STEERING ARM LENGTH?!!!

I suspect that you are a student - one rule of this forum is to not help with homework/school; however, these links should help you:

http://eprints.usq.edu.au/2272/1/Freeman_Barry_2006.pdf

http://machinedesign.com/ContentItem/70954/Theinsandoutsofrackandpinionsteering.aspx

http://www.zakgear.com/Rack.html

P.S. - I also suspect that this is urgent because you have neglected to spend the time upfront. But I was once a mech eng student and understand the time constraints.

thanks for the help sir. you are right for every word. yes i am a 3rd year mech student. actually its not my homework or assignment, but a sae competition that we are participating into. i m responsible for the steering system and am trying to gather as much info about it as possible. as far as giving time is concerned, i have been able to give some amount of time for the study, but have certain doubts. its for clearing those doubts that i m here. it would be gr8 if seniors like u cud help us students. i also gratefully welcome any kind of further help. thanks again.

Often you will be limited by the rack available. In other words you have to make a particular rack work. But virtually any rack can be made to work, for the reason you imply in your post title: if the rack travel is small, then the steering arm must be short, etc.

Most of this can be figured out graphically. For example, you can draw a circle to represent the desired steering circle. Plot the wheelbase on that circle, and you can see the road wheel angle required to follow that circle. You can also see the difference in angle required at the outer and inner wheel. However, because the load on the wheels in cornering will be different from side to side, you might not want the two wheels to follow the apparently correct arcs (in other words to have full Ackerman effect).

You will want to keep turns from lock-to-lock to about 2.5 to 3, probably. (Much less than 2.5, and the steering starts to feel "twitchy" to people not accustomed to driving go carts, and the effort may be too high, also.) You can put stops on the rack itself (sturdy nylon rings just inboard from the ball joint that connects the rack shaft itself to the inner tie rod) to limit rack travel, if necessary.

If you know the rack travel and the desired steering angle, then trig will tell you steering arm length.

Steering ratio is the number of turns of the steering wheel to (partial) turns on the steering axis or the road wheels. 15:1 is fairly common, meaning that turning the road wheels through 1/6 of a full turn (60 degrees, which a fairly typical total left-right steering angle) would require 15/6 turns: 2.5 turns. But there are many things to consider.

You will want to read up on bump steer, which will limit the location of the rack, fore and aft and up and down. (Bump steer is the tendency of a deflected wheel to steer in or out because the arc that the wheel moves through has a different center than the arc the tie rod moves through.) If the rack is incorrectly located the amount of bump steer can be dramatic, with the wheels toeing out (for example) several degrees when both wheels hit a bump.

Playing around with sticks (etc.) to represent linkages can be beneficial to get a feel for what you need. There are computer programs for dealing with this stuff, but they are not as instructive as actually building a crude model out of sticks and scotch tape, in my opinion.

To avoid impaling your driver should a crash occur, (and for other obvious reasons) you will want the steering shaft to have a couple u-joints. Read up on these, to understand why the output can turn at a different rate than the input (and means to compensate.)

Tie rod length, you'll have to find out the distance from your inner rod-end to the outer one, also allow distance at the ends for the rod-eyes or knuckles that join the tie rod to either end.

you have 2 options (doubt there are more) for the center mounting location point to your rack.

This can be either a centre mount, or a end mount, end mouting is getting very common on cars, where the tie-rods exit the ends of the racks then its a short distance to your wheels, or the centre mount style that there is a plate protruding from the centre of the rack and allows the t-rods to be mounted as close to one another as possible, this is the style I'm using on my Off road buggy, it also minimises "bump steer" the travel on road cars is quite small so the bump steer is also minimised (your going over large bumps at low speed so its less noticeable)

Below are pictures of my setup (most likely more travel than your looking for)

Travel of the tie-rods is limited to the number of turns on your rack, the length of travel on the rack and the angular travel permitted by the front end.

Upon setting the thing up, place a laser or some pointing device on a flat surface, and check the amount of bump steer thru the whole travel, the position of the steering box to the wheel will affect the amount of bump steer.

also, make sure that thru the full travel, you have full travel clearance for the tie rods to the suspension components and to the frame (and steering box) that your running, also lastly, the end ball joints can also travel the required andgles for the travels above.

"total rack travel from lock to lock"

if your pinion has a circumference of 2" and your pitch on your rack is 1/4" and the rack is 10" long you would have 40 teeth on rack

so 40 teeth/2" circumference would be 20 " of travel

Question...

With 10" of rack, how do you get 20" of travel?

I would agree if the 10" was from centre to endstop, then lock2lock - 20" but if the rack is 10" long, you only have 10" to travel, otherwise your pinion will not be meshing with teeth?

thanks a lot guyz!!! ur responses have been really gr8 and helpful. keep posting the ideas. i m really grateful to u all for the help. thanks once again!!!

Is there any formula for finding steering arm lenght, tie rod length, steering arm angle for rack and pinion.