Hot Rods adjusting wheelbase with a triangulated four bar?

A good friend has a triangulated four bar suspension on the rear of his '33 chevy. He needs to move the axle forward about 1/2". The lower bars are level and it stands to reason that shortening the bar 1/2" will accomplish what he is looking to do. But the angled top bars are a different matter. Is there a set ratio for shortening the top bars and not change the pinion angle? By that I mean if you turn the ends say 6 turns in on the lowers how many would the top bars need?

I have never dealt with a triangulated four bar so my knowledge is practically nil.

Thanks

 

Ted,your thought process sounds valid,I would think dropping the drive shaft and determining the angle at the universal joint you should be able to adjust the top bars to bring everything back to alignment. HRP

 

Measure the angle of the upper bars to a line parallel to the lower bars. I don't mean projected horizontally or vertically but in the actual plane of the upper bars. This isn't always the same but if the axle is set up properly it should be somewhere between 25° and 45°. Divide the distance you want the axle to move by the cosine of this angle. That'll give you the amount by which to shorten the upper bars. It should be around ⅞"-1".

 

Simple trigonometry (assuming the uppers and lowers are in the same plane. If the uppers are both angled horizontally and vertically, then the math gets a bit more complicated). If they are in the same plane, you would multiply the amount you shortened the lowers by the ratio of their lengths. For example, if the lowers are 20 inches long, and the uppers are 25 in, and you shorten the lowers by 0.5 in. You would shorten the uppers by 5/8".

0.5 * (25/20) = 0.625

This isn't 100% accurate as it assumes no angle change. Because there can't be a change in length of of the distance between the mounts (3rd leg of the triangle), there will be a small angle change. However, this method should get you close. Check pinion angles before and after and adjust as needed.

Good Luck.

 

That'll only work if the upper and lower bars have the same projected fore-aft length.

 

True. You need to measure the lengths of the triangles as created in the diagram below.



A setup like this would be alot more difficult to calculate the distance needed on the back of a napkin.



Or screw it. Measure the pinion angle before and after and adjust as needed.

 

I'm just a dumb Southerner and I don't have a degree in engineering or quantum physics but I believe this job can be accomplished by the average hot rodder.

Probably why I stick to Henry's design and leave the none traditional suspension to guys that know more about it than me. HRP

 

Like the latter part of frozenmercs post...Don't over think it, get the bottom set, use an angle finder on the pinion, adjust the top bar lengths equally until the bolt slides in.

 

Like Roadsir said it's not that hard. Take the top bars loose. Shorten the bottom bars the desired amount. Reconnect the top bars and set pinion angle. You're done.

 

Yep, when you get into higher math, quantum physics and the like I am pretty lost but I do understand how an angle finder works. Thanks to all for the replies.