Speculations on Triumph’s 250 Motocross Engine
6 min read
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In accordance with established Web advertising rules, Triumph has revealed a couple of particulars and images of its new four-stroke 250 motocross engine, with broad hinting that it’ll in time serve in different roles.
As some have already commented, its basic look is like that of the present KTM 250, with forward-facing exhaust, main drive on the suitable, and chain-driven DOHC. Additionally just like the KTM the crankshaft’s main pinion additionally drives a jackshaft straight above it, the place the two:1 discount for the cams happens. The water pump is on the outer finish of this shaft.
This produces some confusion, as with the underside cam-chain sprocket turning reverse to the crank, the cam-chain tensioner is now on the entrance of the chain housing, giving the preliminary (however false) impression that the engine spins reverse to the wheels—within the case of Yamaha’s first M1 990cc MotoGP engine of 2002, the tensioners being on the entrance gave right info—that engine did spin reverse to the wheels.
The brand new Triumph’s bore and stroke haven’t been revealed however the numbers for this 12 months’s different 250s are crowded up near a 1.5 ratio of bore over stroke:
KTM: 81 x 48.5 (identical as its RC250 GP engine for Moto3)
Honda: 79 x 50.9
Kawasaki: 78 x 52.3
Suzuki: 77 x 53.6
Yamaha: 77 x 53.6
We will additionally evaluate these 250s with one cylinder from both a World Superbike or a MotoGP bike. Whereas World Supers engines sometimes peak at shut to fifteen,000 and MotoGP nearer to 18,000, these 250 MX engines are redlined at round 13,500. In Moto3 roadracing, redline is at the moment set at 13,500, which is suggestive as a result of this restrict was supposedly set to forestall an costly “rpm race” of the sort that drove System 1 V-10s and V-8s to twenty,000. As a result of dynamic stress rises because the sq. of rpm, the pistons, valves, and con-rods of 13,000 rpm engines see solely 75 p.c of the stress traditional in Supers, and 60 p.c of that in MotoGP.
One other doable motive for rpm moderation is that this Triumph has a pressed-together curler crankshaft—like others in its class. Unforgettable within the proprietor’s service e-book for Honda’s authentic 450 MX bike was the recommendation to exchange the crankshaft each 10 hours of racing.
Motocross just isn’t roadracing. With all of the elbow-to-elbow happening, it’s important to have an engine that pulls throughout a large rpm vary. It’s additionally doubtlessly good for gross sales, as on the murky, typically coyly out-of-focus Triumph teaser video it’s claimed that you simply don’t need to be an knowledgeable rider to take pleasure in this bike.
A main booster of engine torque is a excessive compression ratio, so all these 250s have numbers within the vary 13.75 (Suzuki) as much as 14.5 (KTM). The bigger the bore, the nearer the piston should come to the top, making it unattainable to mix a number of valve overlap (which requires slicing the piston crown for valve clearance) with excessive compression. Which means has Triumph chosen to compromise? Bigger bores—particularly with excessive compression—can gradual combustion. But in addition they present room for large valves. Yamaha’s 250, not too long ago very profitable, has the smallest bore (77mm). However there’s way more to MX success than the engine.
On this previous decade a number of work has gone into making engines breathe deeply with little or no valve overlap and with consumption closure shortly after backside useless middle (BDC). It started in roadracing, the place marginal tire grip throughout off-corner acceleration wants very easy and controllable engine torque.
On the identical time, right here got here the European Fee, setting the exhaust emissions commonplace we all know and love as Euro 5. As a result of valve overlap (each intakes and exhausts barely open collectively within the neighborhood of TDC on the finish of the exhaust stroke) permits recent cost to flee by the exhaust valves, it has been deeply chopped to fulfill the brand new commonplace. Luckily, this had already been carried out in roadracing, however to flatten the torque curve, to not reduce emissions.
Because of this super-lightweight finger followers have changed the heavier inverted bucket followers in latest engines. To compensate for the additional circulate previously offered by valve overlap and late consumption closure, designers have elevated elevate. OK, that makes circulate sense, however should you elevate the valves increased in shorter complete valve-open timing, doesn’t that significantly improve the valve acceleration that have to be used?
You wager it does, and that’s why so many engines (doubtless together with this Triumph) use the lightest doable valve prepare components to permit valve springs of acceptable stress to precisely management valve motion. And this 12 months’s KTM has titanium valves—solely 70 p.c of the load of metal. We’re not informed what materials is on this Triumph’s valves. And I’m solely guessing once I counsel finger followers.
Right here’s the rule: The shorter the valve-open time, the extra your engine acts like a easy air pump, and the flatter and smoother its torque curve turns into. Good for avenue, good for racing, good for emissions.
However the longer you make valve timing (lifting the intakes many levels earlier than prime middle [BTDC] and shutting them 50 or extra levels ABDC) the extra rpm-dependent engine torque turns into. At low and mid rpm, the consumption and exhaust waves that make such lengthy timings work close to peak are absent, and so is engine torque. So the longer the valve timing, the peakier the torque supply. You possibly can simply make a number of peak horsepower this manner, but it surely tends to be the hard-to-use old-time energy we known as “mild change.”
One photograph reveals valve prepare components, which embody a gentle “beehive” valve spring (the coils on the prime spiral with a decreasing diameter, giving a beehive look). Such springs resist coil resonance, the place the coils bounce quickly, finish to finish, significantly growing the variety of fatigue cycles per minute, thereby shortening spring life.
We’re additionally given an edgewise view of a piston: Like all trendy pistons it’s little greater than a disc to help the rings, a pair of wrist pin bosses, very brief skirts to maintain the disc sq. to the bore, and stiffening ribs below the disc or crown.
Piston shaking pressure will increase because the sq. of rpm, so a small and lightweight engine like this one actually hammers every little thing connected to it. That requires a stability shaft.
For those who pull up the specs on the engines on this class, you discover claims from 46 to 50 or extra peak horsepower. Then up comes Cycle World’s 2022 KTM dyno test: 39.8 hp at 13,400 rpm, with peak torque of 19.1 lb.-ft. far down at 8,700 revs. OK, manufacturing unit numbers are taken on the crankshaft, whereas CW’s are on the rear wheel. Within the latter case we have now main and transmission gear losses, chain loss, and possibly largest of all, rolling resistance from a tire strapped down firmly to drive a curler dyno. Effectively, people, the racetrack additionally measures on the rear wheel, and a tire churning by grime eats energy too.
Fascinating to notice that KTM’s Moto3 250 provides its peak torque a lot increased, at 10,500, indicating the extent to which circumstances in MX require robust midrange.
What has Triumph informed us? As little as doable save that the looks of its engine and the internals revealed is standard, and that the crew growing it are effectively conscious of the necessity for robust bottom-end and midrange. Bear in mind the sensible phrases of the late Don Tilley: “Them younger fellers love the large numbers however we gained a lotta races by maximizing horsepower averaged over the rpm vary truly used.
Triumph has gained a number of patrons for its street bikes by giving them broad, rideable torque moderately than the slender “supersport” energy that punishes errors. Not a nasty technique it doesn’t matter what type of bike you’re designing.
