Yamaha It250/400 Steering Fix
Yamaha IT250/400 Steering Fix
Pulling Back For Better Bite
by Len Vucci
We tested the Yamaha IT250E for our April, 1978 issue, and liked it very much, so much that we secured one for a long-term project bike. The bike will remain basically stock, in order to evaluate its long-range reliability and overall demeanor. But we'll be making various bolt-on modifications, representative of the changes an owner would make, to tailor the bike for serious off-road riding without limiting its utility as a weekend playbike. As we stated in the test, the E model IT is an improved version of its predecessors, but still has several shortcomings which can be fixed by the resourceful owner.
One major complaint involves the steering, which isn’t as responsive as it could be. The factory’s choices of 31.5° steering head angle and 5.5 in. of trail are a sign that the designers were more interested in straightline stability than in zipping through tight turns. This isn’t a bad choice; many IT buyers ride for fun and a stable bike is fine for cowtrailing and exploring via fire breaks and power line roads.
Still, the IT250 is powerful. We know from informal comparisons that the IT250E will out-drag the YZ250E. Further, the YZE is fine at top speed in a straight line, this despite the YZ’s greater height and its rake/trail of 30.5° and 5.28 in. Steering excepted, the IT250E handles well at speed and thus appeals to serious enduro riders, whose use of the power and wheel travel is hampered by the lack of response. Ridden hard, the stock IT front wheel washes out, especially on loose surfaces, and in twisty sections the rider spends most of the turn on the tank, forcing the front wheel down. The IT250 needs more bite.
(Incidentally, there may be some rethinking going on at the plant. Word has it the F-series IT line will have the rake steepened some, probably to what the YZs have now.)
Meanwhile, there are a couple ways to speed up the steering of a motorcycle, most of which don’t lend themselves to the IT250.
The surest changes involve steeper head angles. Quickest of these is to slide the stanchion tubes up in the triple clamps, which has the effect of pulling the front wheel back toward the rest of the bike. But on the IT that would reduce ground clearance and put the front tire too close to the fender at full compression.
Next, raising the rear of the bike tips the steering head. But in the IT’s case, modifying the monoshock is tricky and the bike is already fairly tall. Strike that option.
The most extreme modification would be to cut the frame backbone and downtubes at the back of the steering head and change the head’s angle to the frame and thus to the ground. Major surgery. Doing complicated work right requires a higher level of skill and confidence than we or the average owner can boast.
So much for the usual modifications. Instead, we did something different. Profab, makers of various suspension pieces for competition machines, has just gone into production with pulled-back triple clamps for the Yamaha YZ.
By installing a different set of triple clamps, we can alter the geometry to effect an improvement. The Profab clamps bring the stanchion tubes 10mm closer to the steering head axis. The bike’s wheelbase is> shortened by about 0.6 in., and trail is increased by the same amount.
Normally, an increase in trail is just what we would not want for an already slow-steering bike. Still true. But by moving the mass of the fork assembly closer to its pivot axis, we can lower its moment of inertia, which makes the fork more sensitive to changes in direction, and physically easier to turn. We were hoping that the improvement in steering feel would more than offset the increase in trail and would yield an overall improvement in steering characteristics.
The Profab clamps are an across-theboard replacement for the stock items. But in order to reuse the IT’s tapered roller bearings, the bottom bearing must be removed and reinstalled using a hydraulic press. When reassembling the steering, liberally grease the roller bearings. Tighten the stem firmly, but not to the point where binding can be felt as tbe assembly is turned side to side.
Position the stanchion tubes so that each extends above the top clamp 2!/s-in.
The bike was ridden at the sites where it had been tested when stock. Terrain varied from a hard-packed motocross track to desert sandwashes. To a man, each rider felt the bike handled better.
Novice, non-berserko riders claimed the bike was a bit more twitchy when traversing rutted sandwashes and the like, but behaved better everywhere else. But the tradeoff was still in favor of the modified steering.
We’d like to say the modification made the bike’s handling perfect. We can’t. But we can honestly say there is a substantial difference between stock and modified steering. Is the improvement worth the price? This depends on how much money you have to spend, and how serious you are about your riding. Personally, we think an actual steering head modification—if done properly—would be more beneficial, in terms of both results and capital outlay. If time and opportunity allow, we’re going to light up the cutting torch, and pass on the results.
In the interim, there is still a bit of
modifying to be done. When the bike is really pushed hard in corners, the rear wheel doesn’t track as well as it could. We suspect the swing arm is flexing, a condition which seldom happens with the stock steering. In the near future, we’ll be modifying the stock swing arm to provide additional strength and rigidity. If necessary, we’ll swap the stock arm for a stronger aftermarket unit, and reflect the whole project in an upcoming issue.
PRICE AND AVAILABILITY
The triple clamps shown in the photos are machined billet aluminum and retail for $148.29 plus shipping. (California residents add 6% sales tax.) Cast aluminum triple clamps, though unavailable at press time, will be offered soon. They’ll be considerably cheaper, at $ 108.45 plus shipping (plus tax in California). For more information, contact: Profab
Dept. CW 3185 Airway Ave. Costa Mesa, Calif. 92626 (714) 751-6264
