The Stovebolt Site Forums The Shop Area Driveline Rack and Pinion on a Straight Axle???

Buy plenty of life insurance before you install that system!
Jerry

I read some prior discussions here on the stovebolt forum before posting this yesterday. It seems many jump in and predict a horrible death from R&P on a straight axle, but the reasons why are not discussed. I would like to understand the "why".

So far, there have been three negative items mentioned...

1) He saw one on a truck that was done "Mickey Mouse". I have seen some IFS jobs on vintage trucks that could be described the same way. Do it right or don't do it.
OK, you do have a point there. The truck I saw was definitely done with an aftermarket kit. Even then, the whole thing looked very shakey to me at best.

2) "questionable steering column that has to telescope and flex with the suspension" To me, this is the major drawback to R&P on a straight axle. The rack has to be mounted on the axle for it to work, so the steering shaft has to be able to move with the axle. But it's not something that I feel cannot be worked out to be reliable and safe. There are drivelines on all sorts of agricultural equipment that telescope and flex while running at 540rpm all day every day...as long as it is kept greased.
Big difference between a driveshaft and a steering shaft. Loose the driveshaft, you only loose power to whatever is being driven. Loose a steering shaft, you loose all directional control. Also, a beam axle not only flexes up and down, but there is also some side to side motion. Having the steering gear mounted to the axle rather than solid to the frame will lead to bump steer issues because of that flex.

3) "add unsprung weight to the axle" So you remove the tie rod and add a rack. Considering the total weight of tires, rims, rotors, calipers, axle...is the net increase in unsprung weight really that much?
The weight of the brakes, tires, wheels, and axle are all figured in when the suspension was originally designed. The tie rod on these trucks maybe weighs a few pounds. A typical rack and pinion weighs somewhere around 50-80 pounds, depending on whether it is power assist or not. Adding that much weight to the axle as unsprung weight will affect the handling and ride, that's why none of the oem manufacturers mount the steering gear that way, that does apply to larger wheels/tires, heavier brakes, etc.
I am curious what others thing about this. Please jump in.

Tim

My reasons for not are as follows

Why??? whats it gaining you. It doesn't get rid of bumper steer it may well add bump steer into the front end that you didn't have before.

The sliding shaft is going to send a lot of shock and vibrations up the shaft and into your hands now that its location is un-sprung and has no shock suppression. Hit a bump and its tire and what ever little absorption the slide joint offers then your hands.

The relatively short telescoping shaft with a U joint on both ends will have a bad habit of binding at certain angles, particularly if oddball terrain is encountered. The movement of the axle, the springs, and maybe a little frame flex added in can cause the steering column shaft to get into an angular bind sufficient to lock the steering wheel at a deadly dangerous moment. Since the rack and axle move as a unit, bump steer won't be a problem, but the geometry of the telescoping shaft will be unpredictable and dangerous.

BTW, I have run telescoping PTO shafts on my farm equipment for 30-something years, and your argument comparing them to a steering column shaft is totally bogus. There is virtually no comparison since the operating angles are very shallow on PTO equipment and the shaft length is many times what a steering pinion shaft would be.
Jerry

Just keep it theoretical, please. Building one to actually run on the street will be deadly dangerous to yourself and the rest of us who have to share the road with you. 50-something years of being closely involved with dirt track race car building has taught me that there are plenty of people with great plans on paper who should not be given access to the tools to try to make their pipe dreams a reality. At least on the race track, everybody is aware of the risks, and we take our chances willingly. The innocent drivers and/or pedestrians that are put at risk on the public roads by incompetent metal butchers don't deserve to be put in that sort of danger.
Jerry

the real problem is the pinon gear wont stay stationary relative to the steering column not only do yo have to deal with it moving vertically at the same time it can be twisting.
very nasty bump steer

PTO shafts only move in one plane.

straight front axle move a point moves in more then one plane

What is the reason or advantage to put a rack and pinion steering on a straight axle vehicle?

I like it, and as said NO bump steer like other conversions.

"This kit includes new steering arms to help correct accerman[sic] and reduce bump steer."