Author Topic: C15 ss Max speed  (Read 3162 times)

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MICKREEVES

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C15 ss Max speed
« on: 21 February, 2017, 07:55:10 »
Following my previous blog-
My 1964 C15 SS from America, with 10/1 piston, has a disappointing low top speed, 60-65 MPH.
I can find nothing wrong with it.
I wonder if the original road tests 'in the day' were done with the rider  lying flat on the tank, with
the normal handlebars, not clip-ons. How much difference would that make??
I have my bike with higher than normal gearing, which I prefer. This should give more relaxed cruising,
with higher speed available in third gear.
I cant go to a heavier bike. I have not the strength to lift it when it falls over.
Mick

AWJDThumper

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Re: C15 ss Max speed
« Reply #1 on: 21 February, 2017, 09:10:34 »
Mick.

As suggested previously, it would be very interesting to see the results of putting your C15 on a dynamometer. I would expect you'll find it's producing no more than 75% of the power it had when it was new. That reduction in power would almost certainly equate to the reduced top speed you are now getting! That said, 65 mph max on a C15 is probably its safety limit given its handling and brake design!

JulianS

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Re: C15 ss Max speed
« Reply #2 on: 21 February, 2017, 10:04:08 »
I think you are expecting too much from an over 50 year old C15.

Not only the engine and cycle parts but also the build and weight of rider have significant effect on performance of a small bike.

The test bikes in the magazines were probably collected by the tester from the BSA factory and would in all probability been in top condition set up properly by factory staff.

The first photo shows an extract from the road test of a standard C15 from "Motorcycling" February 22 1960. You will see that the rider got 70 mph going down hill.

The second and third photos are an extract from the road test of a C15 SS80 from Motor Cycle June 1st 1961. You will note the tester comments about cruising on the M1.

Reading the tests I think that if you can get a reliable anytime top speed of 60 - 65 then it is not doing too bad.

EDDIE SIMPSON

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Re: C15 ss Max speed
« Reply #3 on: 21 February, 2017, 12:01:57 »
hi, a larger main jet may be needed at top end. if you have choke control apply flat out and feel what happens. you could raise the gearbox sprocket size and alter gearing but the engine will suffer getting up to top speed and you will hate slowing down for rounderbouts etc.

STAR TWIN

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Re: C15 ss Max speed
« Reply #4 on: 21 February, 2017, 14:19:14 »
10:1! Not a good idea in a C15. Or anything these days. Consider fitting a standard piston before something catastrophic occurs.

TTJOHN

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Re: C15 ss Max speed
« Reply #5 on: 22 February, 2017, 08:36:22 »
I see that you say your C15 came from the states, looking at some of their sites, it seems that a lot of US bikes had a 19T sprocket on the gear box, although the standard sprocket was 17T, I have just gone up to a 18T but have not had the chance to try it out yet, I am hoping it will give me a little longer legs, I believe Feck do a 19T but I'll wait and see how the 18 performs.

TTJohn

CRAIG12

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Re: C15 ss Max speed
« Reply #6 on: 23 February, 2017, 16:07:47 »
"I have my bike with higher than normal gearing, which I prefer. This should give more relaxed cruising,
with higher speed available in third gear" ............ sounds good IF the bike will now pull top gear , will it still rev or are you changing down unless on the flat etc

JulianS

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Re: C15 ss Max speed
« Reply #7 on: 23 February, 2017, 16:55:09 »
A larger gearbox sprocket will not increase maximum speed unless the bike was undergeared for the power before the change. You will reduce the acceleration and possibly the top speed unless you reduce the weight of bike and rider significantly.

The blue top service sheet below from 1963 shows that the factory actually reduced the gearbox sprocket from 17 to 16 teeth on the SS80 and got a higher maximum speed.

Looking through the 1964 USA brochure that shows the standard gearbox sprocket as 16 teeth.

There were quite a number of engine gearbox and rear wheel sprocket variations available for the competition models some of which show in the second photo of a US bulletin.

MICKREEVES

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Re: C15 ss Max speed
« Reply #8 on: 17 April, 2017, 16:02:15 »
I found that the standard 1 1/16" new Amal carb on my bike, was feeding to 1" port in the head!
I was advised that any step in the inlet tract would affect the performance.
I filed out the head to match. I fitted the fibre heat insulating plate on to the head with the bolts. I was able to see
how the holes matched very easily.
The holes in the plate were larger than the bolts, so a step would certainly still be  possible.
I made alloy sleeves to glue into the plate, with exact fit on the bolts.
With the plate now unable to be out of line, I could cut the head to match the plate exactly.
With the cylinder head off, I mounted the carb and tightened the fixing bolts, aligning the carb to the plate.
With the head off, I could see through the intake clearly - not possible on the bike.
So, the inlet port was greatly improved. I changed the cams to the scrambles type. the piston is 10/1.
Running on a 18 tooth output sprocket, and using a satnav for checking, my top speed was now-
Top gear 60 mph, with difficulty, at 5200 rpm.
Third gear 60 mph more quickly, at 6600 rpm.
The engine did not seem to want to rev any higher. Very disappointing.
I cant think why this could be so?? All the parts seem to be in good order.
Mick

JulianS

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Re: C15 ss Max speed
« Reply #9 on: 17 April, 2017, 17:06:43 »
I have no doubt that your current overall gearing is much to high for the power of your engine, max speed in top and third being the same means top is just an overdrive.

The manufacturer issued a service sheet which recommended a gearbox sprocket with 16 teeth as giving the best performance and you are using an 18 tooth sprocket .

The service sheet is attached below.

R PAWSON

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Re: C15 ss Max speed
« Reply #10 on: 17 April, 2017, 21:03:13 »
With the Gold Star, according to the development history, they found that a step down from the carb diameter to the port diameter gave an increase in power and to remove the step can be a mistake.
Whilst I understand what you are trying to do getting the thing to perform to keep up with modern traffic, you also need a good set of brakes, my, admittedly limited, experience of C15 s would indicate they are pretty marginal at best and if you do get it to cruise at a speed far above it's anticipated cruising speed when it was designed in the fifties you may well be demonstrating that adrenalin is brown.
Consider that the bikes released to the press for the reports you are reading were carefully put together before being released. 
Also consider as Julian keeps saying revs mean power ( and perhaps a wrecked engine!) so overgearing doesn't help top end if the engine will not rev out.

MICKREEVES

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Re: C15 ss Max speed
« Reply #11 on: 18 April, 2017, 13:32:52 »
I removed the 'Quietener' from the rear of the silencer,. I can now get up to 65 mph. (speedo reading over 70)
About the high gearing -
with 60 mph at 6600 rpm in third gear. this would work that at 80 mph the revs would be 7270, not quite high enough for max power??
but surely near enough to give better power than I am getting??
Mick

JulianS

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Re: C15 ss Max speed
« Reply #12 on: 18 April, 2017, 16:06:15 »
Mick

Maybe the best way forward is to just enjoy riding your C15 as it is. It is not going to be in anyway competative on our roads today.

There is a lot of pleasure to be had just going for a ride without all the pressure and stress of worrying about how fast it will go.

I think the question of gearing has been done to death in this thread and the similar thread on the old forum.

The only thing left to do if you want to wring maximum speed out of it would be to take the advice given by Arthur 2 months ago which was to put the bike on a dynamometer and see what that tells you about the power output and state of timing,and carburation etc.

STAR TWIN

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Re: C15 ss Max speed
« Reply #13 on: 19 April, 2017, 17:07:49 »
Too often over the years I have seen people (usually keen teenagers) try and get their C15s to perform far above their design limits. It always ended in tears and blown engines.

TTJOHN

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Re: C15 ss Max speed
« Reply #14 on: 19 April, 2017, 20:13:51 »
If you want more speed from a small bike buy a jap bike, stop wasting your time and effort trying to get more out of something which was not designed for speed, no matter what you do to, it it's still a C15.