gearbox sprocket Enfield

[Sphere]

Altmann uses a 19t on his Enfield. Does anyone know what Sommer uses? I'm thinking about 20t to get a bit more top-end, but I can't find many people that use it

[Dan J]

Sphere - I'd be careful with going that high. My Yanmar clone (10bhp @ 3600rpm) only just manages on an 18 tooth sprocket and the difference between 3rd and 4th can be a bit of a killer on change up. I know the Hatz has a little more power so why not start with a 19 tooth and see how you get on? It'll be most depressing on a 20 tooth if you're forever changing back to third and the engine simply doesn't have the power to pull back to its peak to get you up to those higher speeds...

[Sphere]

Okie. Well, I'm slightly worried, since my chassis is a 350 so my current sprocket is 16t (or should be); I think my top end won't go over 80km/h if I leave it like that.

[Dan J]

The rev range is so limited on a stationary diesel it does make the correct sprocket size decision hard. 18 tooth for me, on a standard Avon tyre gives 65mph/105kmh at 3600rpm. The engine will often only reach 3200rpm and 56mph/90kmh due to the limitation of the engine power but maybe with the greater power the Hatz has you will reach 3600rpm no problem. I have even been tempted to go to 17 tooth but this I feel would be too slow, 19 tooth would mean 4th gear was nearly useless for me.

Either way, your 16 tooth definitely has to go!

[IgorVigor]

Dan, it might be worth you trying the 17tooth even though it will be theoretically slower.

even though your top speed in theory will be slower, the engine will be able to rev out easier, so you can hold a higher speed.

The other option is to change the wheel sprocket. adding a couple of teeth should do a similar effect.

[Sphere]

I think I may have to buy another final drive chain if I move to 19t. Oh well, I guess I will be fondling Ebay UK to see if I can resist buying a bunch of stuff I don't need

[oldbmw]

As it happens I have spent several days on and off changing the gearbox sprocket on my own bike. The PO had been misled into fitting a 19 tooth sprocket in place of the original 18. Problem was at every little gradient I was swapping gears. I have had three little outings on it since the change and my speed over the ground is now quicker because it will maintain speed up inclines and I am not spending time changing gear. My circular 100 Km route was done especially quick, even my wife noticed I was not aware of it until I got home and saw the time. My advice is to ensure the gearing is low enough for the engine to pull it easily.

[oldbmw]

Additional information.

I have done bout 3000 miles on the new 18 tooth sprocket. The bike really is much nicer to ride. It would appear that the mpg may have risen ever so slightly from 95mpg to 96
What I also did at the same time was retard the ignition slightly. This allows the motor to run smoother and I am now on 95 octane instead of 98. Although in theory my cruising speed should have dropped by about 3mph in fact my progress over the ground is now a lot quicker as it will accelerate up hill in top gear and holds its speed much better.

All in all a very worthy undertaking, putting it back to stock. Think if I was riding in India or on mostly poor or crowded roads I would be tempted to install a 17 tooth sprocket.

[John]

Okie. Well, I'm slightly worried, since my chassis is a 350 so my current sprocket is 16t (or should be); I think my top end won't go over 80km/h if I leave it like that.

Sphere, your speeds in top @ 3600 will be as follows;
16t = 51.86 mph (true gps speed, NOT speedo reading!
17t = 55.07 mph.
18t = 58.31 mph.
19t = 61.5 mph.
20t = 64.9 mph
A 20 sprocket isn't practical even for the bigger stronger Hatz, as it will leave the engine no power in reserve for hills & headwinds, & you will be forever changing down into 3rd, losing mpg & speed. 19t, which is fitted to the Sommer bike, is okay, but still a little ambitious at 61.5 mph for only 10.7 bhp max. Remember that after losses to drive the alternator, 10% loss through the transmission, & rolling resistance, you are left with about 8 bhp for propulsion! - It will certainly do that speed on the level on a calm day, but really you should gear it for a more modest top that will be consistently available at 3600 even in mildly adverse road conditions.
Bear the following in mind; the 462 Hatz can match the 350 petrol engine for torque between 2000-2500, at 19.4 & 19.7 lbs/ft respectively for the diesel & petrol. But at 3600, the Hatz has dropped to 15.62 lbs/ft, due to pumping losses in overcoming the 20.5:1 compression, while the petrol retains its 19.7 peak. This means that at 3600, the diesel is making 10.7 bhp, while the petrol is making 13.5 bhp at the same engine speed. So personally, I would go with the 18 sprocket for the Hatz, & a 17 sprocket with the Yanmar clones.

[Sphere]

Hey John,

I guess we'll find out, the 18t sprocket has already been delivered. Holland has no hills to speak of, but we sure got wind

Thanks for your reply.

[John]

Hey John,

I guess we'll find out, the 18t sprocket has already been delivered. Holland has no hills to speak of, but we sure got wind

Thanks for your reply.

Good luck! Let us know how you get on, & post some details on your cruising & top speeds, mpg, ability to resist headwinds etc. I've just ordered my own diesel bike this week from PricePart motorcycles, which hopefully will be delivered at the end of September. Mine has a 406 cc diesel, (8.8 bhp) we'll compare notes!

[Sphere]

I will. I'm also looking into -good- fairings. Not just keep the rain away, but make the Bullet streamline profile better. That should also help.

[John]

Hey John,

I guess we'll find out, the 18t sprocket has already been delivered. Holland has no hills to speak of, but we sure got wind

Thanks for your reply.

Hi Sphere. I unwittingly got the number of teeth on the clutch sprocket wrong. I have amended the speeds @ 3600 for the various size gearbox sprockets accordingly. Regards, John.

[Sphere]

Thanks I visited a guy with a sommer and I am pretty sure he said he has an 18t. Oh well I will find out in due time how 18 teeth will suit me

[John]

Hi Sphere. Sommer states on his website that his bike's top speed is 99km/hr. (61.8 mph) with a 55 - 57mph cruising speed. Therefore he must be using the 19 sprocket as standard. The guy you spoke to may have had his own bike amended. I have also heard that some Sommer customers have requested 20 tooth sprockets, giving a theoretical 64.9 mph top speed. Years ago, I rode an Enfield Robin fitted with the 20t sprocket. It was fun hearing one bang every lamp post, but the bike would slow down in top on roads where gradients were so gentle as to be invisible to the naked eye! Sometimes requiring a down change. About 55 mph was all the engine could pull on the level, (about 3000 rpm) - I think you have chosen well with the 18t. Top speed of 58.3 should also be your cruising speed with the Hatz pulling it, & you should also have something in reserve on gentle hills. At Sommer's own estimated cruising speed of 55 - 57 with the 19t sprocket, (3336 rpm @ 57 mph) You will have a full 3520 rpm available to pull it, making it far more consistently attainable than the standard Sommer. Sustained cruising speed makes far more impact on your journey times than an arbitrary & only occasionally attainable top speed with ambitious gearing. Relax! You've got it spot on!