15:1 or 18:1 Rack Ratio?

[Charlie M]

What's the preferred steering rack ratio for a street car that may see an occasional autocross, 15:1 or 18:1? Steering racks aren't cheap, I hope to get this right the first time.

I see different units used to describe the rack ratio, a number of turns "lock to lock" or a ratio. What's the relationship between the two units (15:1 = ? turns lock to lock, etc) and is it the same from one manufacturer to another?

Thanks,
Charlie

 

[Phil]

Charlie:
The ratio is expressed as a fuction of degrees of turn.
For example if you put your tire on a degree wheel and turn the rack one full turn and get a value of 36 deg., divide this into 360 degrees and you get a rack ratio of 10 to 1. A pretty fast rack. The higher the number the slower the rack, and less wheel angle produced per turn.
Turns lock-to-lock is exactly what it says, the number of turns required to cycle the rack thru full travel. Each full turn would produce the number of degrees at the wheel specified by the rack ratio.
There are some factors that can affect the wheel travel such as ackerman, length of steering arm and steering toe. Usually the turn is measured at the inside wheel (Because of ackerman), and some guys average the inside and outside wheels to get a figure.

Also some racks are spec'd as inches per turn with rack travel in inches per one turn of the steering wheel. Since the manufacturer doesen't know all of your specs this is the easiest way to spec the rack. Again this needs to be measured at the wheel (In degrees) because of the factor of steering arm length which if longer will effectively lessen the amount of turn and raise the rack ratio, but gives the rack, and the driver more leverage on the upright, and slower steering.(Remember the big boats from the sixties with huge steering wheels?)

If you are trying to choose a rack for your project I would suggest looking at the rack that was used with the original setup, and the steering wheel diameter, and use that as a starting point for reference, then making small changes to suit the chassis.

If you can get a baseline degree reading at your wheel per inch of rack travel then you can apply ratios and travel from different racks to get an idea how fast the steering will respond.
Hope all this makes sense
Cheers
Phil

 

[Charlie M]

Phil,

Thanks for the explanation. I'm planning to use '94 Mustang spindles, Factory Five control arms, and a manual Mustang rack. Basically, the same front suspension FFR uses in their Roadsters and Coupes. A lot of people have gotten those cars to handle well and the parts are plentiful and inexpensive.

I believe that the Mustang racks are 15:1 but they have power steering. I'm concerned that a manual 15:1 rack may require a lot of steering effort at low speed and may be a little "twitchy" in a short wheelbase car at high speeds. I get different opinions from the Cobra crowd; some say the 15:1 rack is fine while others have swapped it out for an 18:1 rack. However, the GT40 has a 5" longer wheelbase and a lighter front end so it may behave differently.

Most of the Cobras are using a 15:1 rack. I could try that as a starting point. If I pick up a used one at least I should be able to sell it and recover the cost if I decide to change. I was hoping to get some opinions here as to what ratio people are actually using.

Thanks,
Charlie

 

[Mike Drew]

Remember, the rack ratio is only part of the equation. The length of the steering arms on the spindles has a huge influence as well. If you have long steering arms, then a given amount of rack movement will have considerably less influence on the front wheels than if the steering arms were shorter.

Philosophically, it's like the gearing in a transmission. Long steering arm = low gear, i.e. less steering effort, and less front wheel movement for a given amount of steering wheel turn. Short steering arm = high gear.

So, you could have a 'quick ratio' rack (i.e. 15:1) with long steering arms, or a 'standard ratio' (i.e. 20:1) with short steering arms, and have exactly the same effect.

In other words, your original question is only half the question...

 

[Charlie M]

Mike,

I'm using Mustang spindles without any alterations to the steering arms so a 15:1 Mustang rack should give me 15:1. That's the ratio most of the Cobra people use with this setup so I'll try that as a starting point.

Charlie