HONDA MT50

Reader Paul Pon­tin ‘goes elec­tric’ with this tid­dler.

Classic Motorcycle Mechanics - - CONTENTS - WORDS AND PHO­TOS: PAUL PON­TIN

Over the years I’ve built sev­eral one-off mo­tor­cy­cles and, like most of us, lusted af­ter more power, but I was strug­gling to find the old ‘joy of rid­ing' that I used to have. Liv­ing in South East Eng­land I fig­ured the den­sity of modern traf­fic had sim­ply ex­tin­guished the ex­cite­ment. I started han­ker­ing af­ter the sim­pler life, some­thing al­to­gether smaller, nim­bler and more fun! Crawl­ing to work one morn­ing through the grind of sub­ur­ban traf­fic in the car, I’d been over­taken four times in two miles by cy­clists. I then had an epiphany: mopeds aren’t such a daft idea! Back in the day rid­ing a moped could be gen­uinely ter­ri­fy­ing, as ur­ban traf­fic was a hell of a lot faster than it is to­day. Nowa­days be­ing lim­ited to around 30mph wouldn’t be so bad, but poor ac­cel­er­a­tion would. That’s when the idea for this project was con­ceived. In an at­tempt to in­ter­est the kids in me­chan­ics I had al­ready cob­bled to­gether a few off-road elec­tric ve­hi­cles, which was enough to as­cer­tain that the 250w limit le­gally im­posed on elec­tric bi­cy­cles is ab­so­lutely no fun at all. What I had in mind was a moped, just like we grew up with – but pow­ered by an elec­tric mo­tor. Three ma­jor ob­jec­tives for the con­ver­sion were con­sid­ered: it should try to re­tain the aes­thetic of the donor bike (elec­tric bikes of­ten look like a cross be­tween a mo­tor­cy­cle and a dog ken­nel!) and it should be light, fun and cheap. So that was the idea – it took three years be­fore it was re­alised! I even­tu­ally ac­quired a knack­ered 1993 Honda MT50-SJ from ebay for £127. I wanted an MT50 as the twin side frame rails gave space in which to house the vul­ner­a­ble bat­ter­ies, with col­li­sion pro­tec­tion be­ing af­forded by the chas­sis con­fig­u­ra­tion. The ini­tial idea was to keep the frame and run­ning gear largely un­changed, build an elec­tric mo­tor assem­bly that was a ‘drop-in’ re­place­ment for the MT50 en­gine and then stuff all the bat­ter­ies and elec­tron­ics un­der the orig­i­nal seat and tank. I had just semi-re­tired and I was look­ing for­ward to this project as a gen­tle run down from full time work in elec­tri­cal en­gi­neer­ing. Gen­tle? Ha! The con­ver­sion ended up be­ing way more com­plex

So you’re semi-re­tired and de­cide to turn an old moped into an elec­tric moped. Where do you even start?

than orig­i­nally planned. It in­volved mod­i­fi­ca­tions to the frame, plus the man­u­fac­ture of one-off fi­bre­glass moulds to pro­duce a tank, rear mud­guard and seat unit. All the run­ning gear was ei­ther mod­i­fied or re­built. In ad­di­tion, nearly all the an­cil­lary bits re­quired fab­ri­cat­ing from scratch. Around 250 in­di­vid­ual metal parts had to be ma­chined along with all the sur­face fin­ish­ing, zinc plat­ing and paint­ing. I even set up an an­o­dis­ing plant in my sum­mer­house be­cause the cost to get the mass of parts done pro­fes­sion­ally was pro­hib­i­tive. By the end the bike was al­most en­tirely cus­tom built. Ul­ti­mately the project took over 2500 hours to com­plete. Full-time work was never this hard! Mid-mount elec­tric mo­tors do ex­ist for mo­tor­cy­cle use; but they are big, heavy and rather ex­pen­sive, so I started hunt­ing around for some­thing else that could be pressed into ser­vice. The so­lu­tion came from an un­likely source. The mo­tor was orig­i­nally in­tended to power large scale ra­dio con­trolled planes; such mo­tors fea­ture a huge power-to-weight ra­tio but, un­sur­pris­ingly, are also highly stressed. Specif­i­cally built to spin a pro­pel­ler, they are blessed with a boun­ti­ful air sup­ply to keep tem­per­a­tures un­der con­trol. Pro­duc­ing up to 6kw (8hp) and weigh­ing in at just 1.8kg the po­ten­tial was just too good to pass up. These mo­tors pos­sess vir­tu­ally no safety mar­gin and will ea­gerly con­sume huge cur­rents for a sec­ond or so be­fore lit­er­ally burst­ing into flames. Equally, the higher the volt­age it is fed, the faster it spins un­til it cen­trifuges it­self into obliv­ion. It’s like hav­ing an in­ter­nal com­bus­tion en­gine that just keeps mak­ing more in­sane power un­til it ei­ther melts or blows up! On any elec­tric drive sys­tem the gear­ing used to re­duce mo­tor speed down to rear wheel speed is crit­i­cal. Hours were spent try­ing to get the volt­age and gear­ing right. Elec­tric mo­tors have an ef­fi­ciency curve; rule of thumb – they are at their most ef­fi­cient at around 80% of no-load speed. Any in­ef­fi­ciency ap­pears as heat, so if you drop into the in­ef­fi­cient speed re­gion then the tiny mo­tor soon heats up and POOOF! Without a pro­pel­ler to keep it cool I had to come up with an­other plan. The an­swer came in the form of a 3in fan unit (de­signed to pump air from the bilge of a boat) fit­ted into a ma­chined piece of toi­let waste pipe. A two-stage re­duc­tion sys­tem was em­ployed to gear the 7800rpm of the mo­tor down to the rear wheel. The first stage is sim­i­lar to the pri­mary drive of a nor­mal en­gine and em­ploys a chain to drive a lay-shaft that sits behind the mo­tor. The lay-shaft trans­fers drive over to the op­po­site side and then via the se­condary chain to the rear wheel, just like a nor­mal en­gine. The mo­tor had to be heav­ily mod­i­fied be­fore it could be used. Mag­netic po­si­tion sen­sors were fit­ted in­side the mo­tor which fa­cil­i­tate much bet­ter tim­ing of the power to the sta­tor coils, al­low­ing some fancy elec­tron­ics to avoid ex­plod­ing the mo­tor. A tem­per­a­ture sen­sor mounted to the coils is used to limit things if the elec­tron­ics get it all wrong. The mo­tor was fit­ted with rel­a­tively small bear­ings, be­ing all that’s re­quired to spin a nicely bal­anced pro­pel­ler. A large out­rig­ger bear­ing had to be mounted on the drive side to take the con­sid­er­able ad­di­tional ra­dial load im­posed by a chain drive. All of these com­po­nents were mounted be­tween 8mm thick alu­minium en­gine plates, which were la­bo­ri­ously hack­sawed to shape. Spac­ers be­tween

the plates kept ev­ery­thing rigid and holes drilled in the plates lined up with the orig­i­nal en­gine mounts on the frame. The idea was to just swap out the MT50 mo­tor and bolt in the elec­tric re­place­ment. Even the fi­nal drive sprocket is in the same place as the orig­i­nal, mean­ing sus­pen­sion and chain align­ment is un­af­fected. Pri­mary and se­condary chain-cases were fab­ri­cated from welded sheet alu­minium to keep ev­ery­thing neat. The com­pleted mo­tor weighs 6.2kg, sav­ing a tidy 16.2kg from the the orig­i­nal MT50 en­gine which weighs 22.4kg! Next, the bike had to be loaded up with bat­ter­ies. Lithium poly­mer bat­ter­ies were used – again in­tended for ra­dio-con­trolled planes. Pro­duc­ing huge cur­rents with min­i­mum volt­age sag, these make for a very punchy power de­liv­ery. Range was set at around 15 miles en­abling cost and weight to be min­imised. The range may seem pa­thetic, but the aim was purely to see if mak­ing a func­tion­ing road-le­gal moped from such parts is even pos­si­ble. I could ex­tend the range later if re­quired. There are two bat­tery boxes. The front one sits be­tween the frame rails on top of the en­gine plates hold­ing a to­tal of eight bat­tery packs, four on each side. The other, mounted to the frame rails un­der the rear mud­guard/seat assem­bly holds a fur­ther four bat­tery packs. An electrics plate was at­tached to brack­ets welded to the top frame rail. Form­ing the lid of the front bat­tery box this plate keeps ev­ery­thing rigidly mounted to the chas­sis and houses the speed con­troller, con­tac­tors, fuses etc. Run­ning at the bat­tery volt­age (nom­i­nally 55.5V), this wiring han­dles all the high-power trac­tion du­ties. Lastly, a top plate is fit­ted to hold all the low volt­age elec­tron­ics, in­clud­ing logic to mon­i­tor the bat­ter­ies and pro­tect them from un­der or over volt­age con­di­tions, as well as a slew of elec­tron­ics to mon­i­tor, con­trol and data-log all the var­i­ous pro­tec­tion sig­nals re­quired to stop the dreaded mo­tor an­ni­hi­la­tion. Hook­ing this lot to­gether was a ma­jor headache as there are mul­ti­ple pro­ces­sors and it all had to be de­signed, built and pro­grammed specif­i­cally for this set-up. Get­ting the aes­thet­ics right was prob­a­bly the hard­est as­pect of the de­sign. I tried to use the orig­i­nal tank, seat and rear mud-guard, but in the end none of them were right, so I de­cided to mould my own from fi­bre­glass. I am still not en­tirely happy, but hav­ing tried five dif­fer­ent de­signs for the tank and three for the rear guard, I had to stop some­where! The process for the tank was to first make a plug. In my case a wooden ver­sion of the fi­nal de­sign was made from MDF and steam-bent ply­wood. Any sur­face im­per­fec­tion in this will ap­pear in the fin­ished mould­ing so a great deal of time was spent fill­ing, sand­ing, paint­ing and gen­er­ally get­ting it as smooth as pos­si­ble. Next, a split fe­male mould of the plug was taken. The mould needed to be split into two parts as the an­gles on the tank mean that it would not re­lease cleanly from a one-part mould. Fi­nally, the split mould was bolted to­gether and fi­bre-glass laid up on the in­side. When it has cured and the mould is un­bolted the fin­ished tank can fi­nally be re­moved. Sim­i­lar moulds had to be made for the seat and the rear mud­guard. The front mud­guard was the only bit of body­work I pur­chased: £5 from Kemp­ton au­to­jum­ble! To fin­ish the seat I had to cut and lam­i­nate var­i­ous bits of foam to get the re­quired den­sity and con­tours and then cut and stitch the seat fab­ric. That took ages! So what is it like to ride? On pa­per a dream: the torque avail­able at 0rpm is about the same as a 2016 Honda CBR125R at 8000rpm. This means that when­ever you touch the throt­tle it wants to wheelie! All fan­tas­tic fun when I was off-road, but I had to change the gear­ing and dial the torque back

when I nearly per­formed an in­vol­un­tary loop while ex­e­cut­ing a feet-up U-turn! I am work­ing on a pro­gram­mable throt­tle ramp to fix this, but for the time be­ing it is fine and bet­ter than it was. It is nim­ble too: kerb weight is 75.5kg vs 90kg for the orig­i­nal, en­sur­ing the light­ness/fun goal has been met. The after­mar­ket rear shocks are rea­son­ably well damped, while the orig­i­nal forks are the weak link. Thicker oil and some preload spac­ers for the springs may help. All the foot con­trols have been re­moved to save weight and make ev­ery­thing as slim as pos­si­ble. The rear brake has been mod­i­fied to al­low cable op­er­a­tion from the left hand lever: it’s like a mod­i­fied push bike! I have the power set to a max­i­mum of around 3.8kw (5hp). This gives a speed of 30-35mph with plenty of power to go up­hill without drop­ping speed. In nor­mal flat run­ning it av­er­ages around 1.8kw which keeps the mo­tor tem­per­a­ture down: these power set­tings keep it within the DVLA re­quire­ments for moped clas­si­fi­ca­tion. Power de­liv­ery is the ad­dic­tive part of elec­tric bike own­er­ship; hav­ing all that torque and no gears has no equiv­a­lent in a petrol en­gine. I’ve had mo­tor­cy­cle rid­ing friends laugh at it, but once they ride it they un­der­stand! There are very few DIY elec­tric con­verted bikes in the UK and the range makes it pretty use­less for any­thing other than pop­ping to the shops. I’ve not done this to make money: the re­wards are gained from solv­ing tech­ni­cal prob­lems, mak­ing things with your own in­ge­nu­ity and own­ing some­thing truly unique. If you fancy a chal­lenge, think of a to­tally ridicu­lous one-off build… and give it a go!

Yes, it’s an MT50, cap­tain, but not as we know it. Paul Pon­tin has boldly gone where few have gone be­fore!

The orig­i­nal ma­chine as bought from ebay.

All the elec­tric gub­bins fit­ted to the bike.

ABOVE: Just some of the bits fab­ri­cated for the build!

BE­LOW: Still an MT50 at heart! Well...

Com­pleted en­gine assem­bly tray with front bat­tery boxes.

1 4 6 2 5 7 3 8 THE BUILD: 1/ Ma­chin­ing a se­condary sprocket. 2/ Drilling the en­gine plates. 3/ De­tail­ing on the pri­mary drive. 4/ Mak­ing the mould for the seat. 5/ Shap­ing the plug for the rear mud­guard. 6/ Here’s the tank split-mould. 7/ Mo­tor and fan-cool­ing ar­range­ment. 8/ An amaz­ing 8bhp from this!

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