Greg's flat tracker (Ro-dy cranks, now retired)
- Thread starter jseng1
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concours
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Sporting TWO BIG Mikuni VM's.Here's a shot of Greg's gorgeous lightweight flat tracker.
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Just saw your list of bikes. My latest project is a DT400B.Sporting TWO BIG Mikuni VM's.
concours
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The ignition system is a "one hit wonder"
Only used that year, IIRC.
SteveA
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Norton established the 80.4mm stroke with the 77mm bore to address F750 road race rules for their road racers. Though of course they would have been aware of flat track rules as well.
So, yes of course it was then useful to US Flat trackers. But I reckon an 828 with it's mid-range would have been good too!
Of course, Harley might have had a problem with that!
(the photo in my avatar is of my Rickman with an 80.4 stroke 750, Maney cases and crank, JSM pistons and long rods and Fullauto head)
Is s/stroke max. torque really lower than max torque for the longer stroke engine? I don't think so, rather the opposite, but the rev at which it manifests itself will be higher up. Might be less favourable for a flat tracker.So, yes of course it was then useful to US Flat trackers. But I reckon an 828 with it's mid-range would have been good too!
Of course, Harley might have had a problem with that!
Do you have figures for your engine?
- Knut
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Yes indeed, very nice!!!
SteveA
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I have no useful figures to offer, so I will not 'create' any. Just offer a couple of anecdotes.
I have only run the 750 short stroke on a dyno once, without revving past 6500 when it was new. The rear wheel HP figure was respectable and impressed the dyno tester as the most powerful British twin he had tested, but testing British twins wasn't his bread and butter. We were doing basic jetting before running in, so we weren't looking for a 'big' figure.
It revs a lot more than 6500, possibly 8000. It is very rideable, with peak torque probably 1000rpm higher than an equivalent 850. I love how it builds between 6000 and 7000, change up, repeat.....
The crank is lightweight Maney and combined with the JSM long rods and pistons and a suitable balance factor the power delivery is smooth and vibration rarely observed!
I ran an 850 motor built from short stroke parts including head cam and pistons, but with an 89mm stroke in the 1970s. This was also a very rideable machine, you could be in too high a gear and not care much! It was a lot faster than I was and would easily pull away from not only 750 Nortons (long stroke), but any other 4 stroke twin it came up against, including an 840 Weslake run by John Caffrey. Caffrey complained he had paid 3 1/2 times what I paid for my engine parts and was told it would be the fastest 4 stroke twin in the UK at the time.
Clearly it wasn't, I gave him hard evidence on that, but sincerely I doubt mine was either. I suspect that as far as road racing was concerned, that was built in Thruxton, either by the race shop, or later by Thruxton Motorcycles, but mine did fly.
Other evidence shows that US tuners achieved a little more with flat track motors than Thruxton did, but it is a different discipline.
The short stoke 750 has no shortage of torque as far as I am concerned! But it is true to say that when the 750 and 850 motors were compared in the '70s at Thruxton the 850 delivered a lot more mid-range!
I think either of the 750 configurations would have been good for flat track, and I think there is plenty of 'evidence' that was true in the results lists.
