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Post by mattb on Nov 8, 2007 4:25:51 GMT -5
I have only the most basic theoretical knowledge of engines, so the answers might be obvious, but:
If one is doing work to the bike, including considering mods to the engine, what can one do to make it lower reving? ...and would the difference achieved be perceptible enough to be worth it? I'm quite happy with the power produced by the SR, and so as long as it does not result in a loss of power, my interest in working the engine is rather on getting that low-reving feel such as old British singles had. I assume for a start that increasing the cubic capacity would do this? Anything else, and if just the cubic capacity, just rebore? shorter piston? fancy piston? Any thoughts on the lowest cruising revs one might achieve in doing this?
Regards, Matt.
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Post by StewRoss on Nov 8, 2007 5:51:43 GMT -5
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Post by mattb on Nov 8, 2007 9:08:19 GMT -5
Cheers Stew, some good stuff there, looks like it would do the trick. At $2000 dollars for the crank assembly, and fly wheel and cover, it might however be a bit beyond the budget for the next few years, but the sprocket kit looks good, and affordable: "About 2000RPM at 55 km". Just what I want! Does that look like something I could approximate by some locally available sprocket? (At $143 I might not do better locally, anyway!). I wonder if this would effect my acceleration too adversly? (Inner-city traffic dweller, I like to filter to the front and then gun it - the one situation in which I care about power.)
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Post by colinjay on Nov 8, 2007 21:26:04 GMT -5
Hi Matt,
You can raise the gearing a bit from std, but you will lose out a bit on acceleration and the bike caqn get a bit "snatchy" to ride at low speed in the higher gears. As Stew has aluded too, the problem is basically due to the small diameter and low weight of the SR flywheels.
THe flywheel issue has always been the bigest trade-off with big singles.
Big and heavy = good pulling power, great low speed cruising and no topend performance.
Small and light = great acceleration and topend performance but no bottom end pulling power.
Middle of the road (as per the SR) = compromise, this was one of the complaints about the SR (and XT)from the "traditional" big single riders in the 70's. It didn't have the low end pull of your traditional british single nor the performance of the sporting singles like the BSA Goldstar. The other part of the compromise is the use of the XT/TT trailbike gearbox ratios in the SR.
The flywheels in my 1949 500cc Matchless are big and heavy enough to use a anchors for an aircraft carrier, and it will happily crawl along at 10mph in top gear and accelerate (if you can call a slow, gradual increase in speed acceleration) away from that speed happily without any chain snatch. Having manual ignition advance/retard, and retarding the ignition help too.
So it is basically a case of 1 tooth up on the C/S sprocket and maybe a couple of teath off of the rear sprocket and live with the compromise.
CJ
CJ
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Post by joneill4 on Nov 9, 2007 9:47:24 GMT -5
"About 2000RPM at 55 km"
I don't know about your SR, but mine makes about 3 horse power at 2000 RPM. I take the opposite tack. I keep the RPMs at about 5500 RPM. I will admit though, I am rattling my bike to pieces. Seems like every day, I lose another part. Most recently, my chrome SuperTrapp muffler (the prettiest part on the whole bike) cracked in four or five places.
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