Very good James.
I might add, whether, as the result of bump steer the car darts in one
direction or not, the driver may feel the effects of bump steer when the
steering wheel "jerks" when hitting a bump. On an MGB if the lowering
is accomplished by shorter springs you will feel more jerk when hitting
the same bump. In the extreme, the car can be uncontrollable.
While one reply to the original question indicates the condition of the
suspension is the reason for bump steer, this is not truly accurate.
The reason is as James explains, and it occurs in varying amounts based
on the relationship of the A arms and the tie rods. That relationship
is changed by modifying spring heights, and therefore the amount of bump
steer is affected. Now to CMA I must add, it's likely bump steer will
be noticed more when components wear out, but worn components are not
the reason for bump steer.
Larry Hoy
>-----Original Message-----
>On Thu, Mar 22, 2001 at 03:14:13PM -0500, Edwin McCarroll
>profoundly declared:
>> But can anyone tell me the answer to : 'What is bump steer?'
>
>Change in toe settings during suspension travel.
>
>> Is it something I want?
>
>NO
>
>>Something I need?
>
>See above
>
>>Something I need to acquire?
>
>no
>
>>If so, where do I get it?
>
>You already have it.
>
>>How do I know if I have it?
>
>All front suspension systems have some
>
>>Please help with my edification. Is this topic mg list
>compatible?
>
>I hope so
>
>>So many questions ... so few answers.
>
>Ok, now that I've addressed your questions. You set your toe
>when the car sits at normal ride height, if you measure it at
>any other position in suspension travel it will be different,
>maybe not noticable to you but it will be different. If you
>were to draw these measurements on a piece of paper, you will
>see an arc of toe settings, the one you wanted will be in the
>center of the arc, and you will be at max toe in/out
>(depending on design) at full droop and compression. The idea
>is to make the radius of the arc as large as possible, i.e.
>make the line look straight, and keep your suspension as close
>to your toe setting as possible during suspension movement.
>If you hit a bump or rut in the road and the two sides of the
>car compress equally then you won't feel this, but if you hit
>a bump and don't compress equally (i.e. most bumps) you will
>feel this as the car sharply pulling off the line you were
>traveling. The second place you feel this is also in braking.
> Assuming !
>that your brakes work evenly, i.e. you keep going straight on
>a smooth road when you apply the brakes. In both cases the
>problem comes from your toe setting changing due to bump
>steer, and your unequally loaded front suspension has more
>grip on the heavy side and that tends to determine where you go.
>
>Ideally toe is intended only to compensate for forces incurred
>in driving so that your tires stay square. When traveling in
>a straight line, there is rolling and aerodynamic resistance
>to your tires, these things compress rear a-arm bushes, and
>stretch fronts, etc. The toe setting is ideally intended to
>compensate for that movement, so that when all forces set in,
>the flex will be cancelled out by the toe in. It can be used
>for other minor influences in handling, but that is/was it's
>primary intention.
>
>In summary, bump steer is bad, but you have to live with some,
>just like the rest of us. It this is one reason that people
>always tell you not to lower a car with cut springs, as you
>get further away from the design ride height, the rate the
>bump steer sets in increases, this means near full compression
>the rate might change 10x more per inch of wheel travel, than
>it did near design ride height.
>--
>James Nazarian Jr
///
/// mgs@autox.team.net mailing list
/// (If they are dupes, this trailer may also catch them.)
|