Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/126471
The first really big trick was to lower the chassis. It needed to be lowered so that when the bike was sliding, the center of gravity would be low enough that the bike wouldn't high side. It aliO neeaea to be lower so that the rider could comfonably slide with one foot'OD the ground, and also hopefully, eliminate the bike's awful tendency to wheelie at the slightest provocation. And horsepower wasn't causing the wheelieing. It was (ready for the buzz won:ls?) the combination of the center of gravity location and the Instantaneous Force Center common on all motocrosaers. The IFC is a composite of the forces acting through a severely angled swingarm , intersecting the forces acting and extending through the tangent points of the countershaft sprocket and the rear wheel sprocket. It's called anti-squat, and it 's too much anti-squat in this motocross bike's case . Calculating the weight transfer on this bike stock, using a gravity ac celeration of .71 , the total weight on the front wheel under acceleration is 21 pounds with a 154 pound rider in the forward position. It's trying to wheelie just sitting there. Not too swift. After I lowered it, the bike had niorestatic weight on the front end, arid IeM weight transfer under acceleration. In fact. it developed a pronounced case of pro-squat coming off of the corners. I took it from too much weight transfer to not enough. That wasn't so good either, but it sure beat the heck out of looping the bike on the starting line and ha ving the handlebars meet your teeth on every gear change. The pro-squat manifested itself by partialIy unloading the rear tire under accel• eration. But, sitting further back on the' seat. compensated for it. That's what made the two 250cc riders uncomfortable at first . Lawson and I , easily compensated for it by moving back about three inches. Lowering the chassis was easy. First, the tall stock rear shocks were removed. A pair of 15,75 -inch S&W gas cell shocks were installed to lower the ride height. And since the available travel in the new shock was grea ter than the remaining available travel at the raised rear wheel , a stack-up of travel limiters had to "b e installed on the shock shafts to keep the wheel from bottoming on the f~~der. Happily, S&W makes those too. Use two each ~ -inch and one each l-2 -inch limiter on each shock shaft. That makes for a two inch limiter on each shock. Also , narrow bushings are required in each shock eye to fit the Honda shock mount. Those have to be ordered from SAW too. Order two kits. And speaking of mounting the shocks, Do not mount these shocks upside down. They yill not valve properly upside down as will some of the other brands. O rder the dual rate springs listed in the pans list in the article. They'll cover up to a 175 lb . rider. The shocks and spring rates were another compromise which were to be lived with, rather than to be chosen by optimum design. They are not valved heavily enough for this combination of leverage ratio and short travel. It's not the shock's fault; they were designed to be stroked faster, and therefore become stiffer in valving characteristics. But since the piston accelera tion rate in this circumstance is slow, the shocks ' act lazy. And to keep the springs from overcoming the valving, thesprings have to be softer than conventionally used in smooth dirt track applications. However, the combination does work, and there is no chance of wearing out the shock in a lifetime. And boy , does it give a cushy ride on a ratty track. Plus, it's got a right price on it. Now that the back end if lowered , the front end has to come down in a matching fashion. It's a straightforward deal. Remove the handlebars, unbolt _ the fork caps , unbolt the damper rods, and pull out the fork springs. Cut six inches off the fork springs and be sure to cut from the end with the coils spaced closely together. That way , you'll be cutting off the coils from the low rate portion of the progressive rate spring. Heat and flatten the spring end, Since we want to stiffen the front end of the motorcycle . this is one way of accomplishing both the lowering and rate stiffening at the same time. At the same time we would like to have the front forks absorb the slight bumps in the track. To do this I took one CR125 Honda upper fork spring and made a pair of two inch long springs, one to sit on top of each shortened CR250R fork spring. Now the total free length of both springs stacked up Perts Uat TT Honda CR250R Project Bike QaUlltity 1 ea. 1 ea. 1 ea. 1 ea. 1 pro 1 pro 2ki~ !kit 2kiu !kit 1 ea. 1 ea. 1 ea. l ea . lea. Item 1979CR.125upper l ork spriJlI 1980CR.125Steel .bifl1eftr R.otor puller Supplier Pan No . RODda Dealer 51402' ftC 000 $9 ea. RODda Dealer 2f701-4fi6.eOO $8.50 ea. 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