In an effort to find the best bike for riding everything from off-road to motocross (in Florida—not for racing, but for fun) I settled on a KTM 450 EXC, similar to Karel Kramer’s KTM 525 MXC. I thoroughly enjoy the bike and find that the worse conditions get, the better it performs. I even ride it on motocross tracks after adjusting the clickers significantly.
In my search for the best all-around bike I have ridden CRF450s, YZ250s, CR250s, etc. to compare the bikes with mine on various terrain. I have found that the motocross bikes handle the off-road sand whoops better, but ride harshly over roots and logs. I understand this is a factor of the suspension being much more firm, thus the reason my off-road bike cannot handle large jumps.
However, something that perplexes me is that the motocross bikes corner significantly better. When coming into a tight, sandy turn, the motocross bikes such as the CRF450R and YZ250 seem to feel lower and just track through the turn amazingly well, whereas my KTM wants to push through the berm (basically go straight through the turn rather than around it).What dynamics cause this? I want to find out so I can either modify my suspension, learn to deal with the differences, or consider switching bikes if I find this turning feature to be the most important attribute to me. I am extremely interested in your knowledge on this matter.
Jarrod ThompsonJarrod,There are a couple of potential factors that may affecting the turning of your KTM. The first is weight and it’s placement. Your bike weighs about 255 and new 250F models weight under 220. The additional weight makes the bike harder to stop and turn, especially in sand. The old “an object in motion tends to stay in motion” thing.In addition, and motorcycles are affected by rotating mass in the engine. The rotation—especially of the crankshaft—of parts in the engine generate a gyro effect. As you may remember from playing with tops or those little gyro toys that used to be sold, when you get a gyro spinning it resists movement.Also, the length of the engine’s stroke is especially critical. The KTM 400 and 450 are virtually the same bike, and share the same bore, but the 400 has a shorter stroke. The 400 feels lighter and flicks from side to side easier. An EXC also has a heavier flywheel-weight as part of the ignition, and a starter clutch that rotates at all times the engine is running. Those parts add to the gyro effect that must be overcome when turning. The added mass also helps the suspension deal with roots and rocks and greatly enhances traction.Suspension set-up on the KTM is critical to its turning manners. KTM says the bike should have between 30 and 35 millimeters of rear suspension sag for the bike without a rider. That means that a measurement between the rear fender and the axle should be taken with the bike on a stand (with the rear wheel off the ground), then measure the same points with the bike sitting on level ground.The KTM should also have 110mm of sag with the rider on board. For example, if you have 33mm of bike sag, but 90mm of sag with you on the bike, the shock spring is too stiff. If you have more than 110mm then the shock spring is too soft. If you are close, I’d preload the shock spring more and go with less bike sag. That will hold the rear end up a little better.Also, it is easy to get carried away going stiffer (clicks in, or clockwise) on the adjuster at the bottom of the shock. That makes the bike feel more calm entering turns since the feel of the suspension has less hop to it. But the KTMs like to feel a little loose, so a rebound setting close to stock (usually around 24 clicks out from all the way in) will help the bike turn. If you slow the rear shock rebound, the rear squats and doesn’t fully return. As a result of the bike squatting, the forks get raked out. That will make the front feel like it is pushing through the turns rather than tracking.For sandy terrain you may have good results with having a suspension company modify the suspension. Ask local racers and see if there is someone in your area who is good with sand set-ups.Hope this helps.Karel Kramer