wolfe10 wrote:
Bump steer most commonly occurs when the axle is inadequately located/attached to the chassis.
So, one front wheel hits a bump. It drives that side of the axle back causing it to steer in that direction.
Particularly common on Safari coaches with Torsilastic suspension, as there is only ONE forward link (driver's side). So, if right wheel hits a bump that side of the axle is driven rearward, which IS a steering input. Easy fix is to add a forward link on the right side. Created quite a cottage industry for Ralph Andrews/Pioneer Metals in WA state.
Bump-steer is not officially defined as "steering because one has hit a bump which
directly forces the wheel out of desired alignment", although this is often the intuitive interpretation.
"Bump" is defined, in engineering terms, as any vertical compression of the sprung suspension and is the opposite of "droop" which any decompression of the sprung suspension. "Bump", the wheel goes up, "droop", the wheel goes down. The important difference is bump-steer is when the swing of the wheel going-up swings in a different arc than does the "tie-rod" link (so to speak). If the tie-rod arc does not track the wheel arc, the tie-rod seemingly gets shorter or longer and adds
indirect unwanted steering input. The unwanted steering input is often a toe-in/toe-out change.
Thanks for the tip on Safari coaches with Torsilastic suspension!
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427435 wrote:
It's apples and oranges, but I recently had the suspension components all replaced on my 67 Corvette. It steered like **** when I got it back-------all over the road. This was after the shop that replaced the suspension parts had it aligned. The shop that aligned it, used standard 67 alignment specs. Those were set up for bias tires back in the day. It now has (and has had) radial tires. I took it to another shop that increased the caster as is normal for cars with radial tires. The car now steers nicely.
As a downside, radial tires act like a normal bias tire that is running separate within a stiff steel hoop that actually touches the pavement. The stiff hoop can tend to wander on it's own depending on road tilt and/or varying lateral cornering forces. Greater caster muscle-guides the hoop a bit more to follow the aimed wheel direction (the wheel always remains dead center on it's own caster). The hoop is, of course, the steel belt.
The plus is that the hoop/belt better avoids conflict losses of tire tread squirm, delivering much less wear and maximum uniform traction over the face of the tread.
Wes
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