Steering on a 6x6 design

[geoguvna]

Hi all, I'm experimenting with a 6x6 chassis design (3 motors, one powering each axle independently) and I want it to have steering on two axles. Not an engineer but I've Googled a lot of 6x6 designs, but it's not clear to me which arrangement provides a tighter turn radius - front/rear or front/ middle. The front/middle seems most common in military designs (like the Stalwart). But I find my prototype turns pretty well. 

[allanp]

Front and rear would give you a tighter turn radius.

[Jundis]

Front and rear will give a tighter radius. Front and middle is better for fast moving machines like wheeled tanks.

[geoguvna]

5 minutes ago, allanp said:

Front and rear would give you a tighter turn radius.

Thanks. I wonder why the Alvis design doesn't use that? Simplicity? 

2 minutes ago, Jundis said:

Front and rear will give a tighter radius. Front and middle is better for fast moving machines like wheeled tanks.

Thanks, that makes sense. My vehicle won't be going fast

[JintaiZ]

I like your progress so far, but I think you can add suspension if you don't want it to go too fast.

Also, you can make your pictures bigger by including a direct link.

[Saberwing40k]

7 hours ago, geoguvna said:

Thanks. I wonder why the Alvis design doesn't use that? Simplicity? 

 

I think it might be human factors engineering. A 6x6 vehicle with front/middle axle steering behaves a lot more like a normal truck than a vehicle with front and rear steering. I think it has to do more with simplicity, though. A Stalwart needs only 2 extra links to get the axles to steer, whereas having the front and rear axle steer would require a second steering box, and a long steering shaft.

You can see the steering link between the 2 front axles.

 

Your design does look cool, though.

[MinusAndy]

Could you make it so that front steers 2x middle and rear mirrors middle?

[pdw]

22 hours ago, geoguvna said:

Thanks. I wonder why the Alvis design doesn't use that? Simplicity? 

Front wheel steering is naturally stable, whereas rear wheel steering is unstable.  If a rear wheel steer vehicle starts to turn, its momentum will act to make it turn more.  A front and rear-steered vehicle is likely to close to neutral, depending on where the CoG is relative to the mid point between the steering axles.  This means that if it starts to turn it'll carry on turning, but it won't get tighter as a RWS vehicle would.  A vehicle that is positively stable will be easier to drive.  Front-and-rear steering is used where manoeuvrability is essential, such as telehandlers, and even there they often have a 2WS mode for driving along - even on a vehicle with a 40kph max speed.

The Alvis may not be very quick but it is about getting from A to B, rather than constant low speed manoeuvring.  The additional axle for improved off-road ability, and the second axle is steered to avoid tyre scrub.  Turn radius is the same as if the middle axle wasn't there.   A front-and-rear steered 6 wheeler has the same turn radius as if the rear axle wasn't there i.e. it has half the wheel base, and thus a tighter turn radius.

 

 

[Void_S]

On 2/26/2021 at 10:41 PM, pdw said:

The Alvis may not be very quick but it is about getting from A to B, rather than constant low speed manoeuvring.  The additional axle for improved off-road ability, and the second axle is steered to avoid tyre scrub.  Turn radius is the same as if the middle axle wasn't there.   A front-and-rear steered 6 wheeler has the same turn radius as if the rear axle wasn't there i.e. it has half the wheel base, and thus a tighter turn radius.

Exactly! Moreover, the front-rear steered machines have a different wheelbase calculation - it equals the biggest distance between steer and non-steer axles. So trucks and other construction or military machinery have exactly the same parameters as if they had no rear  (steer) axle at all.

So, the thumb rule is here: leave only the frontmost steer axle plus non-steer axles and you will clearly see how the vehicle turns.
Additional steer axles only help with the weight distribution of off-road capabilities.

Edited March 1, 2021 by Void_S