Technology Honda CRF 450 R

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Technology Honda CRF 450 R

Technology Honda CRF 450 R
For the sake of simplicity

Why complicated when it can be made easy? This is what the Honda technicians thought and designed a cylinder head for the four-stroke Crosser CRF 450 R with different controls for the inlet and outlet valves.

Gert Thole

06/11/2001

Two overhead camshafts, abbreviated dohc (double overhead camshaft) ?? This is so natural with modern high-performance engines that the manufacturers no longer explicitly mention it in the brochures. Today, most motorcycle engines have two camshafts rotating in the cylinder head. The only exceptions are the single-cylinder off-road vehicles. For two reasons: Firstly, in many cases the structures are old. Second, ohc engines have advantages (see box on page 50) that are particularly useful in enduros and crossers.
In addition, until recently, off-road machines were less dependent on high liter output and corresponding speeds. One drew from the full, for 60 hp from 600 cm3 displacement and more, speeds around 8000 / min are completely sufficient. The turning point came in 1997 when Yamaha competed in the half-liter world championship with a high-revving four-stroke engine measuring just 400 cm3 and revolutionized the world of cross-country skiing. Of course, the engine had a dohc head, the only way to achieve speeds of over 11,000 rpm with such large cylinder units.
The competition was shocked, only now is Honda responding to the Yamaha offensive with a four-stroke crosser. All experts expected a two-cam design like that of Yamaha. The experts were all the more astonished when Honda entered a prototype of the new four-stroke engine in the Japanese championship in autumn 2000 (MOTORRAD 24/2000, page 144), whose unusual shape of the cylinder head already indicated that it was by no means a dohc engine could act.
Now Honda is revealing the secret and presenting the production four-stroke crosser CRF 450 R, which will be available from the end of the year. The key data initially correspond to common values: a stroke of 62.1 millimeters and a bore of 96 millimeters result in a displacement of 449 cm3. So for a future class the bike would be 250 two-stroke / 450 four-stroke ?? in the USA already common? well equipped. The special feature can be found under the unusually shaped valve cover. Only a single camshaft, driven by a toothed chain, rotates there. However, not centrally between the inlet and outlet valves, as usual, but exactly above the former. It operates it directly via bucket tappets. The advantage: The higher moving masses of an ohc drive are elegantly bypassed on the inlet side with the inherently larger and therefore heavier valves. In addition, the 36 millimeter components are made of titanium, which, according to Honda, saves 15 grams each compared to steel. The specified maximum speed of 11200 rpm does not pose a problem at all for such a valve train.
But how do you get the same speed stability for the exhaust area, where a longer rocker arm is then inevitably required? Certainly only with a light yet stiff lever. To do this, Honda used two tricks. Firstly, a record-breaking valve angle of only 21.5 degrees, which reduces the distance between the valve stems and thus shortens the length of the rocker arm. Second, the designers designed the component as an angle lever. The camshaft no longer acts on it from below but from the side. A roller reduces the friction, which would otherwise be very high due to the large gear ratio and which would put a lot of stress on the sliding surfaces. Light adjustment plates replace the heavier adjustment screws. The spark plug could be cleverly positioned centrally between the fork of the rocker arm.
This mixed construction, so Honda, combines the advantages of both variants, namely little mass and a compact design. The technicians have calculated a weight saving of 380 grams compared to a dohc design. Since part of the moving masses is omitted, the Honda single should hang on the gas more lively and rev up more explosively. The Unicam design also makes the cylinder head extremely compact and lowers the center of gravity of the engine.
Kick-off is made easier by an elaborate decompression device, which opens the exhaust valves minimally when kicking off by means of a centrifugal mechanism. If necessary, it can also be operated using a lever on the handlebar. While all European four-stroke bicycles have similar starting aids, Yamaha has so far not done so. Amazing, because an automatic deco is not only helpful when starting, but also makes it easier to start a stalled engine.
Like Cannondale, Honda uses a double oil circuit with two separate wet sump systems. The gearbox and clutch are sealed off from the crankcase. One pump lubricates the gearbox and clutch, the other the engine’s crank and valve train. The advantage of this principle is that the abrasion of the clutch does not get into the engine circuit. Dispensing with an oil tank impairs cooling, but reduces weight.
Weight was the top priority anyway, because this is where the Honda marketing strategists identified a weak point in the successful Yamaha four-stroke engines. With consistent lightweight construction on every part, such as a titanium manifold, Honda wants to achieve a dry weight of 102 kilograms. Rumors from the USA speak of a ready-to-drive combat weight of 106 kilograms. This would mean the number one stage: The direct competitor, the 111 kilogram Yamaha YZ 426 F, is ?? at least in this discipline – beaten by far.

Twice better?

There are still camshafts lying underneath (Harley), but overhead camshafts are common today, either in a single (ohc) or double (dohc) version. In ohc engines, the cylinder head is flatter and the drive is also simpler. Rocker arms transmit the cam lift to the valves. The necessary play is set using small screws. the rocker arm usually works with a translation, the cam lift is then smaller than the valve lift. The disadvantage is the indirect transmission and the higher accelerated mass due to the swinging rocker arms. This is why the valve train usually determines the speed limit in ohc engines. Dohc motors are more stable in terms of speed, in which only valves, springs and bucket tappets and adjusting plates are accelerated. The bucket tappets absorb lateral forces without transferring them to the valve. To adjust the play, the camshafts usually have to be removed. The direct transfer of the cam lift to the valve ensures precise control times up to the highest speed regions. In addition to their storage, the two shafts need a lot of space above the valves, and the space required to drive the two shafts is greater than with the ohc arrangement.

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