ASK THE MXPERTS: THE TALL & SHORT OF MODERN CHASSIS DESIGN

On Mar 22, 2023 Last updated Mar 22, 2023

The rider triangle of the lowered Husky FC450 is identical to that of the taller KTM 450SXF. How is that possible?

Dear MXA,

I believe that MXA should stop whining about KTMs being too tall. It is a good thing that KTM caters to everyone—KTM is for the tall riders and Husqvarna is for small riders. People get taller every year, and it is easier to lower a bike than to scale it up.

You need to get a grasp on the dynamics of chassis design. Taller is not better. It raises the center of gravity, and all objects behave as though their mass is concentrated at a point called their center of gravity. The basic facts are that the lower the center of gravity, the easier it is to keep your balance. If you’re sitting on a chair, you can lean over more than if you’re standing on it. With your center of gravity low, you can lean further to one side or the other without creating enough turning force to tip you over. That’s why motocross bikes, like Formula 1 cars, should be designed with as low a center of gravity as possible. The lower the mass is to the ground, the less risk of tipping over. Balance is an important factor in how to make a motorcycle go faster. Think of it this way: if you stand up in a canoe, you raise the center of gravity. Within a few seconds, you start wobbling and end up wet. Stay low in a canoe, and balance is easier to maintain. The same goes for a motorcycle.

Taller riders have advantages in the whoops thanks to their long legs, but the standard issue rider triangle does force them rearward—which accounts for taller bar mounts, lowered foot pegs and higher seats.

Contrary to what you think, it is not easier to lower a bike properly than it is to make a bike taller. When Husqvarna’s engineers lowered the Husqvarna motocross bikes, they had to manufacture shorter cartridge rods for the fork’s internals and shorter stanchion tubes. If they hadn’t, they would have moved the fork’s internal piston too far away from the center of the stroke and WP’s cross-over slot. On the rear, they modified the shock’s stroke, lengthened the shock linkage arms, changed the bell crank’s rising-rate geometry and designed a taller seal cap.

Shorter riders are pulled forward more and benefit from their lower center of gravity, but they often can’t touch the ground.

What they didn’t do was change the rider triangle. The handlebars, seat and footpegs remained the same distance apart, meaning that whether the rider was tall or short, the three contact point (seat/bars/pegs) were no different on the taller KTM or lower Husqvarna. A side effect of lowering the 2021-23 Husqvarnas was that riders with short legs could touch the ground.

As for your assertion that people get taller every year, science does not support that conclusion. For example, from around 1710 to 1830, the average height of an adult American man was 68 inches tall. Over the next 60 years (1830 to 1890), the average height actually dropped down to 65 inches. Then, over the next 40 years (1890 to 1930), the average height climbed back up to 69 inches. Today, the average height of adult American men is still the same as it was in 1890.