Another short stroke

A word on the crank. As an opinion, it cannot be more, but it is unlikely it was in the used short stroke motor purchased from Thruxton!

I say this because I shared the picture with Norman White, and he says he never saw it before! I don't think much happened in the race shop he didn't see!

My guess at the moment would be that someone in France made it to go in that motor! But again, it is no more than a guess.
 
SteveA said:
A word on the crank. As an opinion, it cannot be more, but it is unlikely it was in the used short stroke motor purchased from Thruxton!

I say this because I shared the picture with Norman White, and he says he never saw it before! I don't think much happened in the race shop he didn't see!

My guess at the moment would be that someone in France made it to go in that motor! But again, it is no more than a guess.
Click to expand...
I should Ask Martin ! I know a Friend of him.
 
SteveA said:
A word on the crank. As an opinion, it cannot be more, but it is unlikely it was in the used short stroke motor purchased from Thruxton!

I say this because I shared the picture with Norman White, and he says he never saw it before! I don't think much happened in the race shop he didn't see!

Well, Mr. White might have a closer look at the picture as a similar crankshaft was fitted in the 1974 Space frame works racer ( https://shedtime.wordpress.com/2021...eum-in-best-the-netherlands/#jp-carousel-2213 )
Click to expand...
 
It is probably impossible to know what something such as a shorter stroke crank in a Commando motor actually does, until you work with it and adjust everything associated with it, to suit a race circuit which you know very well. A short stroke crank usually runs at higher revs and causes the motor to be peaky. That probably suits riders who are situationalist rather than systematic. But the faster we go. the more being smooth and systematic becomes important.
I suggest the long stroke Commando crank is an alternative concept to conventional thinking. With close ratio gears and high overall gearing, it works very well.
 
That is really interesting. Norman said he has never seen this crank before. So I guess that also means he didn't make it, or see someone make it.

The picture shows the crank outside the motor. So I guess another crank is in the bike?

So there is another crank similar to the one in France, did you measure the stroke?

Regardless, this indicates at least two of these cranks were made, but where?

In that picture there are other parts, short stroke style pistons and a standard alloy rod that would not fit those pistons unless they were machined from blank for the smaller gudgeon pin size of that rod. I have a pair of those pistons and ran them in a 850 with a short stroke head. The rods were standard alloy and the crank and cases were MKIII. The picture is me on that bike at Cadwell Park in around May of 1976.

Norton ran similar 850s in UK 501 to 1000 races in '74, and Dave Croxford has said the space frame with an 850 was the best configuration he had ridden.

It is said that Norton did not actually race a 750 short stroke! Which may be true, and certainly externally you would not have known which crank was in it.

Is it possible to get more information from the museum?
 

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  • Another short stroke
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acotrel said:
It is probably impossible to know what something such as a shorter stroke crank in a Commando motor actually does, until you work with it and adjust everything associated with it, to suit a race circuit which you know very well. A short stroke crank usually runs at higher revs and causes the motor to be peaky. That probably suits riders who are situationalist rather than systematic. But the faster we go. the more being smooth and systematic becomes important.
I suggest the long stroke Commando crank is an alternative concept to conventional thinking. With close ratio gears and high overall gearing, it works very well.
Click to expand...
Whatever may usually be the case, a 750 short stroke Norton is not peaky and doesn't really rev that much higher than a long stroke!
 
SteveA said:
That is really interesting. Norman said he has never seen this crank before. So I guess that also means he didn't make it, or see someone make it.

The picture shows the crank outside the motor. So I guess another crank is in the bike?

So there is another crank similar to the one in France, did you measure the stroke?

Regardless, this indicates at least two of these cranks were made, but where?

In that picture there are other parts, short stroke style pistons and a standard alloy rod that would not fit those pistons unless they were machined from blank for the smaller gudgeon pin size of that rod. I have a pair of those pistons and ran them in a 850 with a short stroke head. The rods were standard alloy and the crank and cases were MKIII. The picture is me on that bike at Cadwell Park in around May of 1976.

Norton ran similar 850s in UK 501 to 1000 races in '74, and Dave Croxford has said the space frame with an 850 was the best configuration he had ridden.

It is said that Norton did not actually race a 750 short stroke! Which may be true, and certainly externally you would not have known which crank was in it.

Is it possible to get more information from the museum?
Click to expand...
Well, it makes 3 of these, 'cause Martin has one too 😉
 
How much crankpin offset difference is there (generally) between long stroke and short stroke commando engines? I'm curious as to how small of a change is made and how much of a difference it makes... and if the difference is a known result (meaning, you gain more of one thing at the expense of something else...)
 
A very short stroke motor can make a motorcycle more difficult to ride without making mistakes. Whatever the stroke might be, the gearbox must accommodate it. When developing the bike, I only ever do it on one circuit which I know very well. Even then, it is not possible to know how well I have done, until I actually race the bike.
It is like a lot of these things, there are sets of variables which interact to give an effect. When you change one the rest are often adjusted to suit, and you can end up in almost exactly the same situation which you had before you began making changes. In my case, the major problem was I did not know what the Seeley frame could do. I only found out by systematically advancing the transition point in corners during 3 races at one meeting. Then I simply flicked the bike into a corner while braking behind the 3 leaders, and blasted under and past them.
I am and was too old to be doing those sorts of things. But it was so easy - I kid you not. I do everything slowly, even the fast stuff.
 
SteveA said:
Whatever may usually be the case, a 750 short stroke Norton is not peaky and doesn't really rev that much higher than a long stroke!
Click to expand...
Where is the advantage ? I loved my 500cc short stroke Triton. It was the nastiest motorcycle I ever rode, but I was too lousy to buy a 6 speed gearbox for it. I used to choose where I was going to lose a race. I only ever really lowered the gearing on it once, and I knew what was about to happen. I had to stand it up in a corner and spear-off to miss three guys who had passed me towards the end of the preceding straight.
 
o0norton0o said:
How much crankpin offset difference is there (generally) between long stroke and short stroke commando engines? I'm curious as to how small of a change is made and how much of a difference it makes... and if the difference is a known result (meaning, you gain more of one thing at the expense of something else...)
Click to expand...
Long stroke Atlas/Commando is 89mm, short stroke, as used by Norton for a 77mm bore 750 (748.7848), was 80.4mm.

(89-80.4)/2 = 4.3mm (specifically)

If that is the question you are asking?
 
acotrel said:
Where is the advantage ? I loved my 500cc short stroke Triton. It was the nastiest motorcycle I ever rode, but I was too lousy to buy a 6 speed gearbox for it. I used to choose where I was going to lose a race. I only ever really lowered the gearing on it once, and I knew what was about to happen. I had to stand it up in a corner and spear-off to miss three guys who had passed me towards the end of the preceding straight.
Click to expand...
I just find it a very sweet engine to use. Rider friendly. Mind you, so was my 850 (89mm crank) with a short stroke head and pistons.

The lack of significant power advantage on the dyno is probably exactly why Norton spend a fair bit of dyno time with them but didn't rush to race them!
 
SteveA said:
Long stroke Atlas/Commando is 89mm, short stroke, as used by Norton for a 77mm bore 750 (748.7848), was 80.4mm.

(89-80.4)/2 = 4.3mm (specifically)

If that is the question you are asking?
Click to expand...
Yes, that's the question I was asking... thank you. What change in the engine do you expect from that?
 
SteveA said:
I just find it a very sweet engine to use. Rider friendly. Mind you, so was my 850 (89mm crank) with a short stroke head and pistons.

The lack of significant power advantage on the dyno is probably exactly why Norton spend a fair bit of dyno time with them but didn't rush to race them!
Click to expand...
Often the revs are limited by the valve gear and the cam, even if the crank can safely be revved higher. I am surprised that my 89mm crank revs so freely, but a low balance factor can also stop a motor from revving high. A BF of 72% might be about the minimum to get high revs. More revs = more horsepower but often less torque.
 
o0norton0o said:
Yes, that's the question I was asking... thank you. What change in the engine do you expect from that?
Click to expand...
I will answer the question perhaps a little obliquely, but it's probably fair to say my reasons for going short stroke didn't fit your question! o_O

I raced a Rickman chassis with an 850 sourced from ex factory parts in the '70s. I have described this engine earlier, basically a 750 short stroke with an 89mm crank. I sold it in 1980.

In 2009 I was able to buy the frame for a second time. It had had a hard life and needed repair. The engine removed from it before I bought it was a 1007 long stroke. After repair, I wanted to race it, but also give it as easy a life as possible. Available race classes in the UK at the time were 1300 twins and F750, with the prospect of F750 racing if I rode in Europe.

After thinking about it for a while, and recognising that the cost of building a race engine from scratch doesn't vary too much from 750 to 1007, I reasoned that smaller shocks at higher frequency were likely to be less damaging than larger shocks at lower frequency!

Anyway, I selected 750 as the capacity likely to give me most racing opportunities, and indeed in the first season raced it in two classes at each race meeting I rode when I only took one bike.

Having decided on the 750 capacity, I contacted Steve Maney to purchase a set of cases. The prospect of an actual 750 short stroke appealed emotionally due to the history of the bike I ran in the '70s, so I enquired about the prospect of a crank.

When I spoke to Steve he had one short stroke (80.4) crank on the shelf and told me he made cranks in batches, with no further batches of any stroke due for manufacture. So I bought that.

I 'expected' to sacrifice some power in the mid range and gain some rpm at the top. I didn't expect much difference in max power.

In reality, as a rider and it being 25 years between riding the 2 engines, I can't make a direct comparison. But, definitely more rpm, probably less power at 4000rpm, good power at 7000rpm.

I love this engine, but I loved the one I had in the 1970s too.
 
Ron Wood's 84 hp short stroke used standard alum rods because he didn't like the heavy factory steel ones. He had to mill the head, top and bottom of the cylinders and the cases to make it work. He said it ran smoother because of the weight savings. I had to mill my components in a similar way to use the first lightweight (and shorter) pistons back in the 1980s. I was able to use 850 sized pistons that weighed less than 750 pistons and things finally stopped breaking in my long stroke. See those early pistons below. Note the top of the cylinder is milled below the top fin.
Another short stroke
 

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