I read about it on another forum but i dont know what it is. Tried searching and nothing came up either.

 

Hello Thunder_x..here is something for you too look at for your answer.
http://en.wikipedia.org/wiki/Bump_Steer

WheelBrokerAng

 

yea, i read that but i dont fully understand.

"Bump Steer is the term for the tendency of a wheel to steer as it moves upwards into jounce."

So if i am experiencing bump steer: I would go over a bump on the street and the steering wheel will jerk/move to one side randomly?

 

Read more in that link I put in there and also click on the highlighted areas that explain it more in depth, If you still dont understand what it's saying, go to a front end alignment Expert at say your local Firestone Tire Dealer and they can set up your car on there machines and show you what it's all about...

WheelBrokerAng [sm=exactly.gif]

[IMG]local://upfiles/22/342232881F494C4B85B1655809D08FCE.gif[/IMG]

 

I read that too, and didn't find it particularly clear. Here's something I wrote a few days ago for a post on the 'Ozhonda' site in answer to someones question, hope it helps:

From my understanding bad dampers and / or poor alignment couldn't be a (significant) cause of bump steer, but might increase the severity of an existing bump steer.

To understand why we need to understand what bump steer actually is. True bump steer is caused by dynamic toe change with bump or rebound (which is different to momentary torque steer effects that might feel similar to bump steer). This is typically caused by the steering rack being mounted too high or low, the tie rod end being mounted too high or low, or the tie rod itself being too long or too short. Any of these conditions will cause bump steer in some degree, from severe to not noticable. And, some of these things might only need to a bit 'off' to cause a noticable problem, and that problem may manifest to a greater or lesser degree depending on other factors such as tyre characteristics (i.e. a less directionally sensistive tyre will be less affected by a given bump steer / toe changes because it will 'smother' small steering inputs due to being unresponsive in nature).

As an e.g.;
Looking at a double wishbone suspension from front on we'll see the upper and lower arms pivotted on the chassis pivots, and each arm will rise and fall with the outer end describing an arc (and these arcs will be different depending on the length of the arm). We'll also see the tie rod rise and fall pivotted at the rack end joint, and the tie rod end describing an arc. The arc in which the tie rod end rises / falls should match the arc of the suspension at the point on the strut or upright at which the tie rod end attaches to it. To achieve this the rack and tie rod end must be at the correct heights and the tie rod at the correct length (though this is typically more or less impossible to acheive exactly due to toe being adjusted by means of changing tie rod length, though it will typically be quite close to the correct length within the range of likely toe adjustment).

If any of the above is wrong then bump steer will occur because as each component rises and falls in arcs the effective distances (of the ends of the arms and tie rod) from the centres of pivot become laterally longer or shorter, and if these distances don't become longer or shorter in a manner that is 'matched' to the arcs of the other components then the ends of the tie rod will move in or out (laterally) relative to the steering axis and the toe will change with bump and / or rebound. Got that?! This is hard to describe!

Possibly the most likely reason for introduction of excessive bump steer is modification of suspension / steering geometry. E.g. if you substantially increase caster by moving the top of a Mac strut or upright (of a double wishbone suspension) backward then you'll also tend to lower the height of the tie rod end because you will have 'rotated' the strut or upright around a point centred on the lower ball joint, which will cause the end of the steering arm to be lowered to some degree. On the other hand if you increase caster by moving the bottom of the upright or strut forward then any increase in bump steer will be less because you move the strut / upright in an arc centred on the top of the strut or upper ball joint, and the end of the steering arm will lower a lot less (maybe not at all, depending...) than if we'd moved the top of the strut or upright backward.

It's very difficult to design and build a suspension / steering set up that has absolutely no bump steer at all (with some suspensions it's utterly impossible, e.g. front trailing arms on old VWs where the suspension doesn't move in arcs but straight up and own but with the tie rods moving in arcs), but the designer will (should) attempt to achieve as close as possible to zero or minimal bump steer at / near the static ride height. Due to manufacturing tolerances it wouldn't be unlikely for a car to have some degree of significant bump steer from the factory, but unlikely to be a substantial problem most of the time (especially with less responsive tyres fitted). Having said that, a lot of effort might be put into dialling out any significant bump steer on a chassis intended for serious competition.

Even on a well set up car it wouldn't be unusual though for bump steer to become significantly worse the further into bump or rebound you go. So, if you have a bad damper you might expect to see more adventures deeper into bump (and rebound to a lesser degree), possibly resulting in noticably increased bump steer.

Poor toe alignment could in theory cause the tie rod to be too long or too short, and thus introduce some bump steer, but I suspect it would have to be quite a lot wrong to cause a significant problem (I could be wrong). Soft suspension bushes might also increase bump steer tendencies, though the mechanics aren't the same as above, it's more to do with the bushes compressing and allowing whole suspension arm to move inward and outward while the tie rod end doesn't.

 

JohnL you've got some wonderfull, informative posts but my goodness you must type fast *L*

 

Not nearly as fast as I'd like, but the more you type the faster you get! I'm glad someone is finding some of my stuff interesting, I find it more entertaining to write this stuff than watching rubbish on the box, and the box is filled with rubbish at this time of the year. If I were less lazy I suppose I should be readingsome 'worthy' literature, maybe War and Peace etc etc etc etc...

Back to bump steer, that article suggested that bump steer was deliberately designed into the front end as a means of introducing roll understeer, but IMO this is wrong. Bump steer in the front end will have affects in roll, but because the steering is free to be steered, what you would get is not so much 'roll understeer' as 'roll need more steering input', i.e the driver would just unconsciously wind on more steering to keep the car on the desired line as it rolls in the corner, and it will have no true affect on understeer characteristics.

I think it much more likely that it's simply that zero bump steer is hard to achieve on a mass production line, and that the manufacturer will assume some bump steer will occur so err toward a bump steer that when cornering will tend to momentarily widen the steered line rather than tighten it when you hit a bump mid corner (which might induce an oversteer in a more extreme situation), but a perfect set up would have zero bump steer.

The rear is a different story, toe change with roll is deliberate and carefully designed to induce roll understeer (good safety thing, but only for unskilled drivers), and can do so because the rear end can't be 'steered' by the driver. The down side is that this introduces bump steer, which is really an unwantedby-product of the intended purpose of the roll toe change (i.e. roll understeer). Any bump steer front or rear is undesireable, and for a 'performance' car any roll steer is probably not a good thing, unless it's introduced as a means to couteract some underlying deficiency in the basic suspension design (possibly more common than we'd like to think). I know most racing car designers go to pains to ensure as much 'linearity' as possible in suspension behavior, and this includes elimination of bump steer front and rear (and thus by implication the elimination of roll steer).

One of the less recognised benefits of stiffer springs (and dampers) and increasing roll stiffness in general is that it lessens suspension motion, and as such will reduce any bump steer and roll steer affects simply because the suspension moves in bump and rebound less.

 

thanks, very helpful