Tie rods

[shah]

hello all,, because of the new rule for the 2009,, we had to significantly change our design, as a result our steering rack is connected to steering arm via tie-rod which are at an inclination of 16deg. Previously we had horizontal connection...
How does this geometry effect the bump steer, ackerman steering , and consequently stability..,.
all replies welcomed ,,

[shah]

hello all,, because of the new rule for the 2009,, we had to significantly change our design, as a result our steering rack is connected to steering arm via tie-rod which are at an inclination of 16deg. Previously we had horizontal connection...
How does this geometry effect the bump steer, ackerman steering , and consequently stability..,.
all replies welcomed ,,

[lporter]

another failure of the search function....

[Grant Mahler]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by lporter:
another failure of the search function.... </div></BLOCKQUOTE>

No kidding...Google must be down too. And damn...the library is all out of books.

[shah]

hey guys!!!
i did search but i did come accross any elevated tie rod topics.. so if you can help i would be great ful

[Alastair Clarke]

Shah,

In an effort to be slightly more constructive - are you doing some kind of suspension modelling, like in ADAMS or some other suspension package? If so, why not change the orientation of your tie-rods on that, and do some simulations to assess the effect on bump steer and ackermann? Failing that, if you have a solid model of your suspension, try putting it through varying amounts of bump and see how much bump steer you get.

Alastair

[Brian Evans]

Rack height relative to steering arm height is one of the key determinants of bump steer. So having the rack well above the steering arm will probably result is significant bump steer, with no other changes. Maybe you can figure out a way to reduce the effect on bumpsteer, with longer or shorter arms or something like that...

Brian

[JHarshbarger]

Bump Steer: Your tie rod should be roughly in the plane of the two a-arms. Once you're close to that, you can fine tune bump steer. There's no other means for reduce bump steer.

Ackermann Steering: Won't affect it, unless it is angled forward or backward, and that still doesn't really decide Ackermann geometry.

Stability: No difference, beside bump steer problem. However, your tie rods will have more force running through them the steeper your incline is from horizontal.

[shah]

tnx for replies
I thought so the bump steer might be a problem.. lets what happens we are making a moke design and will play around with it.. Adams hummm we got the software but unfortunately there is no one this year to guide us ...

[Grant Mahler]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by shah:
tnx for replies
I thought so the bump steer might be a problem.. lets what happens we are making a moke design and will play around with it.. Adams hummm we got the software but unfortunately there is no one this year to guide us ... </div></BLOCKQUOTE>

No one to guide anyone else either. Just sit down and play with it for a day.

[exFSAE]

Harshbarger -

I've found that the sweep of the tie rods forward or backward has a major effect on Ackerman progression. Ie you can have a pro Ackerman "slope" at small steer angles but then progress to reverse Ackerman at large steering inputs, or vice versa.

[Composites Guy]

It is inexcusable at the college level to do no modelling of your suspension. There are several free computer software packages out there which are more simple to use than Adams. You could write your own code. You could use a CAD assembly. Or... if you have no access to computers at all, you could build a physical model with changeable member lengths.

[1975BMW2002]

In our first year we didn't have any modeling software. So we modeled stuff in Excel using lots of equations we found in books, some equations we derived on our own, some tricks to do iterative solutions, and other stuff. It didn't perform a simulation of a car driving any particular geometry, but we could change parameters, and see how it effects ackerman, bump steer, roll centers, roll migration, and other stuff. It was very primitive, but it was good enough for mid fifties finishes on skid pad and autocross with two drivers that had never driven the car before that day.

So find a way to model it.

[Christopher Catto]

Shah,

you will find most of us get frustrated about questions fired off without research. but if you have the time to put into FSAE, and you should have if you want a good shot at it then:

1) make an ACCURATE mockup (no cardboard! try and tig or mig steel pieces together to get good component stiffness so you can get good measurements

2) if you have few books to guide you on suspension, try and model the problem in Excel using simple trigonometry. I know, it's in 3 planes so it may require matrices in the more advanced stages if you want to compute things quickly

3) if you use some simple CAD (Autocad will not do, use Solidworks or Solidedge if you're on a budget) then you can gather some data from moving the rack a certain distance, say a total of 120mm in 5mm increments.

good luck

[James Morris]

You can download optimumK demo, a pretty good kinamatics program from optimumg.com
This should be able to answer your questions.

James Morris
SMU Racing
Swansea Metropolitan University