.

The top, front pivot bracket is usually fabricated from 14-12 ga. mild steel, bridged across the top and secured to the tunnel with heliarc or braze over a 3" x 6" area on both sides.

There was a BMC booklet showing how Group 44 did this on the MGB. They drilled the mounting pad with 3/8" holes on 1" centers, and brazed it to the tunnel. It looked pretty ugly, but worked on the MGB. Probably much fitting to get the pads on each side of the bracket to contact the tunnel, but distributed the forces over a large area.

Then some sort of fabricated cover as this ends up with a suspension member in the drivers compartment.

Try to find a copy of the BMC booklet. Other good stuff in there.

RJS
 
For a car with as little power as a spridget I feel the length and angle of the top center link is basically irrelevant....there are no anti dive or anti squat effects worth considering.

The 2 lower links must be absoulutely parallel to the ground at ride height. Think this through in your head or on paper....you want both lower arms to get effectively shorter by the same amount (otherwise the rear axle steers left or right a little instead of pointing perfectly straight) when the car rolls. This can only happen when they start out parallel to the ground at ride ht.
 
This implies either the front(easiest) or rear pivot location must be adjustable vertically.....I guess that's obvious.
 
Classic connection for the center "upper" arm is to weld a tab to the center top of the axle banjo and another tab on the center of the drive shaft hump inside the cockpit. Tab should be long/wide enough to allow enough weld area for the connection.
 
John,

For what it's worth, I did a lot of research on rear suspension development on our Spridget, and got a lot of advice from a number of people who know far more about these things than I do. The consensus was that messing with the rear (other than a Panhard bar) generally caused more problems than it solved in terms of overall handling, and I chose to focus any experimentation on the front end. However, those of us who are tired of looking at your car disappearing into the distance would be glad to encourage any such questionable modifications!

James Wiley
#72 HP Midget
 
Thanks all and Mike Miller (offline) for your help.

Here's what I settled on:

Fabbed a 1" square tube across the rear bulkhead and tied it into the original shock mount holes on both sides using 1/8 plate welded to the tube. The shelf tray had already been cut to allow the upper trailing arm to extend into the passenger compartment, so welded a 2" wide 1/8 strap onto the tube which extended just beyond where the front heim joint was. Cut a piece of 2" square tubing to make the heim joint mount and joined the 2" tubing, the strap welded to the 1" tubing and another piece of 2" wide 1/8 strap bent to conform to the tunnel with 3/8 grade 8 bolts (six of them). Backed the underside of the tunnel with another piece of strap and large washers. Hasn't moved yet!

The pinion angle is the same as the angle off the back of the tranny - less than 2 degrees different with my primative measuring tool.

Covered the whole thinng with a box of sheet aluminum painted to match the car body.

Headed to CMP this weekend. I'll post a post race analysis on the whole thing.

Thanks again.

John
 
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