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Discussion Starter #1
G,day

I,d like some advice on gearing please

I have 1971 Firebird scrambler that I've bought back to life after it had a fourty year nap

Unceazed the clutch , drained all the water out of the gearbox and got it moving again

Seezed oil ring and big rust Mark on the bore , so new 3 size over piston and rings

It goes like a champ and I love it to death but to me it seems to rev way to much

Standard it was doin 6000rpm to do 60mph , which seemed to me way to high , I didn't want to blow it up just keeping up with traffic , not that there is much where I live

I changed the front 18 tooth sprocket for a 20

It's way better , 5000rpm at 60 mph but it still feels to high

The back sprocket is a standard 47

Can I change it for a smaller one , do they even make one ?

Or should I just put up with it , smile and wave ?



CHEERS Mick


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I use a 20 tooth sprocket on my A65 Spitfire and it seems suitable. When i got it, it was fitted with an 18 tooth. With a 20 tooth fitted, i need to be at over 35mph before i use 4th gear.
 

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Discussion Starter #3
Yeah , Rambo

I put a 20 on the front , standard on the scrambler was an 18

Maybe the gearbox internals are different from the other A65s as well ?

I don't feel a lot of difference between 3rd and 4th

Or maybe I'm just expecting to much from a 50 year old bike

Most of my riding is done on a Ducati ST4s or a 990 Superduke

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Hi Mick, What size is crank sprocket? What size is clutch basket sprocket?

Smaller rear sprocket is not made for conical hub. I did calculation to machine center of 46 rear from Triumph dry frame. Besides ID of sprocket, you'll need to cut hex of original bolts or use Allen head bolts or chain will hit bolt head. At same time remove a little metal off the hub casting on back side or chain will hit bolt bosses. 1 tooth in rear does make difference.

Triumph has crank 29t. clutch 58t. 2:1. Transmission is direct drive in high gear. 1:1. No matter 4 or 5 speed. Triumph always 29t on crank unit twin.

BSA A65 can have 28 or 29t on crank, at least that I know of. I'm under the impression the clutch basket 58t like Triumph. ?? Verify that.

What is your crank sprocket size? That may be where you are getting the high rpm from? I would think you'd be closer to 4000 at 60 like the Triumphs. 6k @ 60mph long term will blow the motor up. At least wear it out really fast.

I'd have to look at my chart to know exact rpm at 60 on my bike. There are online calculators for motor rpm by gear ratio. Measure tire radius to rear axle sitting on bike to get tire rpm @ 60.
Don
 

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Discussion Starter #5
Ok Don , you are way outta my league

I'm just a dumb carpenter

Is the crank sprocket you are talkin about the front sprocket on the final drive chain ?

If so mines a 20 tooth one now , changed from an 18

If not , I'm in over my head

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The clutch basket is identical to a triumph one and interchangable as is the primary chain. The A65 do tend to rev high at around 80 mph but about the same as a T120.
If you have a 20 tooth chain sprocket, that will suit it best.
 

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Discussion Starter #7
Ok , thanks Rambo

I'll just stick to what I've got and relax a bit more



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All the sprocket in your bike are correct, you could use 21 gearbox sprocket to make it feel more relaxed.
I believe your tacho reads too high and seat of the pants feel is A65 revs higher than Triumph so you think 6 th revs you see is reality, but it isn't.
Another cause of this could be slipping clutch, but I believe you'd feel it.
 

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The clutch baskets are not the same between the A65 and T120. A65 has triplex chain, T120 has duplex. Plus, the offset is different.
I believe the primary sprockets are 28/58. You cannot change either sprocket by one tooth and still have the chain fit.
Calculate the speed from the RPM, sprockets and tyre diameter.
MPH = RPM/60*28/58*20/47*tyre diam/12*3.1419526*60/88
So at 5000 RPM with a 25.5 inch rear tyre gives 77.9 MPH.
Tachometer must be off.
 

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Yeah , Rambo

I put a 20 on the front , standard on the scrambler was an 18

Maybe the gearbox internals are different from the other A65s as well ?

I don't feel a lot of difference between 3rd and 4th

Or maybe I'm just expecting to much from a 50 year old bike

Most of my riding is done on a Ducati ST4s or a 990 Superduke

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I also have a Ducati...An old Brit bike will never be a Ducati just as the Ducati will never be a BSA...So just try to appreciate what each offers on it's own terms...
 

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somebody has been in there in the past with a 21 behind that trap door. the casez have been notched to make it go through.
 

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somebody has been in there in the past with a 21 behind that trap door. the casez have been notched to make it go through.
From memory when I worked on my Dad's A65 I'm sure the trap door was notched from the factory but not enough to fit a 21 tooth sprocket?
 

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Discussion Starter #14
Thanks everyone

The clutch is not slipping , the bike powers up hills like a powerful thing

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From memory when I worked on my Dad's A65 I'm sure the trap door was notched from the factory but not enough to fit a 21 tooth sprocket?

those look to me to be cut with a file, not machined imjust assuming the sprocket sizes are similar for similar tooth count. i do the same thing on triumphs to fit 21s:





i can get a 20-tooth sprocket through the triumph trap door but a 21 takes notching.

on my 1972 OIF T120, the stock 19/47 gives me 93.5mph at 6000 rpm. with a 21/47, it will be doing 103.3. with a 21, you WILL use that gearshift lever, or else.
 

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That is correct, a t120 does have a duplex. The T140 clutch will be the same as the A65. I have noticed my A65 primary chain has a removable link. My cases have the notching for a 21 tooth sprocket but sticking with a 20 tooth. My Spitfire does have a close ratio gearbox and is high geared. At the time the A65 Spitfire was made, it was recorded to do a 120 mph top speed.
 

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Hi Mick, The only online shop manual I could access was the '62-66 A65 manual. BSA had more options that Triumph for sprockets.

From what I could gather A65 could have either 18 or 19T crank sprocket. Clutch basket is always 58T.

I suspect by '71 the motors had 19T front, but you'll have to count them. Do you have more photos for crank sprocket you could count?

Trans sprocket would have been 18 from new, You now have 20. 21 was available, I have no idea if you had to notch case or not.

Supposing your crank sprocket is 19T. Rear are 20x47. 4.00-19 rear tire like Dunlop k70 RPM at 60mph would be 3762 RPM. 65+4076, 70=4309 according to chart I made up. The difference in sound & feel of motor between 60 & 70 is very dramatic. 60 is loafing. 70 is buzzing.

My tach used to read spot on. For the last several years it reads ok at lower RPM, but goes way higher above 3k or so. At 65 will read 5000-5200. The just riding along at 65, I'll look down & tach will be very close to 4100. Will work good like that for many miles including going through stop signs & back up to 65. If stop motor for fuel, restart, it goes back to 5000. 95% of time it reads 5000, then goes correct. I can't explain this for anything.

Remember in top gear all the Transmissions are 1:1 so the ratios inside the trans don't have any effect in top gear. It's just the primary drive sprocket sizes & rear chain drive sprocket sizes, actual radius of rear tire with you sitting on bike, tire pressure set.
Don
 

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Discussion Starter #20
I didn't have to notch the case , Don

Here are some more pics


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