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[gnasher]

i am finishing the rebuild on a 350 redditch bullet, the gearbox sprocket that was fitted was a 20t, i have looked in the workshop manual and this says 20t and yet when read these pages i see people have mainly 15 or 16t, why the large discrepancy and what do other redditch owners have fitted please.

[stinkwheel]

Those will be people with Indian bullets. Reddich bullets have a larger rear sprocket with 46 teeth where the Indian ones have 38 teeth.

[gnasher]

thanks Stinkwheel, it is very confusing as redditch owners are also saying they are fitting 15t, 20t now ordered, thanks again

[Adrian]

Ah, but WHICH Redditch Bullet owners, gnasher? Pre or post '56?

The older Redditch Bullets (which were the ones the Indians effectively copied, apart from the later-style engine) had the 38T three-vane rear sprocket/brake drum, rather than the later English Bullet's 46T, so 15 or 16T would have been perfectly valid for the gearbox sprocket on those.

I recently bought a '61 350 Bullet in sort of trail trim. The back wheel has at some stage been re-laced with an 18"xWM3 rim and fitted with a fat 4" section trail bike tyre, factory fitment would have been a 17" rim with a 3.25" SM MkII, so it will be interesting to see what size gearbox sprocket the previous owner had fitted, as the larger wheel/tyre has effectively raised the gearing a tad. Probably threw the speedo drive accuracy out, too!

A.

[gnasher]

interesting, mine is also in green lane trim and also a 61 with 18" rear wheel, i was thinking of fitting an 18t sprocket but i'll stick with the 20 while running in and see how she goes, it's not a massive job to swap over if it needs it.

[Wheaters]

I decided to change the standard 16/38 drive on my Indian Home Market 350 to 16/46, for off road work.

I initially worried about the speedo calibration until I realised that of course it’s driven off the front wheel and totally unaffected by the rear transmission ratios……

It now has 18/42 final drive, same deal.

[Adrian]

The Redditch and early Indian Bullets have their speedo drive on the rear wheel. If you'd fitted a 21" front wheel instead of the 19" on yours it would have caused the same problem.

On mine the speedometer would have been calibrated according to the size of tyre fitted at the factory, which is what the 1692 figure on the dial is about. The larger circumference on the 4x18 tyre compared to the er... puny 3.25x17 means the wheel will turn fewer revolutions per mile, meaning that without a speedo recalibration or a speedo drive unit with a different ratio from the normal 2:1, the speedo will be under-reading somewhat.

A visit to Avon's web site gives the respective diameters. Circumference C = πd . Once you have that and have worked out how many inches there are to a mile, you can work out how many times each respective wheel and tyre will rotate over a mile. For a calibration figure of 1692, divided by 2 for a 2:1 speedo drive, I'd expect the wheel with the 17x3.25 tyre to rotate 846 times over a mile, or at least somewhere close.

A.

[Wheaters]

Yes, I’m well aware of the effect of tyre rolling radius. I built my Liege using a gearbox cable driven speedometer and had to get the new calibration figures for it; the accuracy is tested during the SVA test (IVA these days). The gearbox came off a car with 10” wheels, the Liege has 16”. I got Speedograph Richfield Ltd to rebuild the speedometer to get it within the SVA limits and reset the odometer to zero. It was in effect a brand new car, with a fully rebuilt engine and transmission, so I wanted to reflect that.

Calibrating it involves fitting a basic pointer to the speedo end of the drive cable and working out how many revolutions per mile it does. The vehicle needs to be pushed along on its wheels for this to be very accurate, due to the effect of the weight of the vehicle on the rolling radius.

When I fitted the Suzuki power plant and transmission after thirteen years I had to fit an electronic speedometer using a Hall sensor on the prop shaft because there is no mechanical drive cable. Getting that sorted was quite tricky, especially as the car has different sized tyres for road and competition use. I spent quite some time on the motorway checking the distance run between the distance marker posts so that the speedo could be set up properly. Not so easy now many have been removed during the so-called “upgrades” to four lanes…

I’ve often read accounts of how inaccurate the RE speedos are….mine seems reasonably good but maybe the above company could recalibrate them.

[Adrian]

Nottingham seems to be THE place for speedo reconditioning. Your calibration process sounds very similar to one of those used on the Puca-Smiths digital reproductions of the old Smiths instruments.

A.

[Wheaters]

There’s also a place on Chesterfield Road in Mansfield, specialists in Smiths instruments.