Rack and Pinion

[gambler]

Ive been wondering about the likelyhood of a progresive ratio
steering box (rack). Where the top 5 degrees would be slow like
15:1, then toward 15 degrees left or right it ramp up to 6:1.
My reasoning being on the long straits a driver wouldn't inadvertantly
scuff off speed by (sawing on the wheel), but when making the tight
hairpin turns that extra wheel would be there.
Has anyone tryed this on a simulator?

[RacingManiac]

I believe this concept is actually used on production cars for sometime now....until the recent advent of the variable ratio steering.

[gambler]

Which automobile are you speaking of?

[RacingManiac]

I'll have to dig it up, but I remember reading it in an issue of SAE Automotive Engineering.

[RacingManiac]

Annoyingly Google cannot turn up the same picture, but the system is used in a lot of the new Mercedes as an option called the "Direct Steer" which has slower ratio on the first 5 degree and faster one beyond through the use of a worm gear incorporated in the system. It makes more sense looking at the pic but I can't find it...

[Carlos]

Variable ratio rack and pinion is a feature of many current American GM,Chysler and Ford products, a variety of European manufactureres and Japanese cars. You'll will have to research specific models and option lists. it's becoming a common feature. It's an idea that has been offered in hydraulic and electric power steering systems since the '60's, for instance - In the late 1960s, General Motors offered a variable ratio power steering system as an option on Pontiac and other vehicles.I remember it as on option on GM Camaros of the era.

" In 2000, Honda launched the S2000 Type V equipped with the world's first electric power variable gear ratio steering (VGS) system.[7][8] In 2002, Toyota introduced their own "Variable Gear Ratio Steering (VGRS)" system introduced on the Lexus LX 470 and Landcruiser Cygnus, and also incorporated the electronic stability control system to alter steering gear ratios and steering assist levels. In 2003, BMW introduced their "Active Steering" system on the 5-series.[9]
This system should not be confused with variable assist power steering which varies steering assist torque not steering ratios, nor with systems where the gear ratio is only varied as a function of steering angle." Wikipedia

[RacingManiac]

I think though the system you mentioned varies input from the driver to the output to the rack, I am guessing through a gearbox of sort. This actually has a rack that's cut with varying pitch. Though practically I guess they do the same work though.

[gambler]

Thanks, Ive got a direction to head in now.
Im sure the gearcutter fussed and cussed the engineer
thoroughly upon its inception.

[DaveKillens]

[safeaschuck]

If engineering were religion, that would be an abomination,

[tc9604]

I guess it's easier to incorporate a variable rate lever/linkage into the hub/track rod end assembly than to try to adjust the linear rate of the rack

[Jersey Tom]

I suspect you'd have to be very careful integrating the specifics of that steering box with the general vehicle dynamics package... what the driver (handwheel) cornering gains are gonna be through a range of speeds and lateral acceleration. Kind of cool though.

Not sure why your driver would be sawing at the wheel going down a straight though...

[gambler]

I know it would be worth the while to do it on the sim first.
It may feel so wierd... the worst would be an understeer situation
where the driver would have to come back through the slow part
to right the car.

[RacingManiac]

Dave, that was the exact pic I was looking for!

[Woteva]

There are a LOT of cars on the road today with variable ratio steering by the way. Almost the entire mercedes benz range has it STANDARD, with some models getting an option for an even more radical version. The 997 porsche has it for instance, as does Audi, BMW (although their system as stated above uses a special gearbox, while the other ones are just using a variable ratio rack. Several fiat and opel models get it also, and I'm sure a lot more.

F1 uses it extensively. These guys supply it I believe.

http://www.nevodevelopments.com

Because the ratio is changing as the steering wheel is turned, it does have the tendency to feel a bit weird because the steering wheel angle is going to be different when compared to a constant ratio system. Hence the driver often feels like the car is understeering simply because the steering wheel angle is greater than previous. However, this is usually solved by explaining to the driver what is happening. The biggest benefit of it I think is the reduction in the sensitivity of the car in high speed situations, ie straight ahead, allowing a more agressive aero to be used, but still having a ratio that is useful when negotiating slower corners.