My A65 Outfit is fitted with a Spitfire engine. It has a 17 tooth gearbox and 47 rear wheel sprocket. As you would expect the gearing is not right for an oufit, I often have to change down to 3rd when I should be able to stay in 4th . The thought of removing all the primary drive etc whilst leaning over the bike seat with my head upside down to change the gearbox sprocket to 16 teeth doesn't appeal to me. Doing the job from the sidecar side is not really practical either. Therefore my plan is to increase the teeth on the rear sprocket to 49 teeth or possibly 50 or 51 teeth. The rear wheel is a single sided QD one with a bolted on rear sprocket. Is the better option to have a complete rear sprocket made or use an old sprocket and have the new sprocket ring grafted on, once the existing teeth have been removed. Also can anyone suggest a company that make sprockets.
Thanks Rob
Hi Rob,
Considering that the std. solo sprocket is 20t I would have thought that the sidecar 17t sprocket would have been enough. At what speed/revs are you having to change down to 3rd?
I reckon that one tooth on the gear box is equivalent to about 2.5 to 3 on the rear.
Dave.
Thanks Dave
My A65 Thunderbolt used for sidecar work has a 17 tooth gearbox and a 47 tooth rear sprocket. The engine will pull a fully loaded outfit in 4th gear with no trouble holding 55-60mph. The Mk11 Spitfire engine reaches 50 -55 mph in 4th and will not go any faster. According to the 1966 data book, the Spitfire has a 21 tooth gearbox sprocket as standard, no sidecar sprocket size given, compared to 20 standard and 17 sidecar for the Thunderbolt, however as the gear ratios are different between the 2 I'm thinking there is a need to take another look at the sprockets. Just for interest the Spitfire engine has plus 80 10.5 to 1 pistons and uses Pazon electronic ignition. Gear ratios below:
Spitfire Thunderbolt Thunderbolt Sidecar
Standard 4 4.64 4.87 5.41
Standard 3 5.32 5.58 6.2
Standard 2 7.43 7.8 8.67
Standard 1 11.69 12.27 13.6
Hi Rob,
I would have thought that the extra power of the Spitfire would easily out pull the Thunderbolt on the same gearing. With the plus 80 pistons you probably have something like a 680cc so, in theory, a bit more power again.
What happens if you open the throttle fully? Does it die or does it not make any difference? Does it pick up cleanly from tick over/low revs? Is it worth checking all other settings at the same time as revisiting the sprockets?
Does it have a belt drive as this could mess with the ratios?
Dave.
Whilst I haven't had chance to start the engine today, I have checked that the throttle slides move freely up and down and that the choke slides do the same. There is no belt drive, just the standard triplex primary chain. The clutch does have the SRM seven plate kit together with their pressure plate. I changed to the seven plate arrangement due to the standard clutch plates slipping.
Hi Rob,
It will interesting to hear what you find. The SRM clutch sounds like a good move. Used parts can only be reused so much before it is not possible to make a good job of anything.
Dave.
Hi Rob
Talon Engineering in Yeovil would be able to make you a sprocket. They did one for my special that required a sprocket to fit a Jap wheel but running the right chain. Looks the business too, give me a shout if you need their contact details. They do have a website though so type in Talon Engineering.
Thanks Russ for the information about where to have a sprocket made, I will have a look at them. I will also do as Dave suggests and start the engine and check the throttle openings etc. I will do this early next week. Thanks Rob