littleblackflash Posted July 1, 2013 Report Share Posted July 1, 2013 (edited) I did my first trial on my M92 bultaco yesterday, and the gearing in first is way too high. I'm running a sprocket ratio of 11-42 (corrected from 46) which sounds about right. But 1st gear is like being in 3rd on my mono. The bike has been built form a box of bits so who knows where the parts came from. Is there different primary drives / gearboxes or anything else I should look out for? Edited July 2, 2013 by littleblackflash Quote Link to comment Share on other sites More sharing options...
feetupfun Posted July 1, 2013 Report Share Posted July 1, 2013 The standard gearing on 1970s bikes is much higher than on mono bikes. It was chosen to suit the sections being ridden at the time. Your M92 is best suited to a smooth continuous flowing riding style which can seem quite strange to people used to stop/start riding. If you practice enough using 1970s technique, you will get used to riding it. If you gear it down to make it like a later model bike, it will be too slow in first gear for big jump-ups. The ideal setup is to have the motor running turbine-smooth especially on closed throttle and the transition from closed throttle, and never use the clutch. Idle set so the motor stops when the throttle is closed in neutral. Quote Link to comment Share on other sites More sharing options...
littleblackflash Posted July 2, 2013 Author Report Share Posted July 2, 2013 I agree with what you are saying, but in 1st gear with the clutch out I'm doing 15mph on tickover. I have to slip the clutch in the section to avoid stalliing it on turns. Other Bultacos at the same event were much lower geared, despite having the same sprocket ratio. The over gearing is not a couple of teeth on the rear sprocket, it's out by a factor 2 -3. In my village, there is a flashing sign warning you if you exceed 30mph. I can set this off in 1st gear. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted July 2, 2013 Report Share Posted July 2, 2013 maybe you have Alpina/Matador gear ratios and/or primary drive sprockets inside your M92 motor. If you want to test for this, you can work out the gearbox and primary drive ratios by counting how many turns the motor does for one rotation of the gearbox output sprocket in first gear and compare that to the specs for a M92 Quote Link to comment Share on other sites More sharing options...
littleblackflash Posted July 2, 2013 Author Report Share Posted July 2, 2013 maybe you have Alpina/Matador gear ratios and/or primary drive sprockets inside your M92 motor. Thanks. I guess there are different primary drive ratios. I'll remove the side case tonight and count the engine revolution for one turn of the output shaft. That was I'll know for sure. Quote Link to comment Share on other sites More sharing options...
littleblackflash Posted July 2, 2013 Author Report Share Posted July 2, 2013 (edited) Puzzle is still not solved. I took the flywheel cover off and checked my gear ratios and found Primary Drive - 2.375:1 Gear.................... Ratio.................. Overall Ratio (Gearbox out) 1st....................... 3.8:1....................9.025:1 2nd.......................2.92:1..................6.935:1 3rd.......................2.26:1...................5.368:1 4th.......................1.38:1...................3.278:1 5th..........................1:1.....................2.375:1 With a sprocket ratio 11:42 means at 4000rpm (mid range) in first gear I'm doing 8.7mph (4m/s). It feels much faster but if 8.7mph is my slowest speed, it is geared too high. I have found out that my wheel is off a Pursang, which has sprockets ranging from 40T to 48T. If I change to a 48T is will bring my speed down to 7.6mph (3.4m/s) @4000rpm. I'm thinking this is still too high, but probably worth a try. Anyone else know what ratio's they are running? PS - I think my previous estimate of speeds were a little too high. Edited July 2, 2013 by littleblackflash Quote Link to comment Share on other sites More sharing options...
naichuff Posted July 2, 2013 Report Share Posted July 2, 2013 From what I have read you appear to have the wrong internal gear ratios The primary ratios are all the same but Sherpas use single row chain and Pursang us duplex As a rule of thumb I used to work on 30 to 1 crank to rear wheel on 250's for trials Quote Link to comment Share on other sites More sharing options...
woody Posted July 2, 2013 Report Share Posted July 2, 2013 I'd agree your original speed estimates were too high - I'd struggle to trigger the 30mph sign in 4th on mine, never mind 1st. The early Sherpas were never very quick on the road. Anyway, back to the matter of gearing. Your gear ratios are all correct so there is no problem with the wrong gearbox. The clutch gearing is the same for all Sherpas I think, so I don't think it is possible to alter gearing that way. However, yours is the right ratio so nothing wrong there. The final drive gearing was originally set with a 428 chain on this model and you don't say which chain size you are using. With the 428 chain the gearing should be 13 : 52 (14 : 52 is also quoted) The later bikes had the same gearbox ratios but when they switched to the 528 chain the gearing became 11 : 46 which is what you have. I've just rebuilt my M92 and had a first proper ride on it on Sunday. I think I'm running 11 : 46 as well but I'll have to check. When I got the bike it was on 428 chain, so when rebuilding I think I woould have opted for the standard 528 sizes. First thought was also that it is a bit too high, but I think this was exaggerated by the fact that the bike wasn't running well and was very woolly off the pilot jet. It also had two flat spots at aout two thirds and at full throttle. Ignition or carb but don't know which yet but the affect on throttle response can completely alter the feel of the bike. Trying to pick my way up rocky streams with a woolly throttle response makes the bike feel very lethargic which can also make it feel too long in the gearing. If the bike picks up crisp and cleanly it will feel much lighter and responsive and maybe the illusion of a too high 1st gear will disappear. Sorting out the running is this weekend's job. So on your bike, 11 : 46 is correct. As Feetupfun mentioned, make sure the bike is running correctly and try it again, you may get used to it quite quickly, if not all you can do is try a bigger back sprocket as you can't get a 10 tooth front for the Bultaco The last 5 speed model, the 199a used 11 : 39 according to the info sources, and had a lower first gear, although 2nd, 3rd 4th were the same. Not sure how that works out as 2nd - 4th would be quite high compared to 1st. Quote Link to comment Share on other sites More sharing options...
littleblackflash Posted July 2, 2013 Author Report Share Posted July 2, 2013 1st gear gives 34:1 Crank to Wheel after the sprocket ratio is applied. So maybe it's correct, just far higher than other Bultaco at the twin shock trail I went to last week. All the observers commented on the fact I was in too higher gear, so something needs to change. I have a single chain primany drive, and googling for the gear ratios, my ratios match the sherpa's perfectly. I'm going to change the rear sprocket as a start. I am concerned about going to low on the gear. It still needs to pick up to jump logs / streams etc (I don't ride the easy routes). I've rebuild the engine with a new piston, carb, bearings and seals so it tick's over really smooth and pulls strong from low revs. So fine tuning and playing with sprockets is needed. Quote Link to comment Share on other sites More sharing options...
littleblackflash Posted July 2, 2013 Author Report Share Posted July 2, 2013 Woody - I think our post crossed over. I just corrected my original post with the ratio 11:42T but using a 520 chain. (Came in the box of bits). I just looked on bultacouk's site and they only sell a 52T sprocket which will fit my wheel (6bolts with 3 studs) using a 428 chain. Maybe this is what I should be running? In my excel sheet I've made, a 52T sprocket will bring the speed down from 8.7mph to 7mph (1st gear @4000rpm). It will also make my 2nd gear the same as what my 1st gear is today. I'll see if I can borrow a 428 setup from someone. Quote Link to comment Share on other sites More sharing options...
mrb505 Posted July 2, 2013 Report Share Posted July 2, 2013 Thats standard m92 gearing If your running 11/46 4:181 520 pitch sprockets its slightly lower than standard 13/52 428 pitch 4:1 Let it lug its amazing how low in revs these will go if in good tune it feels like you can actually stop for a few micro seconds then take off again Quote Link to comment Share on other sites More sharing options...
littleblackflash Posted July 2, 2013 Author Report Share Posted July 2, 2013 (edited) Thanks all. At least I know I don't have to take the engine and box apart. I did correct my original post from 11:46 to 11:42T after I double checked once I got home from work. The engine is quite amazing really. With electronic ignition it really does pull from almost zero revs. In fact I think it pulls stronger at low revs than my ex-works sherco 290. Maybe it is my riding? Again - thanks for the help and advice, I'll let you know how I get on. Edited July 2, 2013 by littleblackflash Quote Link to comment Share on other sites More sharing options...
woody Posted July 2, 2013 Report Share Posted July 2, 2013 11 : 42 will feel high on the M92. If you ask Talon they will make you a sprocket in the size you want as they will already be set up to machine sprockets for that wheel. It's just a one-off order for a non-standard size. Quote Link to comment Share on other sites More sharing options...
sherpa325 Posted July 3, 2013 Report Share Posted July 3, 2013 Basically all the larger bore bikes have the same overall 1st gear ratio using the factory supplied gearing. For the M92 using the 1/2 inch chain its 13/52 [or11/46 for 5/8 chain] interestingly the 250's used 14/52 . The 11/46 gearing was used up until the 199/198 series which went to 13/46[as a result of an internal gearing change to the gearbox] The 199a 198a 199b and 198b used the 11/39 gearing. The 199 and 198 can use the 11/39 and have the same overall gearing. The bottomline is that 4 teeth on the rear sprocket is equal to one on the countershaft and 11/42 would feel very tall as Woody said. Some people run an extra couple of teeth on the rear to get that extra bit of control you are after, but this is personal preference. Cheers Greg Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.