Reduction in the number of wheels means less drag?

[Frenemy90210]

If the number of wheels on a vehicle are reduced from four to three or two (assuming problem of balance is taken care of), will it reduce the amount of energy spent on all resistance (such as friction/slippage at tyres, rolling resistance etc) faced by vehicle at the wheels. It is also assumed that aerodynamic drag is of no concern for this query. If yes, by what factor, it will reduce the resistance ?

Also, if answer is yes, then why not reduce the number of wheels per railway car to (front) two removing backside wheels; Balance will be provided by front two wheels of next car.

Edit : There were two kinds of vehicles in mind: one was a railway car and another was a standard 4 wheeler car (such as Honda Civic etc.)

 

[scottdave]

Not enough info to say how much it is reduced. But as to why trains have a certain number of wheels per car. I believe that ia because each axle has a certain amount of weight that its designed to carry. To design larger axles may be possible, but there would be other drawbacks.

 

[scottdave]

Whenever building something, there are considerations other than just the physics. For example there is cost. Safety is another factor. I once saw an 18 wheeler driving and on of the back tires blew. The truck did not react much to the change. In fact the driver kept driving, for a while like normal.

 

[sophiecentaur]

It is common to see unloaded trucks in the UK with one axle raised off the ground so someone, at least, reckons that it saves fuel and have included that design feature.

 

[russ_watters]

Also, if answer is yes, then why not reduce the number of wheels per railway car to (front) two removing backside wheels; Balance will be provided by front two wheels of next car.

Rolling resistance on trains is vanishingly small already, so the small energy savings probably isn't worth the annoyance of the created problem of how to decouple and move individual cars...and the extra cost of beefier wheels and couplings that have to carry weight. It's a big re-design.

 

[sophiecentaur]

Rolling resistance on trains is vanishingly small already, so the small energy savings probably isn't worth the annoyance of the created problem of how to decouple and move individual cars...and the extra cost of beefier wheels and coupling that have to carry weight. It's a big re-design.

Braking could also be a consideration. Railway carriages are pretty heavy, even when empty so the brakes would have similar requirement. The suspension involves four wheeled bogies and they are required to point along the rails all the time. I think the geometry of single axles bodies could compromise cornering on rails; each axel of the pair steers the other axle to keep it parallel with the rail and avoids it climbing up over the rail on the wheel flange.

 

[OCR]

It is common to see unloaded trucks in the UK with one axle raised off the ground so someone, at least, reckons that it saves fuel and have included that design feature.

Very common in the US also, and for the very reason you stated....
A lift axle can also save on tire ( tyre ?... ) wear, and in certain circumstances they need to be raised to maintain drive axle traction... for instance, very uneven ground.

 

[256bits]

I had always thought that the lift axle was meant to be utilized so that the truck/trailer could carry more weight, or distribute the weight amongst another set of wheels and axle, rather than due to any consideration to fuel savings. With the axle down, turning is more cumbersome, so when not needed it is put back up for maneuverability.

 

[256bits]

Braking could also be a consideration. Railway carriages are pretty heavy, even when empty so the brakes would have similar requirement. The suspension involves four wheeled bogies and they are required to point along the rails all the time. I think the geometry of single axles bodies could compromise cornering on rails; each axel of the pair steers the other axle to keep it parallel with the rail and avoids it climbing up over the rail on the wheel flange.

The bogies on each end of the railway car act independently of one another. A railway car is just a container sitting 'loosely' on top of each bogie - the container holding the two bogies together. With only one axle at each end, well that would be quite a long bogie, and each axle would have to be 'strongly' attached, and designed to withstand twisting moments. As you say the bogies have a design feature to point along the track. Oscillations from the wheels trying to ride up the rails might not be as self limiting with only 2 axles at each end of the car..

 

[sophiecentaur]

designed to withstand twisting moments.

If the truck is long compared with the curvature of the track and there is just one axle at each end, the leading axle would de-rail. It is essential to keep the axles as near as possible perpendicular to the track so that the separation of wheels on each axle is near enough equal to the spacing of the track. (Short bogies and long curves satisfy this.)

 

[OCR]

Oscillations from the wheels trying to ride up the rails might not be as self limiting with only 2 axles at each end of the car..

Somewhat complicated, but interesting...

Hunting oscillation...

 

[sophiecentaur]

The bogies on each end of the railway car act independently of one another. A railway car is just a container sitting 'loosely' on top of each bogie - the container holding the two bogies together. With only one axle at each end, well that would be quite a long bogie, and each axle would have to be 'strongly' attached, and designed to withstand twisting moments. As you say the bogies have a design feature to point along the track. Oscillations from the wheels trying to ride up the rails might not be as self limiting with only 2 axles at each end of the car..

I've been thinking about this again. Without some servo to keep a single axle perpendicular to the rails, the system could not work. A sprung system could help a bit but it would limit the curve radius / flange height allowable. Two axle bogies do it for me!
Also, there cannot be more than two bogies on a carriage unless they are allowed to slide laterally under the body to follow the curve. The three fixed bogies would have to lie on a cord of the curve and there are only two points where they can lie on a non-straight line. That's true in most cases with no motion (it would need an S shape) but pretty well all cases if you want the carriage to move along the track.
Just think - all this was sorted out hundreds of years ago by the early railway engineers.

 

[256bits]

@sophiecentaur

Oscillations from the wheels trying to ride up the rails might not be as self limiting with only 2 axles at each end of the car..

That sentence was confusing to me when I re-read it.
It should have read:
Oscillations from the wheels trying to ride up the rails might not be as self limiting with only 2 axles, with one axle at each end of the car.
The twisting action is reference by the second picture given in reference by @OCR.

I think we are on the same page.

 

[sophiecentaur]

I can't help thinking that a single axle on a curve would need a lot of controlling to keep it perpendicular to the rails. Any significant angle and the outside wheel would fall off the rail. Perhaps a telescopic axle would avoid that but a pair of axles on a bogey avoids all that problem. Totally passive, rigid system.

 

[haruspex]

rolling resistance

Consider a very simple, and somewhat extreme, model. A load can be supported above a rigid surface by either one or two identical elastic columns. The elastic is maximally lossy, i.e. the elastic coefficient as compression occurs is some constant k, but as decompression occurs it falls to almost zero. So all the work done on it during compression is lost.
If the load is W, with one support it compresses by W/k, doing work -W²/(2k). With two columns each compresses by W/(2k), doing total work -W²/(4k).
This suggests more wheels would reduce rolling resistance, but at greater investment cost. The weight of the extra axles would also figure.

 

[Frenemy90210]

This suggests more wheels would reduce rolling resistance.

This contradicts with the reason why bicycles are so efficient (compared to tricycles/quadricycles) , isn't it ? The reason bicycles have only two wheels, is thought to reduce both rolling resistance and friction due to slippage. IMO, bicycles will loose efficiency if additional wheels are added. I could be wrong. Also bicycle wheels are so large compared to car/scooter etc is it lessens friction at axle. As per my experience, as we add more wheels to a bicycles, it takes more effort to ride it.

 

[haruspex]

This contradicts with the reason why bicycles are so efficient (compared to tricycles/quadricycles)

It may contradict some presumed reasons, but I find that most online techno info on bicycles is put together by expert cyclists unburdened by any actual expertise in physics.
Recumbents suffer more rolling resistance because of the smaller wheels.
I can imagine that full sized trikes are less efficient for a number of reasons: the increased weight, maybe lower tyre pressure, differential gearing.
Edit: and more drag.

 

[skeptic2]

I work at a railway research and testing facility and I have seen a light rail transit train as you described with the exception that the missing two wheels were at the front of the car instead of the rear. (What if there were no car behind it to help balance the load?) I don't know why it was done but my guess is cost.

 

[256bits]

I can't help thinking that a single axle on a curve would need a lot of controlling to keep it perpendicular to the rails. Any significant angle and the outside wheel would fall off the rail. Perhaps a telescopic axle would avoid that but a pair of axles on a bogey avoids all that problem. Totally passive, rigid system.

double axle = Totally passive, tried and true, and with the taper of the bearing surface of the wheel going around a corner causes the outside wheel to cover more distance than the inner. And even a passive tilting there also to help making it through the curve. Speaking of a solid axle here turning both wheels of the wheelset at the same rpm. Certainly there will be some wear on the rail and wheel flange, but if it works, don't fix it.
But of course, mechatronics is finding its place in everything these days.

I work at a railway research and testing facility and I have seen a light rail transit train as you described with the exception that the missing two wheels were at the front of the car instead of the rear. (What if there were no car behind it to help balance the load?) I don't know why it was done but my guess is cost.

whose's "You?" in you described.
http://www.ejrcf.or.jp/jrtr/jrtr18/pdf/f52_technology.pdf
A bit dated, 1998.ut it gives a picture fig 2 of a single axle vs double going over a bump.
Single axle needs a different suspension system.
Cars can share a bogie, as long as weight "restrictions for the rails and rail bed are taken into consideration.
Active steering, either mechanical or with added electronic sensors and actuators would/could give some benefit to the right.

Weight reduction = fuel saving, and of course cost saving ( maybe ), as newer systems can get highly expensive, and of course, delays in development and delivery can make costs skyrocket for the transit systems and the light transit manufacturers.

 

[skeptic2]

double axle = Totally passive, tried and true, and with the taper of the bearing surface of the wheel going around a corner causes the outside wheel to cover more distance than the inner. And even a passive tilting there also to help making it through the curve.

This is visible in the link by 256bits. In fact the gradient may be even more than what is shown in the drawing. As a train goes around a curve the cars slide to the outside of the track. The part of the outside wheel in contact with the track is the part next to the flange and has the largest diameter. The other wheel slides away from the flange where the wheel diameter in minimum. This is what allows cars to go around curves with a minimum of grinding.