Friday, 2 September 2011

Tourqe is cheap

Well it will be if I can sort out how to stop twisting on an Imps standard drive shafts.They are 3/4" and in cast Iorn,suitable for the standard Hillman Imps 38hp.When the sport version of the engine was released in 1966 the Rootes Car Co,changed the output shafts to a 1" diameter,whcih was sufficient to handle the new output of 55bhp.

Two standard Hillman Imp 3/4"drive shafts,check the top painted one,its showing signs of twisting and stress,clearly used with some sort of gusto in a car with more than the standard engines output.

I have an idea and possibly there is some person out there who can give an answer?

A mail that I sent to the boat designer Dudley Dix,he is also keen on cars and is rebuilding a Mk2 Lotus Europa right now.

Hello Dudley,

I was going to ask Graham about this but you have worked with cars also.

The two shafts are Mk1 and Mk2 Hillman Imp transaxle drive shafts,they are cast and 3/4" 
When the later Sport versions came out the power went up from 38bhp to 55bhp and the shafts were then made as a 1" version,they did not twist as much and were fine for normal use.They are now becoming  scarce and the ones I have are about to be shipped back to Scotland as part of the race Imp I have been building up.

Do you think that a bandage of 50mm biaxial carbon cloth and epoxy will stop the twist,what epoxy grade would I use,it would save a lot of old shafts if I can come up with a plan?



Hi Roy,

 I don't think it will work. You will have the same problem that they had with stainless steel rudder shafts in the 90s when they tried wrapping them in carbon to make them lighter and stiffer. Although stronger on paper, they broke. The problem is that the carbon takes all the load because it is on the outside. The carbon was designed to stiffen but not  to carry the full load, so it broke. That suddenly transferred all of the load to the stainless, which was also not designed to carry the full load, so that immediately broke as well. If you were to do this, you would have to design the carbon to carry the full load and use the cast iron shaft only as a core, or make completely new shafts totally from carbon over a foam core.


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