Bump Steer on a lowered 240

[cmathews]

I was wondering if anyone knew how much the bump steer was affected by changing the ride height (lowering with Eibach pro springs 1.8in drop) on the s13 240sx. I am reading "How to make your car handle" by Fred Puhn and it looks like there is an easy way to adjust it if the change is significant, but I have never heard anyone outside of the mod car scene talk about bump steer-- also, this is the first time I ever understood what it was.

Chuck

[DSC]

I think bump steer would have more to do with stiffness then ride height but I guess we can assume (or hope) those go hand and hand.  I don't have them, so I can't say first hand, but pro-kits aren't really that hard so I doubt you'd see a big increase in bump steer.  Your dampening will also play a big role in bump steer but, again a set of koni's or kyb agx aren't going to be THAT hard so bump steer shouldn't be too much worse.

[drift freaq]

bump steer has to do with suspension travel and ride height as much as stiffness . I.E. you can have a very stiff suspension without bump steer provide your travel is correct in relation to spring height.
Now what is commonly seen on cheap suspension jobs i.e. cut springs is bump steer for a number of reasons one is the rate of the spring has been destroyed by cutting it.
usually when one does this the spring is no longer long enough to handle strut travel which causes the spring to bounce in the perch. It also causes bottoming of the strut insert which in turn causes bump steer. It is actuated when you hit a bump and the shock or strut bottoms the bottoming combined with the bump cause the car to jerk left or right i.e. bump steer.
Now if you lower a car to around 2 inches or below spring wise then you are looking at a travel issue i.e. the strut travels to far for the spring and bottoms because the spring is not there to cushion the travel.This another example of a situation that could cause bump steer. The way to correct this issue is to use shorter travel struts and shocks to compensate the shorter spring travel . they then work together like they are supposed to. so in the basic thing to keep in mind is if you use lowering springs that lower your car 2 inches or more you should look to put in shorter travel struts.
Now something I noticed when putting in my AGX's was the fact that they were shorter travel than the stock struts so they were indeed designed for lowering springs.

hope this helps

[DSC]

From Turnfast.com:

Bump steer-- is the introduction of steering input caused by a bump in the road surface. When a wheel travels up and down over a bump, it actually swings in an arc. If the steering tie-rod does not swing in the same arc as the suspension, the wheel will have an increase or decrease in toe and will result in a slight steering effect. Virtually all suspension geometries on street cars have some degree of bump steer, through it will be very slight. However, modifying a car's suspension by using lowering springs can increase the amount of bump steer. Bump steer causes tire scrub, and loss of traction if experienced during cornering. If you're altering you car's suspension, measures should be taken to minimize or eliminate bump steer.

[tnord]

i would think bumpsteer could also be induced by using underdamped shocks (maybe not in the true-est sense of the word). this would occur because as the tire follows the path of the "bump" up, the tire stays in contact with the surface. after it gets over the crest of the bump, it is now mostly the shocks job to keep the tire on the road. if the shock is dead or not valved appropriately, the tire will lose force against the surface, or if the situation is bad enough, actually lose contact with the surface. of course this results in a loss of grip and the car slides off its intended path.

this is all theory i've just thought up myself. so it could be completely useless.

[bbp]

DSC definition seems right on to me.  There are bump steer gauges out there that will measure this for you.  On a street car I really don't think there is much control over it, since there is limited adjustment.  Plus,  your toe will change under braking and side load due to the use of rubber bushings (both front and rear).  

I have not measured the bump steer on my car and probably will not, unless I learn a way to adjust it.  Don't know much about it on the 240's.

{Hi Jack in Progress}  Let's talk about caster and ackerman steering next!  I would like to learn more about these adjustments....any starters?

[drift freaq]

ok I will take a stab at caster. Caster basic is where your tracking and wheelbase are set front to rear. Now with adjustable caster you can change the length incremently by a small amount which actually shortens or lengthens ones wheelbase. now on extremely short wheelbase cars this abilitly to adjust allows them to lengthen the wheelbase slighlty giving the car different tracking charasistics and sometimes improving handling or the opposite on a overly long wheelbase car shortening the wheelbase through caster adjustment can make the car track better. their is like an optimum wheelbase figure for flat cornering adjustable caster can get a car closer to this figure. Of course it can also affect toe which will make the car turn in differently again another trick racers use to change the handling charasistics of their cars. An example of side to side tracking changes affected by Nissan is the fact that their older sports cars were all narrower than the ones they produce today. Reason being is there is actually somewhat truth to that Pontiac adverstising slogan wider is better. Your car will stay flatter on the ground when the contact patch is widened for the wieght of the car. I.E. a 73 240z weighs the same as our cars but is narrower in its contact patch. Therefor you need to do more suspension work to it to make it corner nice and flat. Wherease our cars have a wider side to side track and corner flatter with less suspension straight from the factory giving it higher skid pad ratings stock. Which means that set up it should out handle a 240z anyday of the week.
Now in racing they always widen the body to put extremely wide wheels and tires on so they can increase the width of the contact patch. Again this is the racer trying to compensate manfactures design decisions to make the car perform better.
Back to caster hehehe. Yes if you change caster you will change your turn in charastics and corner tracking. that is why they mess with it on race cars.
Is it necsassary on street cars? . Not unless you &nbsp;autocross or do stuff like (the dreaded dastardly word comes up) drifting. hehehe, &nbsp;he said, drifting <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'>
Or you have a screwed up front end and need to get it back to stock.
well I don't know about ackerman he was a kid I knew in high school I don't how he would affect suspension besides being a nut . hahahhahahhaha <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/hehe.gif" border="0" valign="absmiddle" alt=':hehe:'> &nbsp;<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/crazy.gif" border="0" valign="absmiddle" alt=':crazy:'> &nbsp;<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/hehe.gif" border="0" valign="absmiddle" alt=':hehe:'>

[cmathews]

Congratulations, DSC was the first one to identify the correct definition for bump steer. The other situations masqurading as bump steer are not "normal" conditions found on a car. They involve bottoming out of suspension components and non-compliance issues.

As for whether or not you can adjust bump steer on a street car-- well, sort of. Basicly, the easiest way to do so is to shim your steering rack. I have not done this on any car, so I don't know how hard it is going to be. My main concern was how much bump steer was introduced by lowering a 240sx (although not slamming it to the ground).

I guess I will just have to measure it myself and then post my results, followed by the procudure for adjusting it and a summary of how much I think it helped.

Next, caster. I have never heard of caster changing the wheelbase or track of a car. (I suppose it could change track a small amount under hard cornering with lots of suspension travel, but I will have to think about that one) Caster is the change in camber (camber is the tilt of the top of the wheel in relation to the bottom of the wheel as you look at it from the front) as you turn your wheel. Generally there is no way to adjust castor on a street car (without heavy modification). My opinion as to why this is--- Caster changes affect the bump steer. Changes in bump steer affect the saftey and driveability of a car. I think that it becomes a legal risk to have that adjustment on most regular cars. Street car suspensions are designed to minimize the mistakes of idiots.

As for the thread on Akerman, I will start that discussion tomorrow in a new topic. I have a pretty solid understanding of it now that I have read several Carrol Smith books and "How to make your car handle" by Fred Puhn. Tomorrow I will have more time to write a good description. For anyone who really wants to know everything they can about it, read "Tune to Win" by Carrol Smith.

Chuck

[tnord]

hey chuck........

you seem like quite the knowledgeable guy. probably more so than i &nbsp;<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/hehe.gif" border="0" valign="absmiddle" alt=':hehe:'> everyday normal people, haven't heard of books by carrol smith and the like. i'm just wondering how old you are and what your background is like. any SCCA experience? SoloI, II, W2W?

i'm not trying to question your knowledge by any means. just curious if you're a racer or what have you.

[DSC]

Travis, I think Mr Gerst got it last month...Chuck should get the newbie of the month award this month &nbsp;<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>

[bbp]

I second the nomination! Nice work Mr. Mathews

edit: got name right <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/hehe.gif" border="0" valign="absmiddle" alt=':hehe:'>

[cmathews]

As for the nomination-- thank you. I learn what I can, when I can, from people who I think know what they are talking about. Then, at times that seem appropriate, I let others know what I have learned.

As for my background, etc. I am 22 and attending Northern Illinois University in DeKalb, IL (hometown of Cindy Crawford). My father is a Nuclear Engineer who has been autocrossing since before I was born. I grew up around racers and race cars. My favorite cars growing up were not the Muscle Cars and other things that normal people admire, but rather Lotus Super 7's, Europas, Nissan 510's, Z cars, Honda CRX's, and other small cars that did well on an autocross course. I have always wanted to be a race car driver. I have also always wanted to know how to design and set them up. In 1992 or somewhere in that time period my father purchased a Lola based Formula Ford for autocrossing. Since then I have helped him set many different suspension settings, however, I didn't always know why I was doing each thing. Now that I am older, I have been going back and learning why and how. Any serious racer has read Carrol Smith's books, that is why I have been reading them. Knowledge is power-- and is often more important than horsepower. I was reading a few days ago (How to make your car handle) and read that bump steer and caster are often overlooked by many racers-- but can often make that last 5% of difference in setup that takes you from mid pack to winning.

Later all,

Chuck