Short stroke engine

[chasbmw]

Web site is gone, also came across sales details for the business, I think that the classic bike article was some time in the mid 90s.......Great pity, the business was started in the 1920s.....

[R85/8]

Been trawling the 'net for more info.

It seems the the big bore short stroke is unreliable if done on a R65 base, but generally those have all been conversions to optimise the power output. However I'm starting with an R100, and I'm not chasing max power.

It has occurred to me that doing this and using longer conrods would mean less alteration to other components, as would pistons with a higher crown (or a combination). Anyone tried that approach? My drawings suggest there's enough clearance to do this.

If I remember right the original R75/5 etc used the same conrods and journal sizes as a BMW car of that era, so maybe there will be the answer in a BMW car part. (Correct me if I'm wrong, this is an old memory here)

Otherwise who will make custom conrods these days? Shiny Titanium maybe

Edit: just been checking the price of custom con rods. Looks like this would be the cheapest way of doing the job - it would save a lot of machining work elsewhere, and no worry about rocker/pushrod angles etc. So it just becomes a job of getting the right spec on the conrod to maintain balance.

[the quinner]

Been trawling the 'net for more info.

...snip...

custom con rods. Looks like this would be the cheapest way of doing the job - it would save a lot of machining work elsewhere, and no worry about rocker/pushrod angles etc.

Exactly.

[R85/8]

Exactly.

Anyone done it this way? The con rods do not need to be much longer, only about 4.55mm, so a custom rod could be built to balance the same.

[RickR90s]

At 70.6mm, I thought most Airheads WERE short stroke motors.

RPG

[the quinner]

At 70.6mm, I thought most Airheads WERE short stroke motors.

RPG

It depends...an R50/5 is undersquare (0.95:1)...relatively speaking, that's a long stroke. An R100 is oversquare (1.33:1)...relatively speaking, that's a short stroke.

[Major Softie]

At 70.6mm, I thought most Airheads WERE short stroke motors.

RPG

It depends...an R50/5 is undersquare (0.95:1)...relatively speaking, that's a long stroke. An R100 is oversquare (1.33:1)...relatively speaking, that's a short stroke.

How short is short? Yamaha R6: 65.5 × 44.5 mm - 1.47:1. R1: 1.49:1.

Formula 1 cars run up around 2.5:1. Now THAT'S short stroke.

The question is: is any oversquare engine short stroke? Is "square" the measure of whether or not a motor is short stroke, or is the measure of whether or not a motor is "short stroke" dependent upon whether or not it is shorter than the average motor of the age? I certainly don't know the answer to that question. I don't pose it as a rhetorical question, but as a genuine one. Boxers tend to be shorter stroke motors. Every additional mm of stroke doubles the width, so they tend to be shorter strokes. On a motorcycle this is especially important, but it's not a design issue that can be ignored with VW's, Porches, or Subarus either.

[Armaguidon13]

Sheck and other BM offroaders have tried short stroke for off road

This is a french discussion about that

http://www.flat-twin-bmw.com/t549-moteu ... rse+courte
Sorry for the language...

[R85/8]

There's loads of theory to digest, but the bore/stroke ratio generally is only really important when you are getting up to high piston speeds. I doubt I'll ever be exploring those limits on a BMW

This conversion will be around 1.52:1

It would be good to hear if someone has already done this conversion using longer conrods because I would like to know if there is an existing conrod I can use before I order a custom built one - although that does give me an excuse to dream about shiny titanium rods (be worth more than the bike ) before sanity prevails.

[barryh]

I have no expert knowledge of this but have you considered the influence of stroke/rod ratio on torque characteristics.

http://www.stahlheaders.com/Lit_Rod%20Length.htm