Optimizing Front Wheel Losses in Rear-Wheel Steered Vehicles

[grey]

I want to know a little about rear-wheel steering mechanisms. Basically, my interest is in the relative losses (due to slip angles) on the front wheels of these vehicle...whether they would be expected to be more, and if yes how?

In the end, i want to design a steering mechanism for a 3-wheel vehicle, with two wheel at the front and one at the back (this is a confiiguration that i have been assigned). I was thinking of having the rear wheel steered (and powered). But, I couldn't find much about effect of turning on the front tires. They would be independant of each other. For an ackerman equivalent, I was thinking that the ideal configuration would be when, during turning, the perpendicular from the rear wheel should meet at the perpendicular of the front wheel...which in this case, would always happen. But when I try to visualize this scenario, it seems as though, there would be a lot of slip...

Make me wise! Thanks!

 

[Mech_Engineer]

I would think a vehicle with rear-steering only would be unstable and dangerous to drive at anything over 5 mph. Have you ever driven a front-wheel steer vehicle backwards for an appreciable distance at faster than running speeds? They're twitchy!

 

[Danger]

As Mech said, rear-steer units are extremely unstable, particularly the 3-wheeled variety, although they are incredibly nimble at low speeds. The serious ones can literally turn in place, as in switch directions without leaving the spot. There are automobiles on the market which feature 4-wheel steering as a luxury item. All of them, however, have the rear wheels restricted to something like 3° from centre. There is a good reason for that.

 

[grey]

I agree with what you guys have said. In fact, I have myself noticed the difference in turning while going forwards and in reverse.

But, aside from the instability, is it 'nice' on the tires (nice having the connotations i have described in the original post)? I mean, obviously there are always trade-offs...so if i get efficient turning, I am willing to play with some danger :-)

 

[Danger]

I am willing to play with some danger :-)

NO ONE PLAYS WITH DANGER!

Sorry, it just sort of felt right to say that. :shy:


Really, a lot more information regarding your requirements is necessary before any sort of advice can be given. I mean, there is a hell of a lot of difference between a warehouse forklift and an Indy racer. You have not defined any parameters. Give us an idea of what we're working on.

 

[Mech_Engineer]

There are lots of 3-wheeled cars out there, concept, production, and home-made. ALL use front steering, whether they have 1 wheel in front or 2. Why do you think this is?

http://www.google.com/images?hl=en&source=imghp&q=3+wheel+car&gbv=2&aq=f&aqi=g10&aql=&oq=&gs_rfai=

 

[AlephZero]

I would think a vehicle with rear-steering only would be unstable and dangerous to drive at anything over 5 mph.

Some large farm machines, e.g. combine harvesters, are rear-wheel-only steering and perfectly stable to drive on roads (though the maximum road speed is usually only about 30 mph).

Have you ever driven a front-wheel steer vehicle backwards for an appreciable distance at faster than running speeds? They're twitchy!

I would guess that is because they are designed to be stable going forwards not backwards, and/or because either your driving position is contorted, or you are steering looking only in the mirrors.

 

[grey]

I think it is because of stability, and obviously, danger.

Apologies danger...but you get the point, right

Anyway, i have a few interests in making it rear wheel steered:
1. I want all wheels inside the body - aerodyanmics
2. The driver will almost be supine with his knees just about at the point, at which (for front steered) tie rods will lie, making it difficult for him
3. There is a chance that the steering wheel/whatever will make it difficult to get in, and possibly affect visibility

There are other alternatives, the four link mechanism I have seen for example, and I want to look into rear-wheel steering as an alternative as well.

The speeds are going to be around 35 kph max. Turning Radius about 6 m.
What can you tell me?

 

[Danger]

Apologies danger...but you get the point, right

Of course; I knew the risks when I registered with that name (but it worked out well because that is the name that I'm known by locally).
Although you didn't quite specify, I get the impression that this is a human-powered vehicle shooting for a speed and/or distance record of some type. If that is the case, then steering shouldn't be a problem since the pilot need only go in a straight line with minor corrections. Do you have to slalom around gophers, or is there something else at work here?

 

[grey]

powered by an electric hub motor (also rear wheel), and yeah...there's supposed to be a course which includes some turns

 

[Danger]

powered by an electric hub motor (also rear wheel), and yeah...there's supposed to be a course which includes some turns

Alright, that changes things a bit, but not much. I would still go with front-wheel steering even with the rear-wheel drive; in fact, it's easier that way. Have you thought about inverting your front-wheel steering geometry, so the tie rods and whatnot are ahead of the front wheels and thus out of your pilot's way? It won't be pretty, but it might work.

 

[i_wish_i_was_smart]

All forklifts are rear wheel turn and the smaller ones are 3 wheeled

http://manitexliftking.com/www/pdf/products_mounted_TA.pdf

Cat also makes some but i can't find them on their main site, which were electric

found it

http://www.cat-lift.com/_cat/index....2500-4000-lb-capacity-3-wheel-pneumatic-tire/

good luck

 

[Jasongreat]

Some large farm machines, e.g. combine harvesters, are rear-wheel-only steering and perfectly stable to drive on roads (though the maximum road speed is usually only about 30 mph).
QUOTE]

Every combine and swather I have ever operated is front wheel steer, the rear wheels are only on pivots to follow the paths of the front wheels. They use hydrostatic transmissions with hydraulic motors on each wheel with a planetary gear. They turn by spinning one of the front wheel faster and one slower, sometimes the wheels even turn in opposite directions if the steering wheel is turned sharply enough. Also, top speeds are well under 20 mph, with swathers being closer to 13 mph.

To the OP if you are concerned about the steering linkage getting in the way up front, you could put all the linkage in the rear and use shafts run down the sides of the vehicle to the front wheels to turn them, but you will probably experience more bump steer and would complicate things quite a bit. Steering linkage doesn't really take up that much space though, especially with a rack and pinion type which are usually only 2 to 3 inches tall. Even if you use rear steering, you will still have to have a steering wheel in front of the driver, unless you use joysticks and a hydraulic system to steer but I have only seen that type of steering in slow moving constuction equipment and its not very precise even then, or airplanes where it isn't so imperitive to stay in ones travel lane.

 

[Averagesupernova]

Every combine and swather I have ever operated is front wheel steer, the rear wheels are only on pivots to follow the paths of the front wheels. They use hydrostatic transmissions with hydraulic motors on each wheel with a planetary gear. They turn by spinning one of the front wheel faster and one slower, sometimes the wheels even turn in opposite directions if the steering wheel is turned sharply enough. Also, top speeds are well under 20 mph, with swathers being closer to 13 mph.

Ummmmm, no. While swathers have always spun one front wheel faster than the other for turning whether mechanical or hydrostatic, combines steer with the rear wheels and as far as I know always have. The forward power is hydrostatic on modern combines but hasn't always been. Top speed on a combine? 20 MPH is not at all uncommon. So my question to you is how many have you operated and I want manufacturer and model.
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Oh yeah, one last thing. Wasn't the Isetta 3 wheeled with the steering wheel in the back? Urkle (sp?) had one in the TV show Family Matters.

 

[Dr Lots-o'watts]

Surely, you are aware of these products:

http://spyder.brp.com/en-US/See/Can-Am/Showroom/Discover.htm
http://www.campagnamotors.com/#

The first is becoming surprisingly more common up here in snowmobile country during summertime.

 

[wmazz]

On motorcycles we increase the rake (castor angle) to reduce steering sensitivity. Perhaps you can do the same thing in reverse. Imagine using chopper style forks pointing towards the rear of your vehicle. This should stabilize your steering and simplify your drive train and steering construction. A CON: It may require more effort to turn?

 

[Danger]

Surely, you are aware of these products:

http://spyder.brp.com/en-US/See/Can-Am/Showroom/Discover.htm
http://www.campagnamotors.com/#

The first is becoming surprisingly more common up here in snowmobile country during summertime.

Do I dare presume from your post that you are a fellow Canuk?

 

[grey]

wow! thanks guys for the awesome posts...

but we seem to have a missed the point slightly. While all of your posts give me the necessary basis for choosing a type of system, they miss one of the key things i was interested in...

the affect of rear wheel steering on the front tires' lateral slip - i want to tune it to max fuel efficiency, removing every bit of friction i can (during turning in this case)...what's your opinion on this aspect?

attaching a photo of a vehicle which has done a similar thing

 

[wmazz]

These guys seem to have that figured out http://www.canosoarus.com/03CalifCommuter/03d.htm

But seriously, why would anybody leave the front wheels perpendicular to the ground while turning? Lean all three wheels into a corner.

I am curious if you are allowed 2 smaller motors driving the front wheels, or for that matter, 3 small motors driving each wheel?

 

[mender]

wow! thanks guys for the awesome posts...

but we seem to have a missed the point slightly. While all of your posts give me the necessary basis for choosing a type of system, they miss one of the key things i was interested in...

the affect of rear wheel steering on the front tires' lateral slip - i want to tune it to max fuel efficiency, removing every bit of friction i can (during turning in this case)...what's your opinion on this aspect?

attaching a photo of a vehicle which has done a similar thing

There will little if any difference in using the rear wheel to turn the vehicle.The main issue is the difference between steering with one wheel vs steering with two re: scrubbing from alignment change as the two wheels were steered. Check out Ackerman steering.

To avoid that altogether as you appear to be wanting (not a bad thing), I'd seriously consider a more conventional single front wheel for steering with two rears to support most of the weight. Since most of the weight would be on the rears, the size of the front would be less critical re: rolling resistance and could be smaller for a lower profile and tighter turning radius within a specific body width (narrower). That would also minimize the amount of force needed to pivot the vehicle by putting the centre of gravity closer to the non-steering wheels and would make the vehicle much harder to tip in evasive maneuvers while retaining straight line stability - which means less steering inputs and better efficiency.

You won't see any soap box derby cars with rear steering.

 

[flyingwheels]

I'm curious, what was the outcome of this exercise? Did you successfully build a three wheeled vehicle that steers with one rear wheel?