Changing a steering ratio using a longer pitman arm. need help.

On my 36 ford pickup im running a vega box with a 20:1 ratio about 6 turns lock to lock with me spinning the box by hand. my elcamino is like this and i hate it, feels way too loose on the highway. so i would like to change it to about 3.5- 4 turns lock to lock. the box isn't mounted yet so i can move it further back according to how long the pitman arm ends up being.

how do i calculate the pitman arm length to get this desired ratio? im guessing around 15:1. on the axle im using 42-48 ford spindles with speedway 3 3/4" drop steering arms.

thanks!

 

I think once you get it mounted, I don't think the Vega box won't move thru it's entire 6 turns to get you from lock to lock. Probably more like 4 or 4 1/2, but I'm sure someone else can confirm.

If you want to increase the effort by say, 25%, you could make the arm 25% longer from center to center. I think there may also be a fast ratio vega box available with a 16:1 ratio.

 

i dont know how you figure out the formula to do that?! some of the dirt track guys around here make their pitman arms teeny tiny for a quicker steer & end up over steering real easy.

ive driven some rods with TOOOOO QUICK of steering & i DO NOT like it on the highway at all! ------- not to hi-jack this post but id like to hear others input on quick boxes or quick vs slow steering.

 

Without knowing the steering wheel dia. it would be hard to calculate. When steering ratio is refered to as 15:1 it means that the wheels will turn 1 deg. for every 15 deg. the steering wheel is turned. I gather that this is for a cross steer set up, so this doesn't really apply, but FWIW- with my side steer vega box I get about 4-4.5 turns lock to lock with a 14.5 in. steering wheel, 6 in. pitt. arm and 5 in. steering arm.

 

regardless of steering wheel diameter your ratio will not change. you will still get 1 degree for every 15. you will just have more or less leverage to turn. one turn is still 360 degrees if you have a 12" wheel and a 24" wheel.

 

Pretty much like putting on different dia. rear tires to change the final ratio just backwards.

 

You have steering box ratio and steering overall ratio...with car not together, I think you need to measure both. Stick a protractor on sector shaft and on steering wheel turn it for enough movement to read easily, calculate box ratio. From their you could measure either by mockup (Plywood arms, improv draglink), mockup on car, or calculate ratio of X" arm pushing a Y" arm. My coffee has worn off. No arithmetic from me.

 

no it's not. TOTALLY DIFFERENT.

the distance your hand travels per rotation does not change the amount of travel of the steering arm per 360 degree rotation of the wheel.

putting a big wheel on a car along with a longer steering arm can theoretically reduce the # of turns lock to lock without needing meathead arms to steer it.

 

Ok, look at it this way: You've got a rear axle with a 3:50 r/p and 30 in. dia. tires. Turn the tire 1 rev. and count how many times the pinion turns. Now put on some 26 in. tires and turn them 1 rev. and count how many times the pinion turns. Gears are gears weather they are worm and sector or ring and pinion. There's a reason that Bville racers want a tall tire, it gives them a lower (numerical) FINAL gear ratio, by the same token oval racers often want a smaller dia. tire to give them a higher (numerical) FINAL gear ratio. Tires, just like a steering wheel, are all a part of the FINAL equation.

 

I'm with revkev. Steering wheel spins the same # of times if its a 14" or a 17"

what is going to change your steering ratio is the distance the pitman arm pivot point that the drag link attaches to moves per degree of rotation. The longer the arm the distange traveled per degree turned.

If you look at the quick sketch attached, it shows that the linear distance of "A" is shorter than "B", thus the steering arm on the knuckle is going to move a shorter distance, which means less turn.

 

helps if I include the pic...

 

1 revolution is still just 1 revolution. The gear ratio does NOT change! what changes is the distance traveled by the car. The gear ratio in your example is still 3.50:1 regardless of whether you put a 35" tire on it and spin it 1 revolution or you turn the naked axle 1 revolution - the pinion will still turn the same number of time! The tire diameter only changes how far the vehicle moves, it does not magically change the mechanical gear ratio which is determined by the number of teeth on the ring gear and pinion gear (or worm and sector in steering)
The longer pitman arm would be like the larger diameter tire, traveling further per each steering input. Steering wheel diameter will only ease or increase effort.

 

see, the problem with your logic is you are looking at it from the wrong end. the final equation you are talking about is applicable to the pitman arm on a steering box. longer/shorter arm changes the speed/effort to turn the wheel per degree of steering rotation. while changing the diameter of the steering wheel only changes the amount of effort it takes to turn.

to put it another way, if you put a bigger wheel the car will be easier to steer but still turn 5 turns lock to lock. if you put a longer arm it will be harder to steer but will only go say 4 turns lock to lock.

 

rootie:
you actually have the right idea, you are just applying it to the wrong part. The tire is on the output side of the differential, the steering wheel is on the input. Your theory does apply to the pitman arm(if you look at my drawing and yours they are very similar).

66 elcam:
getting back to your question, I would need to know the distance the draglink pivot point moves, from lock to lock w/out being attached to the steering box, and the current pitman arm length to figure it all out

 

I didn't word that right. I didn't mean to say it would change the initial 3:50 gear ratio. It will change the FINAL ratio. The dia. of the tires are as much a part of the FINAL ratio as the gears.

 

No I'm not including the pitman arm or any other linkage in the factor. A larger dia. steering wheel will result in a slower steering with all else being equal.

 

i'd would put in more caster if its too light steering on the highway, IMHO vega pitman arms are too long (stock and aftermarket) for most cross steer hot rod setups. one outher thing i run into alot is the steering box not being clocked correctly , with the pitman centered there should be very little if any play , move it off center a few degrees (or out of time) and the play in the steering box really opens up giving the steering that loose feel everyone hates

 

That is correct, but in this scenario the steering wheel is not like the tire, it's like the size of the yoke. like I said above, the tire is like the pitman arm. Changing steering wheel diameter only changes effort to move the pitman arm through the ratio of the box, changing pitman arm length does change the Final ratio of steering movement.

 

(steering) Wheel changes ratio between arm movement and movement at road wheel...it does not change ratio between steering shaft and road wheel, the normal definition of overall ratio. Semantic, perhaps, but wheel size does not change steering ratio as normally used! The change would be hard to translate into degrees, also, as at least two levers...forearm and upper arm...are pivoting in three dimensions about their axes at the joints...and that's oversimplifying things!

 

From experience:

The Vega box in my 32 roadster has 4 1/2 turns lock to lock.
Pitman arm is 5".
Stock pitman length is 6".
Lengths measure eye center to eye center.

The car steers easily, tracks and handles well on the highway.

I suspect the 6" pitman would do just as well.

The 5" length is due to bending the pitman so as to set the draglink end where I wanted.
Nice part is, steering effort is less with the shorter arm.

The 4 1/2 turns lock to lock Vega box seems to be the standard and that's true of all of them I've handled so far.

I understand there is a quicker ratio Vega box out there, but haven't run into one as yet.

With no power steering you'll probably find the standard Vega box with 6" pitman arm combined with standard Ford or SuperBell steering arms works just fine on the highway or in town.

My 32 does fine as noted and low speed steering is easy.

I thought the slower Vega box would be a pain in the aft side since I was driving an 88 Mustang GT with 2 1/2 turn box for a daily at the time, but going back and forth between the cars was not a problem.

Got a pal with a car similar to what you're building?
Would he let you drive it to get a feel for it and the you could decide if you really do like it?

About the only guy here on the HAMB who's driven my 32 is Kerry - of Hemi powered 31 Pontiac coupe fame - and I'm pretty sure he'd vouch for it's good road manners....

 

Maybe it would be easier to understand if you looked at the larger wheel in SFM.
(Surface Feet per Minute.)

Cutting a small OD piece in the lathe works fine.
Put a large OD piece in and the surface is going by much faster even though the lathe is spinning at the same RPM.
HSS cutting bits that worked on the small OD burn up in no time on the large OD workpiece.

Not that I've ever done that . . . more than once or twice anyway....

 

Increasing the length of the pitman will translate into a faster turn clock to clock at the sacrifice of a harder turn. You will have to put more into the wheel to make the turn than you did before. To fix this put in a large diameter wheel. The larger the steering wheel the easier it is to turn.

It's all about the leverage applied, the bigger the diameter steering wheel the more torque delivered in to the steering box from you the operator. And the longer pitman the larger the diameter out, the faster the turn. Just keep in mind that these changes add forces to the mechanism and can cause the box to fail!

 

O.k. lets take the human element and pittman arms out of this. Imagine the worm gear is infinite (goes forever) and the sector gear is circular (a ring and pinion if you will) now, attach a driven pully where the steering wheel would be. Would not different dia. (ratio) driven pullys (the drive pully remains constant) operate at different rpms to turn the sector shaft the same speed/rate?

 

wow, alot of info here. i guess the initial question i should have asked was, when a vega box is used with a cross steer setup, how many turns lock to lock does it take with everything assembled with a similar parts to mine. from what mustansix and C9 have said the 6 1/4" center eye to center eye is probly gonna give me what im looking for. im not to worried about extra effort, im using a 18" wheel and my tires are pretty skinny.

C9, were the length measurements you have the total length or the center to center length?

thanks everyone for the help so far!

 

as for your el camino. if its a 64 -72 you can bolt on a 70-81 trans am stg gear.

 

Center to center.
Stock was 6".
SoCal flat, blanchard ground pitman arm was 6" to start with and 5" after bending.
This arm was used in the 32.

I have another SoCal flat arm in the 31 and the box is fairly level, but may be 'tweaked' - as in slightly bent - for a touch more ground clearance.

I'm really bothered when the Vega box is mounted pointing down - probably in an attempt to get a better U-joint angle - and the pitman arm is pointing down and in fact the lowest thing on the car.

One of those above-grade gas station inspection hatches is gonna tear the steering box off or apart with a setup like that.
Easy to say, "don't drive over them" but sometimes there's no choice.

The 31 should steer just as easy with its 6" arm as the 32 does with its 5" arm since it has narrower tires.

I tried the 31's wheels and tires on the 32 and am pleased at how easy it drove, plus it seemed to ride better with the taller tire (25").

Headed for a narrower tire on the 32.
If I ever get the 4 1/2" slot mags finished.

Fwiw, the 31's front wheel is 5" x 15".
The 32's present front slot mag is 5 1/2" x 14" and the tire is 24" tall.

Ran 30# pressure in both sets of wheels.

 

Maybe I misread the original post, but wouldn't having a quicker steering ratio, like 4 turns end to end instead of six turns make the car feel even loser on the highway?

With less turns, the front wheels move even more everytime you hit a bump and your hand jerks to the side a little...

 

Fwiw, if you haven't found a Vega box yet - although it looks like you have one on hand - here's a later Saginaw box that works on the thin fender cars.
This one on a 29 A Sedan.

Note the "S" shaped pitman arm.
Be careful as well, we found a similar Saginaw that steers reverse rotation.
Probably for the front steer cars.
When you're looking at them keep in mind the pitman arm will go forward in most all hot rod installs.

 

thanks C9, thats the exact info i needed. looks like i will give the 6 1/4" arm a try.

what i mean by looser, when i move the wheel at 6 turns on the highway the car doesnt react quickly, its like i have to aim it down the road. it takes alot of movement of the wheel to make the car do anything. every steering and suspension component on the car is brand new, the box was a good used box with low miles, so i know nothing is worn out.

ive already got my vega box, it will be mounted exactly like the box pictured.

 

Ok, I was thinking in terms of the other way around... Like your hand moves a little and the cars all over the road...

A shorter pitman arm should solve your problem, but then it takes more muscle to steer. But I guess you've got to give a little here to get a little there.