Speculations on Triumph’s 250 Motocross Engine

[Admin]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

In accordance with established Internet marketing principles, Triumph has revealed a few details and photos of its new four-stroke 250 motocross engine, with broad hinting that it will in time serve in other roles.

As some have already commented, its general appearance is like that of the current KTM 250, with forward-facing exhaust, primary drive on the right, and chain-driven DOHC. Also like the KTM the crankshaft’s primary pinion also drives a jackshaft directly above it, where the 2:1 reduction for the cams occurs. The water pump is on the outer end of this shaft.

This produces some confusion, as with the bottom cam-chain sprocket turning opposite to the crank, the cam-chain tensioner is now on the front of the chain housing, giving the initial (but false) impression that the engine spins opposite to the wheels—in the case of Yamaha’s first M1 990cc MotoGP engine of 2002, the tensioners being on the front gave correct information—that engine did spin opposite to the wheels.

The cam-chain tensioner on the front of the engine caused many to believe the engine spins opposite to the wheels, but that is not the case. (Triumph/)

The new Triumph’s bore and stroke have not been revealed but the numbers for this year’s other 250s are crowded up close to a 1.5 ratio of bore over stroke:

KTM: 81 x 48.5 (same 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 can also compare these 250s with one cylinder from either a World Superbike or a MotoGP bike. While World Supers engines typically peak at close to 15,000 and MotoGP closer to 18,000, these 250 MX engines are redlined at around 13,500. In Moto3 roadracing, redline is currently set at 13,500, which is suggestive because this limit was supposedly set to prevent an expensive “rpm race” of the kind that drove Formula 1 V-10s and V-8s to 20,000. Because dynamic stress rises as the square of rpm, the pistons, valves, and con-rods of 13,000 rpm engines see only 75 percent of the stress usual in Supers, and 60 percent of that in MotoGP.

Another possible reason for rpm moderation is that this Triumph has a pressed-together roller crankshaft—like others in its class. Unforgettable in the owner’s service book for Honda’s original 450 MX bike was the advice to replace the crankshaft every 10 hours of racing.

Motocross is not roadracing. With all the elbow-to-elbow going on, it’s essential to have an engine that pulls across a wide rpm range. It’s also potentially good for sales, as on the murky, often coyly out-of-focus Triumph teaser video it is claimed that you don’t have to be an expert rider to enjoy this bike.

A primary booster of engine torque is a high compression ratio, so all these 250s have numbers in the range 13.75 (Suzuki) up to 14.5 (KTM). The larger the bore, the closer the piston must come to the head, making it impossible to combine a lot of valve overlap (which requires cutting the piston crown for valve clearance) with high compression. Which way has Triumph chosen to compromise? Larger bores—especially with high compression—can slow combustion. Yet they also provide room for big valves. Yamaha’s 250, recently very successful, has the smallest bore (77mm). But there’s much more to MX success than the engine.

In this past decade a lot of work has gone into making engines breathe deeply with very little valve overlap and with intake closure shortly after bottom dead center (BDC). It began in roadracing, where marginal tire grip during off-corner acceleration needs very smooth and controllable engine torque.

At the same time, here came the European Commission, setting the exhaust emissions standard we know and love as Euro 5. Because valve overlap (both intakes and exhausts slightly open together in the vicinity of TDC at the end of the exhaust stroke) allows fresh charge to escape through the exhaust valves, it has been deeply chopped to meet the new standard. Fortunately, this had already been done in roadracing, but to flatten the torque curve, not to cut emissions.

This is why super-lightweight finger followers have replaced the heavier inverted bucket followers in recent engines. To compensate for the extra flow formerly provided by valve overlap and late intake closure, designers have increased lift. OK, that makes flow sense, but if you lift the valves higher in shorter total valve-open timing, doesn’t that greatly increase the valve acceleration that must be used?

You bet it does, and that’s why so many engines (likely including this Triumph) use the lightest possible valve train parts to allow valve springs of acceptable pressure to accurately control valve movement. And this year’s KTM has titanium valves—only 70 percent of the weight of steel. We are not told what material is in this Triumph’s valves. And I’m only guessing when I suggest finger followers.

Here’s the rule: The shorter the valve-open time, the more your engine acts like a simple air pump, and the flatter and smoother its torque curve becomes. Good for street, good for racing, good for emissions.

But the longer you make valve timing (lifting the intakes many degrees before top center [BTDC] and closing them 50 or more degrees ABDC) the more rpm-dependent engine torque becomes. At low and mid rpm, the intake and exhaust waves that make such long timings work near peak are absent, and so is engine torque. So the longer the valve timing, the peakier the torque delivery. You can easily make a lot of peak horsepower this way, but it tends to be the hard-to-use old-time power we called “light switch.”

One photo shows valve train parts, which include a mild “beehive” valve spring (the coils at the top spiral with a reducing diameter, giving a beehive look). Such springs resist coil resonance, where the coils bounce rapidly, end to end, greatly increasing the number of fatigue cycles per minute, thereby shortening spring life.

Triumph’s 250cc motocross engine has “beehive” valve springs. (Triumph/)

We are also given an edgewise view of a piston: Like all modern pistons it is little more than a disc to support the rings, a pair of wrist pin bosses, very short skirts to keep the disc square to the bore, and stiffening ribs under the disc or crown.

Piston shaking force increases as the square of rpm, so a small and light engine like this one really hammers everything attached to it. That requires a balance shaft.

If you pull up the specs on the engines in this class, you find claims from 46 to 50 or more 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, factory numbers are taken at the crankshaft, while CW’s are at the rear wheel. In the latter case we have primary and transmission gear losses, chain loss, and probably biggest of all, rolling resistance from a tire strapped down firmly to drive a roller dyno. Well, folks, the racetrack also measures at the rear wheel, and a tire churning through dirt eats power too.

What power will these parts produce when assembled? (Triumph/)

Interesting to note that KTM’s Moto3 250 gives its peak torque much higher, at 10,500, indicating the extent to which conditions in MX require strong midrange.

What has Triumph told us? As little as possible save that the appearance of its engine and the internals revealed is conventional, and that the crew developing it are well aware of the need for strong bottom-end and midrange. Remember the wise words of the late Don Tilley: “Them young fellers love the big numbers but we won a lotta races by maximizing horsepower averaged over the rpm range actually used.

Triumph has won a lot of buyers for its road bikes by giving them broad, rideable torque rather than the narrow “supersport” power that punishes mistakes. Not a bad strategy no matter what kind of motorcycle you’re designing.

View the full article