1970 B25 speed

[Jim_Hines]

Morning all,

Quick question.

I've owned my first BSA since early July. It's a 1970 B25.

I'm certainly not expecting a racer and don't intend on pushing it hard, but what comfortable speed do others reach?

Mine will do a max of about 45mph. Is that about right?

At that speed it's certainly not screaming, but it won't do anymore.

I'm about 14 stone in weight.

Thanks,

Jim

[neil1964]

Most decent C15s will see 75 or so.
Back in the day some people managed to screw over 90mph out of a Starfire, flat on tank in leathers etc.

[MGI]

I once lived next door to a guy who purchased a C12 - every time I saw him on the bike it seemed to be over revving for the speed, I asked how many gears it had and he responded three! When I told him there was four gears he didn't believe me and went off on a trial run, when he returned he had a broad smile on his face! I mention this story because I wondered if you had fallen into the same mistake as 45mph is well below the top speed of your bike!

[bikerbob]

I have a 1970 Starfire I weigh about 12 1/2 stone just been out for a little run  and it cruises nicely at about 55-60mph will obviously go a lot faster but at my age 81 am comfortble at about 55mph.

[Jim_Hines]

Morning all,
Thank you for your replies.

Any ideas what could be going on with my bike?

Could be be that it has the wrong gearing? Is that something that can be altered? I don't know if that's something people ever played with?

When I bought it, it was already fitted with 21" wheels. Could that be making much difference?

The guy I purchased it off wasn't the person who did the rebuild so he can't tell me much.

MGI, thanks for the reply. I'm definitely using all four gears. But it does then feel like it needs another gear at 40-45mph.

Thanks,
Jim

[JulianS]

I suggest check the number of teeth on the gearbox and rear wheel sprocket and compare with the standard numbers.

If the rear wheel has a 21 inch rim (18 inch from the factory) that will raise the gearing, could it be overgeared?  I wonder what else the previuos owner changed.

These engine like to rev.

I owned a 1968 Starfire, standard gearing and wheels, it certainly liked to rev and would show just over 80 on the speedometer on a good day.

[Jim_Hines]

Afternoon all,
I've just finished tinkering for the day.

Think I may have found something.

My rear sprocket has 52 teeth and when I've checked that against the manual it should have 49 teeth...

The front small sprocket has 16 teeth which according to the manual is correct.

Could the extra 3 teeth on the rear sprocket be making a big difference to top end speed?

I would like to put it back to what it should be (49T).

Thanks for all the advice.

Jim

[rhyatt]

so - you have a 16-52 currently as opposed to 16-49

so - you are geared DOWN ie  the engine will spin more for a given road speed

is it running out of revs I wonder - does it sound like it is screaming??

as a ballpark , 1 tooth on the front equates to 2 or 3 at the rear sprocket

is the front crankshaft sprocket standard I wonder ? but you'll have to take the primary cover off to check

I suppose before you do that , is to try the correct sprocket at the rear and see how she goes

[KernowPete]

Well I'm in a sort of similar boat with my recently acquired C15. It's a mix of trials frame and SS80 engine so in standard gearing I guess it should be good for 70 or so. It's had what I think is a Bantam rear wheel on it and the sprocket is a 59 tooth I think. There's no speedo on it so I had no idea what the top speed was. I downloaded a speedo app on my phone, strapped it to the handlebars and just been for a spin - mechanical sympathy tops it out at 37mph  . Time to do some maths and see what I can do with it gearing wise. I'm intending to do some green laning on it so want a reasonable road speed.
Maybe yours had been set up for trials/greenlaning at some point?

[KernowPete]

I also wanted to check the front sprocket size on my C15. Wasn't really into taking the clutch off just to have a look so I set up a torch shining down at the sprocket, cable tied an inspection mirror to the subframe and angled it so I could see the sprocket. I used a long screw (maybe 5-6 inch) and using the mirror scratched a mark on the round body of the sprocket. I then turned the back wheel and counted the teeth until the mark came back round.
My gearing is 14 front, 60 rear. Standard for C15 road use is I believe 19 front, 45 rear so a big difference.
I appreciate it's a different bike to yours but it seems we're on a parallel path so thought it worth mentioning.

[neil1964]

Odd to have 21" rear wheel (normal for off-road front)
More common 18" rear although some GS, clipper and B40 have 19" rear.
The small front sprocket and large rear sprocket would be horribly under geared with 18" rear (even big knobbly tyres).
Can you measure sprockets and rear wheel size then you can calculate gearing and work out what is optimum for your engines state of tune.

[Jim_Hines]

Afternoon all,

Apologies for the delay in replying.

Thank you all for getting back to me.

To start with, my mistake, the bike has a 21" front and 18" rear wheels, both with knobbly tyres.

At the current top speed, it certainly doesn't sound to be screaming at all. It's always loud though as it has the Victor exhaust fitted!

I've been looking at replacement rear sprockets as in my mind that would be first and simplest thing to try??

My question with those though, is I've seen 47T and 49T. Any suggestions which I should try?

Thanks for the time and help so far.

Jim

[Jim_Hines]

Just learned how to compress an image to upload so I've attached a picture for reference!

[KernowPete]

Thats pretty!

[neil1964]

That is a lovely looking machine, as you say like a Victor.
What gearing you go for depends on how you use it.
Under gear and you have to rev its nuts off on a fast road or overgear and it will be both sluggish through the gears and will not pull top.
I think that 49 rear might be sensible as 46 is more like pure road gearing but you will have to make that call!