Can Changing Windshield Height Affect Handling and Low-Speed Stability?

Curtis Johnson’s Triumph Bonneville develops a loose feel after a windshield modification.

windshields, windscreens, triumph, bonneville
©Motorcyclist

Q. Last year, I bought a 2010 Triumph Bonneville with only 1,300 miles. The seller lowered the bike an inch by moving the fork legs upward in the triple trees and adding Progressive fork springs and shocks. It came with a Long Haul quick-release 16-inch windshield, which I switched to a Cee Bailey's 20-inch shield. The bike always handled like it was on rails, and the OEM Metzeler ME Z2/Z4 tires hold their line well in the curves.

Recently, I cut the original windshield down to 11.5 inches and replaced the Cee Bailey’s with that. After a short ride I noticed handling felt “loose,” mainly when decelerating and below 30 mph. The bike now seems to have developed a slight intermittent squirm or wallow when slowing to a stop.

Tire pressures are within spec, but the bike doesn’t seem as sure-footed as it used to. I’ve checked the steering head, swingarm, wheels, and bearings. I jacked up both ends to check the tires for high spots, flat spots, and runout—nothing was amiss. I didn’t think the change in windshields could have this effect, but when I took the shield off completely the squirm seemed slightly reduced but still there. Any ideas what could cause this?

Curtis Johnson
Pinson, AL

Got tech questions that need ANSWERS? Email us at mcmail@bonniercorp.com.

A. Even our resident Bonneville owner is stumped by this one, but there are some things you can try. The windshield is the obvious place to start, since changing it introduced the problem and taking it off reduced it. You might have pinched, trapped, or otherwise interfered with the normal movement of the cables, the brake hose, or switch wiring when you swapped shields—in effect if not in fact tightening the head bearings. Double-check your work here. It might help to look at an otherwise stock Bonnie or check the service manual to see that you have the hose/wire routing correct.

A loose feeling on deceleration is most often related to steering-head bearings, brake condition, and/or tires. Any abnormal wear or cupping on the front tire will promote this feeling, as will high spots on the brake rotor. We’re inclined to think there’s something impinging on the steering, so give the head bearings one more very careful look.