You can also use the rev counter to check if you have a 21 or 22 tooth final drive sprocket. On standard gearing a twin does 4,600 rpm at 100 mph, 2,300 at 50 and so on. If you have a 22 tooth final drive sprocket then the rev measurement will be about 5% lower.
Gear Ratios. Which gears are fitted?
- Thread starter Seán Ó Maoildeirg
- Start date
Just gave it a run out the road and recorded about
4000rpm for 60km/ph in 1st and
2900rpm for 100km/ph in 4th.
Note that the speedo is in KMPH and not MPH and before anyone asks, -- yes the front wheel is 20"
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Something is wrong here. 100 kph is about 62.5 mph which with the standard gearing should mean that the engine is doing 2875 rpm. If you have not made a mistake somewhere (we all do it) then the bike is massively undergeared, not overgeared. Are you sure that the rev counter is correctly geared? From your figures the bike is doing 48.3 rpm per one kph in first and 40 rpm per one kph in top giving a ratio between them of 1.2075:1 That figure should be 2.07:1 for the early Shadows and 2.60:1 for the later ones. Either my brain has fallen out of my bottom and I have become terminally confused (likely) or there is something wrong. Hopefully someone will check my figures!!
Well spotted timetraveller. In my hurry to reply last night I swapped the figures around which should have read
4000rpm/60kmph/1st
2900rpm/100kmph/4th
I'll double check those again later this evening when the roads are quieter. (I might still be wrong)
I may well be wrong, but , I feel that some of your problems may be down to a badly set-up engine. maybe you have a choked idle jet in one cylinder?or, more likely badly synchonised carburettors. If i were to start my twin in second gear at 2000 odd revs It leaves"the line" really quick.and pulls like a train.
I am just after rechecking my figures and as you suspected they were wrong. The rechecked figures are;
1st gear 3000rpm gives 40kmph
2nd gear 2600rpm gives 60kmph
3rd gear 2600rpm gives 80kmph
4th gear 2900rpm gives 100kmph
It could be that my problem is linked to my carb set ups but the above figures say that I should be able to get 5kmph with 375rpm in first gear. The engine would die at those revs. My carbs are brand new. Both needles have been dropped one notch but still run a bit fast on tickover. I should mention that the mag' is fitted with a "Manual Advance Retard" and non auto timing gear.
From your new figures the ratio between first and top is 2.586:1. The book figure is 2.6:1 for a Rapide so you have a Rapide first gear not the original Shadow one. If your speedo and rev counter were absolutely accurate then the revs in top with normal gearing would be 2,875. Your figure of 2,900 is well within the 10% error that many chronometric devices have so it seems likely that you also have a standard ratio on the rear chain.
Many thanks timetraveller. Figuring out gear ratios was never my strong point. I guess I never had a problem before where the ratio was a possible culprit. Now to find someone who is better than myself at setting up carbs'. They are a new matched pair from Surrey Cycles. apart from dropping both needles to reduce idling speed I have not made any other adjustments. The slides have only a 20thou gap when shut but the idling speed is still a bit too high (over 1000rpm) when cold and even higher when thoroughly warmed up. It's as if the slides are slightly raised when hot and this also affects gear changing.
I wonder if you have an air leak on the manifolds ? Also the needles should not upset the tickover, Maybe check float levels, Cheers Bill