LocostUSA.com

look guys, you are designing with a clean sheet so why design in any bump steer.

its all down to rack width and location, look for a rack that could be narrowed or lengthened, my preference is for porsche 914 racks with pacific customs end forks, an inch can be removed from each end or by adjusting the forks made an inch longer at each end, thats plus or minus 4 inches and the pinion is in the center so it can be inverted for front or rear steer.

a separate bar that mirrors the rack bar could be added so that it could have track rods in the center, and with the availability of electric power assist, the world is your oyster.

examples of excessive bump steer.

the boys from Janet Air over at Groome lake, formerly known as area 51, were having trouble with bump steer on an old "Roswell MK1" the problem being, in a tight turn you could completely miss the row of cone shaped asteroids just past Alpha Centori and end up in the damp spot of the Milky Way which is to say the least hostile space, it was all due to someone, probably Paul Greyson, leaving a spare ignition key in one of those magnetic key boxes under the passenger side rocker panel upstetting the guidance system, he keeps loosing keys because his eye sight is pretty piss poor in an oxegen/nitrogen atmosphire kind of like earthlings under water.

at the British museum in London, they have a replica of Bodicia"s charriot with the knives sticking out of the axles, it would appear that the blue painted people that built the original purposfully designed considerable bump steer into the alignment causing it to zig zag all over the place as it passed through the Roman Centurion legions chopping legs off at the knee by the hundred. this is why to this day, there is never enough leg room in an Italian car and scooters always have a pass through.

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this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

Good ones, Mr. H!!!

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JD, father of Quinn, Son of a... Build Log
Quinn the Slotus:Ford 302 Powered, Mallock-Inspired, Tube Frame, Hillclimb Special
"Gonzo and friends: Last night must have been quite a night. Camelot moments, mechanical marvels, Rustoleum launches, flying squirrels, fru-fru tea cuppers, V8 envy, Ensure catch cans -- and it wasn't even a full moon." -- SeattleTom

You ask how important is bump steer and I ask you how important is it to you to be able to steer to exactly where you want to all the time?

Bump steer can make the car scary to drive under certain conditions (when it's happening in other words). And you know what? It always seems to happen at the worst time and leave you with the dirtiest pants if not a bent fender.

If you can design around it. If you can't then you'll have to see if you can live with what you have OR will you have to tear it all down and do it all over again.

I usually find I prefer to do it right the first time cause either I never seem to get around to doing it over or it just too much frickin work to do it over and I get rid of it.

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mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.

There is so much written about bump steer with so many variables between builds and using donor components from various manufacturers. I think most would agree that unless starting with a clean sheet you will get close but not perfect, I personally only check for bump steer in what I consider the "normal" working range of the suspension.

Bob

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Build Log viewtopic.php?f=35&t=16640&p=187700#p187700

Mancave http://s1116.photobucket.com/user/mypic ... ow/mancave

Didn't you redo your suspension due to some unidentified handling issue? I'd give bumpsteer more credit than this.

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-Andrew
Build Log
Youtube

Sorry, no convincing arguments so far.

I did learn a couple of interesting things though. One, bump steer in the front dive condition can decrease front traction enough to lengthen braking distance. Two, bump steer can be used as an offensive weapon in chariot design.

Let’s take a look at suspensions for a moment.

Toe in – Having the toe wrong will completely mess up handling and ruin your tires. This is worse than bump steer.

Camber change during suspension movement – Swing axle cars are a good example of what can go wrong here. Ralph Nader didn’t base his career on bump steer.

Caster – I had a Beetle with nearly none. It was darty and would not return to center. It had to be held straight. Waaay worse than bump steer.

Shocks – How many have driven cars with shock that lost all of their fluid eons ago? Darn Brit lever shocks. They put Lucas Electrics in their cars so people wouldn’t notice the bad shocks.

Springs – Overly soft springs or putting the upper mount in the wrong place will cause the suspension to bottom out. Ask me how I know that. Definitely worse unless you are in NASCAR then it is called Coil Binding and treated as a good thing.

Tires – You can totally transform a car when you replace crappy tires with the right ones.

Sure, bump steer is not a good thing but it seems to get more mention here than anything else leaving the impression that it must be right or don’t bother building your car. Why design in bump steer? I don’t know if anybody actually designs it in except for the previously given example of the car companies. There are 10,000 things on a Locost that need to be designed, built and worried over. Maybe it was 10,003. I lost count somewhere in the nine thousands. There is another thread here about stuckness. It is easy to get stuck in the details to the point where nothing gets finished.

So why didn’t I take the time to check for bump steer on my Locost? The rack I have was free (this is a low cost remember?) It is a Sweet rack that is normally used on modifieds, sprint cars or similar. It is a nice solid rack and the price was right. It is also about 4 inches too short. I have a few Miata racks that I could have used but they would have to be shortened and I wasn’t looking forward to that job. So my car is finished and on the road not waiting for me to get around to a job that I didn’t want to do. It steers and handles great at normal road speeds except for the bottoming part. I’ll fix the bottoming this winter. As for the rack length, well I’m putting a new interior in the Mini and I’ve got another car that I’ll be building once the weather turns colder and the house projects get put aside until spring.

I wasn’t meaning this as an insult to the people that enjoy geeking out on suspension design. If you are racing then getting it right is critical but not everybody is racing. For some people driving around town in a car they built themselves is what they are here at LocostUSA for.

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My car viewtopic.php?f=37&t=16434
1930s Style Sports Car viewtopic.php?f=36&t=16888

my examples, however critical to the dawn of or demise of mankind, were ment to lighten the approach to bump steer and how critical it is or isn't.

a friend of mine has a singer car of pre war design, it has a modern engine and he has added rack and pinion steering, however he had no consept of bump steer and the whole steering thing was a nystery to him.

the first time he drove it, it was uncontrolable, the rack was at least 4" to far forward and at least 2" too long, the steering arms on the uprights were rear steer but used for front steer.

yes, bump steer was the least of his worries, but it just goes to show that what we may consider obvious, to some people is a consept they are just not aware of, just like scrub radius or ackaman, just things you must consider when designing the steering and suspension.

his vehicle was seriously dangerouse, not due to his construction skills but due to things he just had not considered at all until he tried to drive it.

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this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.

My 2 cents, dear old dad used to complain of the bump steer on his old Caterham. The car would dart around on slightly bumpy roads. Personally, I never noticed on the Seven I built to Lotus specs, nor did I notice bump steer on the Eleven I built. I have read that the Europa crowd complain about bump steer on the early cars (series 1, 2 etc). When I put my chassis together and set the rack height to stock there was a fair amount of bump steer. I have mostly corrected it by lowering the steering rack, but its not dialed out completely. There might be a toe change of 1/64" through the suspension travel at zero roll. The rear suspension does show some toe change under bump and droop but the change is toe in.

In regards to trying to rid bump steer completely from a Locost, I think its a good theory but it would eat up a ton of time trying to actually achieve it. And I have to agree with John, if you're building a car from scratch, there should be no reason to build in bump steer. If I were to build another car, I would attempt to keep the toe change under suspension movement to the absolute minimum but by no means spend countless hours trying to sort it out.

Rod

And, if a person wants to take bump steer seriously, he/she should also consider compliance steer.

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Peace, Ron

Trust me, the mental picture of a deadly chariot veering wildly through a crowd of Roman soldiers is priceless. I'll make sure to pay closer attention next time Ben Hur is on TV.

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My car viewtopic.php?f=37&t=16434
1930s Style Sports Car viewtopic.php?f=36&t=16888

I think you can minimize bump steer with a standard book frame and donor rack just by having the correct rack location. Length isn't as critical. IMO there's too much emphasis here on shortening racks. Mine's about 4" too long for the "ideal" design, but I kept my tie rods straight and I don't have any measureable bump steer, at least within the first inch of bump. I think Champion had it right in the book. Minimize it, then don't worry about it.

Run87k, there is one problem with your argument. You are comparing something that is built into the car with things are are either easily replaceable or adjustable. To me, that is reason enough to do it right the first time.

If bump steer was adjustable, would you bother to adjust it?

Bill

The answer is quite simple. At this point it is not a problem. There are still teething troubles that need to be fixed such as the bottoming front suspension. I have two other project cars and the time is better spent there. Everyone assumes (well some of you) that this will be as simple as sliding the rack one way or the other. I'm guessing that new ends for the rack will need to be fabricated and a new bracket for the rack as well. The whole point to this thread is that there seems to be a great emphasis placed on bump steer leading some who are new to all of this to worry more over it than may be necessary.

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My car viewtopic.php?f=37&t=16434
1930s Style Sports Car viewtopic.php?f=36&t=16888

My experience sorting out the B-3 handling causes me to agree wholeheartedly with the above. I have bolded the two which I consider absolutely key.

like you my rack (Porsche 924 manual) is not optimal lengthwise for the control arm lengths, as the inner tie rod pivot point is approximately 4-4.5 inches inside the line defined by the upper and lower control arm pivots.

when I first assembled the front end, I had placed the rack so that the tie rods were level with the ground, but could easily see that the tires would toe out when I jounced the frame downward.

my solution was to measure the effect on toe thru 2 inches of bump, and 1 inch of droop, and then position the rack vertically such that bump - within that range - was as close to zero as I could get it. then I just sprung the front end stiffly enough to ensure that suspension movement rarely exceeded that range.

with all that, the steering was only slightly darty.

BUT, the greatest improvement in steering stability happened when I replaced the bias ply rear tire with a radial. with that change, 80% of the dartiness in steering disappeared. I experience a further reduction in dartiness (which was already minimal) when I increased toe-out this past spring.

I don't notice any kind of directional instability when the suspension is in bump. all the improvements I've seen in steering stability have come thru managing toe, and tire selection.

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The B-3 build log: http://www.locostusa.com/forums/viewtopic.php?f=36&t=13941 unfortunately, all the pictures were lost in the massive server crash

The beginnings of the Jag Special,
https://www.locostusa.com/forums/viewtopic.php?f=36&t=19012
Again, all pictures were lost.

This thread is like someone asking why anyone would paint a Locost red. He asks for reasons why he should consider red to be okay, but belittles and mocks any replies; it's clear he's already made up his mind. It's like that line from the movie "Avatar", "It's hard to fill a cup that's already full." If you want to disagree, fine, but quit claiming that everyone else is wrong, foolish, or ignorant. Most people understand that it's subjective. Some people don't notice it at all, some notice it very easily, and some builders correctly reason that it doesn't make sense to design it in, so they don't. Do what you want, agree to disagree, and move on.

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