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1968 bsa a650 gearing #656573
06/12/16 1:23 pm
06/12/16 1:23 pm
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Posts: 27
north america
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COP242 Offline OP
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Just wondering what everyone is running for gears, I find my bike is revving at slow speed, like to change my rear sprocket but not sure what to.

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Re: 1968 bsa a650 gearing [Re: COP242] #656575
06/12/16 1:39 pm
06/12/16 1:39 pm
Joined: Aug 2001
Posts: 3,919
argyll. scotland, uk
gavin eisler Online content
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argyll. scotland, uk
Rear sprocket is not an easy change, check your gearbox sprocket teeth, 19 lowish, 20 stock, 21 easy cruising less snappy acceleration, one on the front is worth 3 off the rear.
Stock 68 has 20 t front 47 rear ( FS had a 51 rear option. Not heard of smaller rear options.
I use the 21 front and 47 option ( my motor is a 750 and pulls this easily). A 22 can be fitted but it means splitting the cases.


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: 1968 bsa a650 gearing [Re: COP242] #656578
06/12/16 1:53 pm
06/12/16 1:53 pm
Joined: May 2013
Posts: 4,852
Rotherham - S. Yorkshire
Allan Gill Offline

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What year is the bike?

42t drums are available. I run 18:42 however my clutch is like running the bikes as 19:42 ( which is a fraction taller than 21:47)

If your running a conical hub the min is 47

If your running a post 66 bike then your will need new 2 part spindle, different bearing, brake plate, and other parts to make the bike use a 42t drum. Cost me about 200 to change it.

If you have a pre 66 bike or one which already has the steel (chromed?) brake plate then you can switch between a 42t drum or the optional drum without the integral sprocket. But you can only use the drum designed for the steel brake plate.

Hope that makes sense


beerchug
Re: 1968 bsa a650 gearing [Re: COP242] #656579
06/12/16 1:58 pm
06/12/16 1:58 pm
Joined: May 2005
Posts: 3,822
ohio
shel Online content
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ohio
What RPMs are you seeing and at what speeds? I see you are a new member, are you also new to Brit bikes? These aren't like Harleys, they need to rev a bit to be happy.


When given the choice between two evils I picked the one I haven't tried before
Re: 1968 bsa a650 gearing [Re: COP242] #656580
06/12/16 1:59 pm
06/12/16 1:59 pm
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Posts: 3,252
arkansas
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leon bee Offline
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arkansas
I agree, check the front sprocket- there has always been about 5 sizes available. Kinda hard to do.....what I do is, with the bike lifted or on centerstand, reach up there with a stick with some white paint on the end and mark one tooth. Maybe clean that tooth a bit first, paint will stay on long enough to turn wheel and count teeth. Count them more than once to be sure.

Like Gavin said, unless you have an oversize rear sprocket you're kinda locked in on the rear.

Re: 1968 bsa a650 gearing [Re: Allan Gill] #656590
06/12/16 2:35 pm
06/12/16 2:35 pm
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Posts: 27
north america
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COP242 Offline OP
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1968 bsa 650

Re: 1968 bsa a650 gearing [Re: shel] #656591
06/12/16 2:37 pm
06/12/16 2:37 pm
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Posts: 27
north america
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COP242 Offline OP
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at 50 mph it sounds like it is really revving, I'm not sure about rpms I don't have a tech, just seems high, you maybe correct

Re: 1968 bsa a650 gearing [Re: COP242] #656604
06/12/16 3:58 pm
06/12/16 3:58 pm
Joined: May 2013
Posts: 4,852
Rotherham - S. Yorkshire
Allan Gill Offline

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Standard gearing for that bike (20:47) would be 3500 rooms at 50mph.

21:47 - 3000rpm
19:47- 4000rpm


beerchug
Re: 1968 bsa a650 gearing [Re: Allan Gill] #656622
06/12/16 5:47 pm
06/12/16 5:47 pm
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Posts: 27
north america
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COP242 Offline OP
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it does have standard gearing but at 50mph it sounds like it's going to blow, lol

Re: 1968 bsa a650 gearing [Re: COP242] #656623
06/12/16 5:47 pm
06/12/16 5:47 pm
Joined: Aug 2001
Posts: 7,459
scotland
triton thrasher Online content
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Originally Posted By COP242
it does have standard gearing but at 50mph it sounds like it's going to blow, lol


Earplugs.

50 mph/3,500 rpm isn't much over minimum top gear speed.

Last edited by triton thrasher; 06/12/16 5:50 pm.

Amateur Loctite enthusiast.
Re: 1968 bsa a650 gearing [Re: COP242] #656627
06/12/16 7:14 pm
06/12/16 7:14 pm
Joined: May 2013
Posts: 4,852
Rotherham - S. Yorkshire
Allan Gill Offline

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You get used to it, when you get to 7000+ it's gets more interesting wink

Worst part is when you jump on a jap multi and open it up, and they don't come on cam till they get to 8000 rpm. Until that point they sound like a child screaming.


beerchug
Re: 1968 bsa a650 gearing [Re: Allan Gill] #656629
06/12/16 7:30 pm
06/12/16 7:30 pm
Joined: May 2016
Posts: 27
north america
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COP242 Offline OP
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LOL guys, what would dropping the rear sprocket 1 or 2 teethe do

Re: 1968 bsa a650 gearing [Re: COP242] #656634
06/12/16 8:19 pm
06/12/16 8:19 pm
Joined: Jul 2001
Posts: 12,845
Central Virginia
Lannis Offline

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Originally Posted By COP242
LOL guys, what would dropping the rear sprocket 1 or 2 teethe do


You can't drop the rear sprocket one or two teeth. You probably have a 47 tooth now; you can replace it with a 42 tooth. There aren't any intermediate sizes, unless you do a lot of machine work to modify one from another model.

Your bike was designed to run with either a 19, 20, or 21 tooth on the front (depending on what it was to be used for) and a 47 tooth on the back. You could also get a 17 tooth (for pulling a sidecar) or 18 tooth (for racing) but you wouldn't use these on a normal bike.

With a 20 in front and a 47 in the rear, the bike will be turning about < 4000 RPM at 60 MPH, which is right where it wants to be for efficient and effective running.

I've been riding (and watching other people ride) 500 and 650 BSAs since 1974. It's my opinion (just MY opinion) that more BSA engines have been destroyed by people "lugging" them, running them too slow, than for any other reason.

The basic reason was that, back in the day, very many people bought BSAs because they wanted to emulate the Big Man Bikes, the Harley Davidsons. Harleys were DESIGNED to "chug" at 2500 RPM, and would effectively pull hard from that RPM.

You WILL destroy a BSA engine trying to do that. But everyone from your Uncle Bill, to "Chopper" magazine, thought that BSA (and cousin Triumph) engines were "meant" to run like Harleys, yeah boy, she'll chug and pull like a freight train, that BSA, don't want to rev her too high buddy, and don't touch that front brake, she'll throw you over the bars etc etc etc and other nonsense.

No reason in the world not to use one of the three standard gearing sets on your bike. UNLESS the clutch is slipping continuously (and it won't do that for long), your BSA engine SHOULD run in the 3500 - 4000 RPM range, and will run 4500 - 5000 all day long. That's what it's designed for ....

Lannis



I stopped believing for a little while this morning. Journey is really going to be upset.
Re: 1968 bsa a650 gearing [Re: Lannis] #656648
06/12/16 10:21 pm
06/12/16 10:21 pm
Joined: Oct 2001
Posts: 1,144
Sydney, Oz
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Shane in Oz Offline
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Wot 'e said.

Harleys have roller bearings in the big end. These work well at lower speeds but the rollers skid at higher speeds and wear the crankpin and con rod bearing race.

British parallel twins have plain big end bearings which rely on a cushion of oil to prevent metal to metal contact and hence wear. That needs engine speeds to be kept up to maintain oil pressure, and also keeps the oil film distributed more evenly around the bearing.

Re: 1968 bsa a650 gearing [Re: Lannis] #656675
06/13/16 5:16 am
06/13/16 5:16 am
Joined: May 2016
Posts: 27
north america
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COP242 Offline OP
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THANKS LANNIS

Re: 1968 bsa a650 gearing [Re: COP242] #656793
06/13/16 11:16 pm
06/13/16 11:16 pm
Joined: May 2004
Posts: 3,745
Sydney Australia
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BSA_WM20 Offline
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The normal and best way to gear your bike is to the speed you spend most time riding at.

Every engine will have a "sweet spot".
This is the particular engine speed where the vibrations from the engine are at the least.
You gear your bike so the speeds you ride at most times fall into this least vibration range.

No two bikes are identical so what is right for one will not necessarily be right for you.
I spent years riding a low revving high geared A 10 and a hig geared WD B 40.
When I got the A 65 I geared it up way too high.
Thus it is sitting in the shed waiting for a complete bottom end rebuild.


Bike Beesa
Trevor
Re: 1968 bsa a650 gearing [Re: BSA_WM20] #656843
06/14/16 10:22 am
06/14/16 10:22 am
Joined: Jul 2001
Posts: 12,845
Central Virginia
Lannis Offline

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Lannis  Offline

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Central Virginia
Originally Posted By BSA_WM20
The normal and best way to gear your bike is to the speed you spend most time riding at.

Every engine will have a "sweet spot".
This is the particular engine speed where the vibrations from the engine are at the least.
You gear your bike so the speeds you ride at most times fall into this least vibration range.



That's good advice right there ....

Lannis


I stopped believing for a little while this morning. Journey is really going to be upset.
Re: 1968 bsa a650 gearing [Re: COP242] #656846
06/14/16 10:38 am
06/14/16 10:38 am
Joined: May 2013
Posts: 4,852
Rotherham - S. Yorkshire
Allan Gill Offline

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I think a slight adaptation to that might be more apropriate. Gear it so that your preferred highway speed is at the most comfortable spot between 4000-5000 rpm

Just to stop people gearing the bike to be doing 70 at 3000 rpm.


beerchug
Re: 1968 bsa a650 gearing [Re: Allan Gill] #656854
06/14/16 11:11 am
06/14/16 11:11 am
Joined: Jul 2001
Posts: 12,845
Central Virginia
Lannis Offline

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Lannis  Offline

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Originally Posted By Allan Gill
I think a slight adaptation to that might be more apropriate. Gear it so that your preferred highway speed is at the most comfortable spot between 4000-5000 rpm

Just to stop people gearing the bike to be doing 70 at 3000 rpm.


I agree, but in order to interpret "Least Vibration", you have to apply some "mechanical sympathy" to that.

Anyone with a developed sense of "mechanical sympathy" will recognize the pain that the BSA engine is going through when it is asked to pull hard at 3000 RPM. It may not have any "higher frequency" vibration, but it is obviously being tortured .... whereas even though there might be a slight "buzz" at 4K RPM, the motor is "happy" there.

My M21 is a good example. At an indicated 48 MPH, it drops into a "zone" where it will obviously run from here to the moon with excellent gas mileage and no strain on the engine. It'll go faster or slower, but isn't as "happy" there ....

Lannis


I stopped believing for a little while this morning. Journey is really going to be upset.
Re: 1968 bsa a650 gearing [Re: Shane in Oz] #656924
06/14/16 10:12 pm
06/14/16 10:12 pm
Joined: Aug 2001
Posts: 4,472
Owego, NY, USA
Mark Z Offline
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Originally Posted By Shane in Oz
Wot 'e said.

Harleys have roller bearings in the big end. These work well at lower speeds but the rollers skid at higher speeds and wear the crankpin and con rod bearing race.

British parallel twins have plain big end bearings which rely on a cushion of oil to prevent metal to metal contact and hence wear. That needs engine speeds to be kept up to maintain oil pressure, and also keeps the oil film distributed more evenly around the bearing.


HD is also a much longer stroke engine. A10s and Triumph 650s are also of longer stroke than the A65, which is over-square (bore bigger than stroke). Long stroke engines tend to be torque-ier at low rpms (more leverage on the crank), but more rpm-limited because of the greater piston travel. Short stroke engines, generally speaking, develop their power in higher rpm ranges.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
Re: 1968 bsa a650 gearing [Re: Lannis] #657361
06/18/16 7:08 am
06/18/16 7:08 am
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Posts: 3,745
Sydney Australia
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Originally Posted By Lannis


I agree, but in order to interpret "Least Vibration", you have to apply some "mechanical sympathy" to that.

Anyone with a developed sense of "mechanical sympathy" will recognize the pain that the BSA engine is going through when it is asked to pull hard at 3000 RPM. It may not have any "higher frequency" vibration, but it is obviously being tortured .... whereas even though there might be a slight "buzz" at 4K RPM, the motor is "happy" there.

My M21 is a good example. At an indicated 48 MPH, it drops into a "zone" where it will obviously run from here to the moon with excellent gas mileage and no strain on the engine. It'll go faster or slower, but isn't as "happy" there ....

Lannis


Yes, sympathy for your engine.
I always was poking fun at the Notrun owners because their Notruns were always blowing up then a good friend pointe out the reason.
The isolastics, on the rare occasions when they were set up properly isolated the rider from the "one more second at this speed buster & I am going to toss a rod" sort of vibrations that we owners of rigidly monted engines are all familiar with.


Bike Beesa
Trevor
Re: 1968 bsa a650 gearing [Re: COP242] #657453
06/19/16 4:33 am
06/19/16 4:33 am
Joined: Sep 2015
Posts: 312
Kent, England
J
John Alexander Offline
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Kent, England
As a rule of thumb 4,000 revs at 70 MPH would be the gearing to aim for, similar to standard gearing on the 650 triumph,4000 revs at 60 MPH is far too low.
Goldie John.

Re: 1968 bsa a650 gearing [Re: John Alexander] #657456
06/19/16 5:18 am
06/19/16 5:18 am
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Sydney, Oz
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Shane in Oz Offline
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Originally Posted By John Alexander
As a rule of thumb 4,000 revs at 70 MPH would be the gearing to aim for, similar to standard gearing on the 650 triumph,4000 revs at 60 MPH is far too low.
Goldie John.

Not on an A65 it isn't.
4,000 rpm at 100 km/hr is about right for a BSA unit twin, so around 4,400 at 110 (70 mph).
You have to keep an A65 over 3500 in top with any sort of load on th engine.

Your gearing works well for a T140 or an A10.

Re: 1968 bsa a650 gearing [Re: COP242] #657479
06/19/16 11:36 am
06/19/16 11:36 am
Joined: Sep 2015
Posts: 312
Kent, England
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John Alexander Offline
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Kent, England
Standard gearing on a BSA A 65L, at 5,000 revs should be aprox 80 MPH. For comfortable cruising, revs are best kept below 4,500 revs for any sustainable distance, fact of life, this is the limitation of the parallel twin hence the eventual dominance of high performance Japanese motorcycles in the late 1960s.Sports riders on british iron would have at times to replace tappet covers, screw on chrome air filters, fractured mudguard stays, leaking central bolt housing petrol tanks, not to mention rotating speedo/rev counter heads and blown bulbs. Be on the safe side ,keep the revs down, but the speed up. British bikes, i love them to death, they have engines , the Japanese have motors. Nuff said.
Goldie John.

Re: 1968 bsa a650 gearing [Re: COP242] #657579
06/20/16 2:05 am
06/20/16 2:05 am
Joined: May 2013
Posts: 4,852
Rotherham - S. Yorkshire
Allan Gill Offline

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Unfortunately it's not always about 70mph cruising speed. Or what torque the motor can give. I've set my motor up to pull in a high gear (3rd or 4th) from 1500 rpm until it gets to the point where it revs out. Torque isn't an issue. That's with close ratio box and 19:42 gearing. It's great back home where I live and if touring then I can even go a tooth higher still. However the oil pump can not keep up at the lower revs (especially that low) and you eventually bugger something. One thing that I am seeing when doing the rallies is a lot of people rarely ride their bikes back home and the most use the bike gets is the 40-50 mph rides at rallies often not even seeing those speeds. And whilst that is fine, the gearing choice should be made to suit that purpose.


beerchug
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