Rear Wheel Spacing on New Build????

[Carott]

Just starting a build with a BMW R75/7 engine powering a BMW 320i independent rear suspension. <br>

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I need to shorten at least one of the axles to align the output shaft of the transmission with the input of the differential. <br>

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Any input on a suggested track width for the rear wheels would be appreciated.<br>

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Brett

 

[vwbug72501]

What is 320's unaltered track width? Trikes usually run 55" to 60". (Other members please fact check)

How much drive shaft offset do you have with the unaltered irs set up centered on the MC frame? Some driveshaft offset can be compensated for by the U joints.

The reasons for these questions is that I believe that you will run into all kinds of suspension geometry issues when you shorten one half shaft to realign the differential and MC driveshafts. You are going to have either unequal A arms on each side causing weird up and down handling or issues with the location of the A arm pivots vs. the inner CV joint pivot locations causing excessive in and out play in the CV joints.

Just my $.02

Good Luck!

 

[Carott]

VWBUG

I appreciate your reply and the good information.

I am aiming for a direct a line as possible from the transmission output to the differential input to minimize stress on the transmission.

The factory axles are already two different lengths.

My plan is build a new support frame for the differential and create new equal length A-Arms.

This should set me up with equal suspension movement and the variation in axle lengths should not affect how the suspension acts.

Brett

 

[vwbug72501]

What would the track width be with 2 320 short axles installed? If it falls into a typical trike width then a new frame with equal length A arms would be a whole lot easier to design. Also you would be able to use off the shelf parts. (Something to think about when you're far from home and broken.)

Have you looked at the offset angles from the stock configured suspension/diff to the MC trans out yet? If you fall within the operational limits of the universal/CV joints then there shouldn't be undue stresses on the MC trans.

Also consider that a slightly longer wheelbase will generally provide a smoother ride, lengthen the driveshaft, and reduce the CV joint angles.

https://en.wikipedia.org/wiki/Constant-velocity_joint

 

[vwbug72501]

A good resource for suspension design : Google "Formula SAE suspension design resources" (Geek Alert!! This will get technical.)

Here's one that looks interesting:

digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1021&context=mesp