‘NEVER ANY DOUBT THAT IT WOULD BE A PAR­AL­LEL TWIN’

Stu­art Wood,

Motorcycle News (UK) - - New Bikes -

“The very first thing you do when de­sign­ing a new bike is de­cide on the en­gine, it’s the key el­e­ment. You de­sign the whole pack­age to­gether and the heart of any bike, es­pe­cially ones such as the Bon­neville or Thrux­ton is the mo­tor.” “There was never any doubt that it would be a par­al­lel twin. The ca­pac­ity was also set from the start, we had a lot of dis­cus­sion and could have gone big­ger but 1200cc was a nice bal­ance to cre­ate a log­i­cal step up from the 900 and make a fam­ily of Bon­nie mod­els.” “The mo­tor is smaller and nar­rower than the old air-cooled en­gine and has a pre-unit Tri­umph look like the orig­i­nal Bon­nie. Lo­cat­ing the drive train on the right al­lowed us to use sym­met­ri­cal cov­ers and that mim­ics the 1959 bike.” “We used a sin­gle over­head cam but it is driven by a chain and gears. The crank has a gear on its cen­tre and this turns a sprocket on an­other shaft, which then turns the chain that links to the cam. If you didn’t have this ar­range­ment, and ran a di­rect chain from the crank to the cam, you would need a large sprocket on the cam.” “The T120 uses fin­ger fol­low­ers in­stead of a bucket and shim val­ve­train to re­duce the head’s height. This also al­lows the head to be very nar­row and meant we could ex­pose the en­gine studs on ei­ther side, which keeps the en­gine look­ing au­then­ti­cally retro, and also ex­poses the spark plugs.” “The cam is slightly con­cave, un­like a tra­di­tional cam which is con­vex, to op­ti­mise com­bus­tion ef­fi­ciency. It also con­tains a de-com­pres­sor sys­tem. A bob weight on the cam sprocket is forced out by cen­trifu­gal force when the en­gine is turned over on the starter mo­tor and this ac­ti­vates two rods on the ex­haust cams which open the ex­haust valves to de­com­press the en­gine – so we can use a smaller bat­tery.” “The en­gine uses two bal­ancer shafts. You can get away with just one, adding the se­cond com­pletely bal­ances the en­gine and re­moves all vi­bra­tions. The crank has a 270-de­gree off­set to give it more char­ac­ter while the clutch is a ramp-style slip­per clutch which gives a lovely light lever ac­tion.” “The wa­ter pump is un­der the gear­box and the ther­mo­stat is at the top front of the en­gine, so the cool­ing sys­tem only needs one small en­try pipe into the top of the ra­di­a­tor and one small exit at the bot­tom. The fins not only look good, they help cool the mo­tor, mean­ing the ra­di­a­tor can be smaller.” tyres, which look great and work well in the dry, are a bit lim­it­ing when the road is damp, but thank­fully the T120 comes with trac­tion con­trol and ABS should the Bri­tish sum­mer hold true to form. Which brings me to the one part of the Bon­nie I feel own­ers won’t gel with – its brakes.

I rode the T120 on both dry and wet roads and didn’t like the dead feel­ing I ex­pe­ri­enced through the lever from the two-pis­ton slid­ing calipers. The rest of the Bon­nie is so re­as­sur­ing and ac­com­plished that the brakes stand out as be­ing un­der par. I know steel braided lines are a neat fea­ture, but they don’t have that slight bit of re­as­sur­ing give you get on rubber hoses. On a sports­bike such as the Thrux­ton you want in­stant stop­ping power, but on the Bon­nie I was hop­ing for re­fine­ment, and it just isn’t there. It could be a pad is­sue, but I’m more in­clined to point an ac­cus­ing fin­ger at the lines or mas­ter cylin­der. It’s a small blot on an oth­er­wise clean copy­book, but one that I think could rile buy­ers. But the new Bon­nie is cer­tain to be a bike own­ers will grow to love, es­pe­cially if they dip into Tri­umph’s ac­ces­sories cat­a­logue.

If you look at the new Bon­nie and think ‘that’s for me’ then it’s pretty cer­tain that you’ll love it, es­pe­cially if you are step­ping up from one of the older air-cooled mod­els. As well as be­ing re­fined to ride, com­fort­able, and beau­ti­fully styled, it has a gor­geously smooth mo­tor and over­all is a very prac­ti­cal retro for the mod­ern world. It’s a grown up Bon­neville aimed at rid­ers who will ap­pre­ci­ate and ex­ploit this new-found level of ma­tu­rity. How­ever, if you favour per­for­mance over practicality and aren’t quite ready to slow down just yet, the Thrux­ton is a far more sat­is­fy­ing ride, while the T120 Bon­neville could leave you feel­ing a lit­tle short-changed.

Bon­neville Chief En­gi­neer, re­veals the in­ter­nal se­crets of the new en­gine

Heart of the Bon­nie 1 Bon­nie’s par­al­lel lines 2 Smaller on the out­side 3 Clever in­ter­nals 4 Ex­posed plugs 5 Con­cave cam trick Ca­pacit y is up but Tri­umph have en­gi­neered the twin to be compact 6 Added smooth­ness 7 Keep­ing cool

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