10hp (Enfield/Yanmar combo) - some observations on gearing

[Dan J]

I hope this may be of interest to some here – particularly those who own or are in the process of building similar powered bikes.

When my Yanmar clone powered Enfield was being built I requested an 18 tooth gearbox sprocket as I thought this would be the happiest medium between speed and the power the engine delivered. This provides a theoretical 60mph top speed - maybe slightly more. I’ve done 2,500 miles under this arrangement and I’ve never been fully happy with how the bike performs. It will do 60mph on the dead flat and with a bit of wind behind you, but it'd take an age to get there and every hill meant the bike would slow down to 50mph (or sometimes a little worse) and would chug, blowing out black smoke as the governor overfuels the engine to try and catch back up to the requested revs. It was often the case that I’d end up changing down to 3rd which at full revs would do just over 40mph as the bike couldn’t cope in 4th on the worst hills. I decided I needed to try something different so, as I didn’t want to remove the primary as I went down the route of ordering a brake drum you can bolt different (larger) sized sprockets to along with a corresponding 42 tooth sprocket. These are all available from Hithcocks at http://www.hitchcocksmotorcycles.com/ac ... rame-tyres

Now, originally the bike had an 18 tooth sprocket on the front, the standard 38 on the back which had a supposed top speed of 60mph. A ratio (18/38) of 0.474.

I’m now on 18 front, 42 back so a ratio of 0.429. Both on paper and on the road this has reduced the top speed down to 54mph but the biggest difference has been a transformation in how the bike rides. It accelerates noticeably more quickly now which is a help. But better still that huge jump in the Enfield gearbox from 3rd to 4th is no longer a problem. The bike gets to 50mph quickly (around 3200rpm) but doesn’t now slow down every time there’s a hill. There’s a few more mph there as well if I need it and it will run to 55mph without overstressing the engine. But hey, I didn’t get a 10hp Enfield to go fast…

For those building bikes or with the patience to change the gearbox drive sprocket, then going to a 17 tooth (and sticking with the standard 38 tooth brake drum at the back) will give you a ratio of 0.447 and a top speed of 56.7mph. Not quite as low as mine but I think if I was building up a bike myself I would definitely take the 17 tooth cog over 18 (or even worse).

Whatever you might think these engines have in the way of torque they are still small, low powered engines and I can tell you mine is much happier with the lower gear ratio. For me there hasn’t really been any change in top speed either because my engine struggled to pull the 18 tooth cog (and the bike) beyond 50 mph and every hill slowed me down. Now the bike will sit just above 50 all day and get there much faster.

I’ve learnt that higher gearing whilst a nice idea was a false economy. It might be worth thinking about for those building as well. 10hp is never going to take you much beyond 50mph and if you’re not happy with that then I’d advise looking to larger and higher output twins.

Hope that’s of interest – happy diesel biking…

[Sphere]

Thanks for this Dan. Lets see if 60 more cc's will help me to stick with 18t. I hope to be able to give my account fairly soon.

[oldbmw]

A good post Dan.

My own bike an Electra came with the 19 tooth sprocket. It always felt like I was riding uphill all the time. Switching it to an original 18 tooth sprocket transformed it into something really lovely to ride. perversely the mpg went from 95mpg to 96/97mpg.
as an aside, the high gearing stresses the big end and mine failed on the way back from Poland.. I now have the bike back with one of Henry's super upgraded jobs . Had I thought a little more I would have been tempted to install a 17 tooth sprocket which was standard for a 500 for about fifty years. Ideal for pottering about.

the switch to an 18 tooth sprocket has increased my average speed over the ground as i no longer am constantly having to change gear.

[John]

Further to Dan's observations, I now have some practical experience with my own yanclone. Although the engine isn't run in & fully loosened up, & the gearbox is still running in grease, the performance is very representative of what will eventually be available. I have a 19 sprocket fitted, along with close ratio gears. I never thought for a second that the engine could pull top gear for most open road conditions. My thinking was that 3rd would be my effective top gear at a calculated 50 mph @ 3600 by gps, with 4th for gentle 40 mph pottering & getting some speed up on the down hill bits.
Henry set the maximum engine revs down to about 3000 for running in purposes. (although the Yanmar manual actually recommends full throttle & full revs for brief periods in the first hour of running, & simply avoid prolonged full throttle operation for the first 5 hours) - I have sat nav wired onto my bike. This helps to set up the engine & check calculated gear speeds / revs against actual experience. After several tweaks of the screw controlling max engine revs, I have now reached a true 47 mph in 3rd. So I still have a little adjustment to go before the full 3600 & 50 mph is available. At the moment, by gps, the revs are topping out at 3380 @ 47 mph in 3rd.
In 4th, the bike will eventually work up to 45 mph, or 2630 rpm..
The speedo on my bike is calibrated in km's, & is marked to 120 km/hr (75 mph). As Enfield speedo's go, it is incredibly stable, the needle remaining absolutely rock steady. I mention this to offer the following observation; Compared to the gps, the Enfield speedo (at least on my bike) is 7 mph optimistic. So at a true 47 it reads 54 mph, & in 4th at 45, it reads 52.
I believe we have been misled by our speedos as to yanclone capability. Since when I eventually reach a true 50 in 3rd, the speedo will claim 57 mph!
One observation in practice that I had forseen, is that in 3rd at 40 mph on half 'throttle', the bike will ascend some pretty stiff hills at undiminished speed, without my having to move my twist grip hand at all. In other words, the governor is fulfilling its purpose & opening up the engine to maintain engine speed independent from the riders input. Then shutting down the fuelling again on the easier sections, saving fuel. This can't happen when the the bike is overgeared, because the engine then spends all its time on overload fuelling even on the level..
Overall, the gearing is still way too high, since 1st & 2nd & 3rd are effectively operating on a 21 sprocket to the 19T 4th gear, because of the close ratio gears. This makes riding in traffic very difficult. 3rd adds frictional losses since it's not 1:1. So I'm going to fit a 20 tooth primary sprocket to lower the overall gearing by 20%. This will give 20, 30, 40, 50 mph @ 3655 rpm in 1st, 2nd, 3rd, & 4th respectively.
I've only done about 100 miles so far, so it's too early to make observations on mpg.
One disappointing aspect, is that the yanclone hates high loads from idle speed. It 8 strokes before dying off, even trying to pull away in first & giving it some revs. I've set the idle speed to the correct 1200 rpm, but it still does it. Perhaps this is because the flywheel is too light & 1st gear too tall. On the Robin diesel enfield, it would happily pull away in top at idle, just by slipping the clutch a little, despite pulling a 20 sprocket!! The Robin had a 21:1 compression & a flywheel almost twice the weight of a yanclone one. Also it was a long stroke engine to the short stroke yanclone.

[Sphere]

Hi John, would it be possible to do this modification after the running in period, and try it in 4th @ 3600 rpm?

[Dan J]

John - just for reference, the speeds quoted in my post are correct. I replaced my km/h speedo with a mph one and have tested it with GPS many times - it is absolutely spot on. Whilst that's nice from a "knowing my speed" perspective (and for making observations such as the one I made at the start of this post re speeds) it does mean I have to watch my speeds - especially around speed cameras and the like. My V-Strom (DL650) on the other hand I reckon under-reads to the tune of about 10%.

[John]

Hi John, would it be possible to do this modification after the running in period, and try it in 4th @ 3600 rpm?

Hi Sphere. I've been for a run today, & reached an intriguing conclusion. The engine, while reaching an easy 3800 without load, can't seem to exceed 3400 in 3rd (47 mph by gps) (it is geared for 50 @ 3600). Even in 2nd, which is geared at 37 mph @ 3600, the engine seems to creep up to the last 1 or 2 mph to 37, yet will eventually see 39, proving that the engine will go beyond its rated revs under load..(3800 @ 39) & is not being held back by the pump settings. Yet when I change into top, although the engine is producing less power, & is only revving at 2460 at a true 42 mph, the engine seems to have a slightly easier time of pulling the bike. It seems to roll along more freely. Today, I even saw 53 mph by gps in top on a fractionally downhill section ( 100 kms or 62 mph by speedo!) , which sustained at 50 for a short period on the level. In 2nd or 3rd, I detect the engine labouring slightly, & the exhaust bark seems out of proportion to the modest speeds achieved. it should be easy for the engine to reach 50 in 3rd, but it can't. I have concluded that a great deal of energy is being soaked up in the 3 intermediate gears, that is not present in top. Top, of course, doesn't transmit its power through the gears, but simply locks the output shaft directly. I'm still using grease in the box, which will eventually be changed to oil. Top is far too tall for practical use though, & I don't plan on using 3rd as my default top anymore, now that I know how inefficient the box is in the intermediate gears. I think that when I fit a 20 primary (50 mph by gps in 4th @ 3655) or possibly even a 22, (54 @ 3600) all should be well.

[John]

John - just for reference, the speeds quoted in my post are correct. I replaced my km/h speedo with a mph one and have tested it with GPS many times - it is absolutely spot on. Whilst that's nice from a "knowing my speed" perspective (and for making observations such as the one I made at the start of this post re speeds) it does mean I have to watch my speeds - especially around speed cameras and the like. My V-Strom (DL650) on the other hand I reckon under-reads to the tune of about 10%.

Hi Dan, I'm rolling on AM 26's, which may have a smaller radius than the more traditional rubber used. & while the needle is rock steady, I'm not suggesting that the speedo is more accurate because of it! The speedo on my Hayabusa was steady too, but overread by 12%!!!

[Sphere]

Ah yes, grease/making the gearbox oil tight. I think I should order the parts Anorak_Ian suggested and get on with it. I don't intend on stuffing mine with grease again. Also, the bearings in my box are knackered I think.

[John]

An apology to yanclones everywhere!
I'm such an idiot! I have no idea why it never occurred to me before. Perhaps I'm one of those people who assumes complex solutions to problems instead of looking at the simple & obvious ones first. I have new AM 26's fitted to my bike, & it has been in storage in a vac bag over the winter. I was so busy checking governor idle & maximum speed screws, adjusting & freeing off slightly binding rear brakes, & contemplating the resistive effect of a grease filled gearbox, that It never entered my head to check the bloody tyre pressures! - After a 20 minute run & returning to the house, it finally dawned on my reptilian brain that the bike was hard to push as well.. I got out the pressure gauge, & the result, even with the tyres warm was.. 15 psi rear, 13 psi front.. I could have put more pressure into them cold by putting my mouth to the valve!
After pumping them up to 42 psi, I set off again. Even as I paddled the bike with my feet onto the road outside, there was an instant dramatic transformation.. it began to freewheel along! I put her in 1st, let out the clutch & she just took off. 39 mph in 2nd, 53 in 3rd (3800 in both cases) & briefly 54 in top by gps, (3160) on a slightly favourable gentle downhill bit, before it started to rain heavily & I returned home. I could literally feel the massive saving in fuel from the saddle! When I've run out a few tankfuls to get an overall representation, I'll post some figures & compare notes.

[Sphere]

These figures make me want to order a 19t sprocket... ah well, lets see about 18 first. Very nice result indeed.

Please... try... fourth...

[John]

These figures make me want to order a 19t sprocket... ah well, lets see about 18 first. Very nice result indeed.

Please... try... fourth...

Don't get too excited just yet, Sphere! After the 54 in 4th on a very slight down grade, it slowly fell back to 48 on the level. Although I wasn't wringing its neck on this occasion, & won't until I've backed off the fuelling screw a bit; She's still coughing thick black smoke when the governor opens out. I know you've got more power & torque than me, but you've still got less than favourable road conditions & accelleration to contend with. Also, I've got C/R gears, which help a lot in getting the engine on song with excessive gearing (which 19 most certainly is!). Remember that a 19 gives a calculated true 61.5 mph @ 3600, & with the 'throttle' against the stop, all the time that the engine cannot easily hit its rated 3600, the governor is throwing overload fuelling into the engine to compensate. Far better to have the engine easily top out at 58 (18) & 3600 & save yourself maybe 20 mpg into the bargain.

[John]

Please... try... fourth...

P.s Note that on the level my bike topped out in 3rd at 53 mph & 3800, (just 2 mph shy of a 17 sprocket @ 3600), at which the governor then pulled the fuelling right back, & the engine became super smooth, while in 4th it sank down to 48 mph & 2800, even WITH the governor continuously on full overload fuelling, & with nothing left in reserve.. Though to be fair, it was very windy that day. In 3rd I could have maintained that 53 even up a gentle incline, & the high wind didn't present any noticable difficulty..

[Sphere]

Hi John,

With 16t I rode the Enfield last Sunday, it wouldn't go past 80 on the speedo. I can't fit the 18t sprocket, because the chain is too short. How many links do you use for your 19t setup? Unfortunately we didn't have an rpm meter handy, so I don't know if the engine puts out 3600 at max throttle, Nanko set it up using the stroboscopic effect and then some. It pulls nicely and I don't have any problem with the gap between 3rd and 4th except for shifting down. But I consider myself lucky since it decelerates the bike better than the front brake

[John]

With 16t I rode the Enfield last Sunday, it wouldn't go past 80 on the speedo. Unfortunately we didn't have an rpm meter handy, so I don't know if the engine puts out 3600 at max throttle, Nanko set it up using the stroboscopic effect and then some. It pulls nicely and I don't have any problem with the gap between 3rd and 4th except for shifting down. But I consider myself lucky since it decelerates the bike better than the front brake

Hi Sphere.
Sorry for the delay in getting back. I was savaged by a mastiff while on postal delivery at work, & spent a week in hospital! If you cannot exceed 80 km, your engine is not attaining 3600. You should be seeing 84 km @ 3600 with the 16t by gps. I estimate you are getting about 3450 rpm. If your 80 k is by speedo, rather than GPS, your true revs will almost certainly be even lower. Can't help with the number of links for the 19 as Henry did the set up. I'll try & find out & get back. I worked the engine revs & road speeds out very carefully. Starting with the 16 sprocket, which is 51.83 mph @ 3600 (83 km) each successive single addition of teeth on the drive sprocket increases the road speed @ 3600 by 3.24 mph exactly (5.2 km). As long as the road speed under load is correct for the sprocket, I wouldn't be too concerned with the max engine speed without load. Since the big Hatz can easily pull all sprocket sizes up to the 19 to its 3600 peak on the level, then providing the engine doesn't exceed the appropriate road speed in relation to the sprocket, the no load engine speed shouldn't rise more than 200 rpm with the clutch in. The 16t achieves exactly 23 km per 1000 rpm. Hope this helps buddy.

[coachgeo]

... I was savaged by a mastiff while on postal delivery at work, & spent a week in hospital! ...

Sorry to hear about your postal 4legged battle royal. If there is a baddog you dont want to meet.... it's a Mastiff.

Glad your out of the Horse-pital and able to add your .02 here

[dx400]

Hey Dan J,

I am in the States and have a yanmar clone fitted with a cvt. I also experienced smoking issues during acceleration. Are you using the stock yanmar air filter or the Royal Enfield box? I modified the stock yanmar box but cutting the forward facing edge out. This means the faster you go the more air is forced into the box. I also stripped down out the paper filter material and re-covered with a better breathing felt. The mods I made solved the smoking issue and added a bit of power. You may not be getting enough air through the restrictive yanmar clone filter. Hope this helps.

[Sphere]

Hi John, wow some story! That type of dogs... an American Staffordshire almost killed one of my Cairn terriers. It wouldn't let go even after someone drove into it with his scooter. I hope you have a speedy recovery. Make sure you take your time, for some it leaves more than just physical scars.

I don't know what's up with my bike. I swapped 16t for 18t and it did improve top end a bit, but perhaps Dutch wind combined with a 6'5" rider is also a limiting factor On my 350km trip home I managed to reach 80-85 on the speedo and on rare occasions 95. My engine doesn't smoke, so it might be tuned a bit lean.

I plan to do 1000km with it then swap oil and see if it improves in the mean time.

[John]

I swapped 16t for 18t and it did improve top end a bit, but perhaps Dutch wind combined with a 6'5" rider is also a limiting factor On my 350km trip home I managed to reach 80-85 on the speedo and on rare occasions 95. My engine doesn't smoke, so it might be tuned a bit lean.

Hi buddy, thanks for the good wishes. I have had a couple of flashbacks that shot me out of bed like a coiled spring! Hopefully that'll fade in time. You may have a point about the fuelling being too lean. When I wound back the fuelling screw too much on mine, although the engine would eventually top out in 3rd to get full revs, it took an age to get there & the engine lacked torque across the entire rev range. Manufacturers seem to recommend winding in the screw until it very lightly touches the governor lever, but for bike use I found this far too lean to be useful. The 18 would see 58.3 mph (93 km) @ 3600, so 95 would be right. One thing that occurred to me; In order for the engine to see 3600 under load, it would need to see 3800 without load. If the engine revs were set at 3600 without load, it would probably hardly ever see it under load unless conditions were extremely favourable. Try giving it a little more fuelling just to experiment. Mine hits 55-56 mph on the level in calm conditions. I have even seen 62 mph on the level with a strong tail wind, & 64.3 by gps on a very gentle downward stretch with the 19, so yours should easily top out on the level with the 18 even in slightly windy conditions.

[Sphere]

Hey John, it will do 3700-ish rpm without load (on purpose). Unfortunately I don't know how to make it run richer, no screws that I can find, perhaps I need to take off the cover for that. I'll see if the partlist gives me a hint. However, first concern is to fix the clutch; it's shot somehow. I went to the DIY store, but the biggest paint trays are still too small for catching all the oil dripping from the primary case. Need to find something else to catch it

[Dan J]

I went to the DIY store, but the biggest paint trays are still too small for catching all the oil dripping from the primary case. Need to find something else to catch it

Go to your nearest pet store and buy the cheapest cat litter tray you can. It'll be perfect for all future gearbox/primary/sump drains as well

[Sphere]

Good call I am also looking at http://www.ikea.com/nl/nl/catalog/products/40102978 although it's about an inch shorter than the primary case at its widest.

[dieselbikin]

Noticed several mentions of over fueling. I case someone missed it, here is the modification that cured my over fueling problems.

https://www.dieselbikeforum.com/view ... =+governor

Before this mod the engine would belch black smoke while under load when set for max power. If I turned the fuel down enough not to belch black smoke under load I would lose power. With this mod I have max power without fuel overload.