In some cars, I have noticed the rear wheels are located at the extreme rear of the vehicle. However, I have noticed that the rear wheels of buses are always located about 1/4th of the way forward from the rear. What is the reason for this?
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7$\begingroup$ I'm sure there are many factors, but a big one is that the further back the wheels are, the larger area the bus needs to turn. $\endgroup$– Ethan48Apr 8, 2015 at 22:14
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1$\begingroup$ In quite a few cars, the rear wheels are "always located about 1/4th of the way forward from the rear". Let's just call such cars an estate. Estates are long and have a high payload, just like buses. And now compare again to a car with the rear axle at the end of the vehicle, like an Aygo. See the difference? $\endgroup$– AlexanderApr 9, 2015 at 11:13
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$\begingroup$ Is the fact that most buses are speed-limited relevant here? I imagine it might not be a good idea to use this on a high speed road vehicle, but I know next to nussing about engineering and physics. $\endgroup$– DomApr 11, 2015 at 14:01
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8 Answers
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Some ideas:
- Wheel Load Distribution: The load is greater on the rear wheels providing the power; more force on the front ones bring no benefit and would provide less traction.
- Better manoeuvrability from having a shorter wheelbase.
- Better Ground Clearance in some conditions, especially for bumps and or up a increasing slope for instance.
- Better Driving: The front wheels now turn around a point closer to the C.G. than with the rear wheels. Not good with vehicle dynamics but this appears better than the rear end 'trailing' behind.
- Structural: As some people have pointed out, it's better to balance the weights on either side of the axle (think beams). You have a (basically) point load for the engine and a distributed load for the rest of the bus. Rather than placing it all on one side, you can distribute it to reduce the ultimate loading.
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4$\begingroup$ The above are great from a physics standpoint, but from an engineering standpoint there are factors constraining the drivetrain overall length. $\endgroup$ Apr 9, 2015 at 12:30
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$\begingroup$ The load distribution decreases the wear of tires too. What you say, @mikefoxtrot $\endgroup$– BijuApr 9, 2015 at 16:35
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$\begingroup$ Anyway, people experience more jerk or shock sitting in the back seats. $\endgroup$– BijuApr 9, 2015 at 16:49
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1$\begingroup$ @ja72: There are techniques for extending dravetrains to very long runs. After all, we've been building boats with super long drive shafts for ages. The physics makes a shorter drivetrain preferable. Just like all the points above. $\endgroup$ Apr 10, 2015 at 20:35
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1$\begingroup$ You can have a long hollow shaft of large diameter (for stiffness and low weight) in a ship, but size constraints limit the diameter on a bus. So to get the desired stiffness you need to reduce length. $\endgroup$ Apr 10, 2015 at 21:06
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I think one of the reasons is structural. The bottom on the bus is like a bridge with long steel beam running the length, and if the supports are at the ends the stresses and deflections would be too high. By moving the wheels more towards the center of gravity in reduces the flexure of the center of the bus.
In addition the driveshaft can be shorter which means less flexure and less wear.
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2$\begingroup$ In the UK, buses typically have their engine at the rear, so moving the wheels away from there increases the driveshaft length. $\endgroup$ Apr 9, 2015 at 18:50
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$\begingroup$ Those brits, with their left hand screws... ;-) $\endgroup$ Apr 9, 2015 at 18:51
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1$\begingroup$ The shorter unsupported spans require less metal, which reduces cost and weight, while retaining structural strength. This combined with the better handling characteristics (particularly in regards to turning sharp corners and running the side of the bus into people and posts) means that the best position for the rear wheels is well away from the rear, and closer (but not directly under) the center of gravity when loaded with passengers. $\endgroup$ Apr 9, 2015 at 18:58
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Front axle load is an important consideration. It's hard to carry a lot of weight on an axle that steers, as you are effectively limited to single wheels, and you want to minimise the power assistance needed to actually turn the steering wheel.
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The answer is a combination of factors. Weight has a great deal to do with the positioning of the rear axle. If you assume that the bus has a full load of passengers, you'll want the power to the rear wheels closest where there is weight. Larger buses have dual-axles/wheels to accommodate the extra weight, but the positioning of the axles often remains the same. Some heavy-duty dual-wheeled trucks have the rear axle closer to the rear of the vehicle to compensate for the loads they will carry. (trailers, recreational vehicles, etc). Conventional passenger buses need a shorter wheelbase, and a heavy-duty single rear axle. It's a compromise between weight, ride and fuel economy.
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There are lots of factors but I'd say the two major factors are:
- Drive Shaft Length - A very long drive shaft causes various engineering problems so there is some pressure to minimize the length.
- Maneuverability - Longer wheelbases cause problems mostly in making turns in tight quarters. The longer the wheelbase the bigger the difference between the track of the front wheels and the rear wheels. So the fronts go around the corner fine but the rear wheels go up over the curb and take out the light pole.
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1$\begingroup$ In the UK, buses usually have the engine at the rear, so moving the rear wheels forwards increases the driveshaft length. But the rear wheels are still usually some distance from the back of the bus. $\endgroup$ Apr 9, 2015 at 18:52
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Think about how you would support a long, flexible beam (or a 30cm ruler, if you prefer). If you put the supports at the ends, the beam will sag in the middle; but if you put the supports at 1/4 and 3/4 of the length, the weight of the beam between the supports will be counterbalanced by the weight of the beam outside the supports.
Of course, there are a lot of other factors involved in designing a vehicle, but the tiny cars with wheels right at the end are the oddities.
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Its most likely a combination of reasons.
Steering behavior and traffic disruption:
Having the axle at the very end of the bus :
Will drastically change the steering behavior or requirements by making the bus' front and rear axles drive along a smaller radius or tighter turn. This can cause more (quantity and/or severity) disruption to other traffic.
The driver must drive way further forward before starting the turn, otherwise the rear wheels will quickly go beyond the lane the bus was in. This can mean drifting over the center of an intersection (possibly in danger of hitting traffic on the other side) if making a left, or hitting curbs (and possibly more damage to street structures, buildings, people, etc.) if making a right at an intersection. Naturally, having to drive further forward will block more lane(s)/traffic in that same direction until completion of the turn.
Having the axle further up:
Will allow the bus' front and rear axles to travel along a bigger radius. This allows the bus to make "smoother" turns, allows the bus to start turning sooner, and allows it to stay more within its own "lane" while turning.
weight distribution and regulations:
I used to work in the transportation industry.
Axle/bogey location is move-able solely to compensate for weight distribution over the wheels. They must make sure that each wheel carries below a certain threshold level of weight in order to be allowed on the streets. If they can't make it work, then they add more axles to distribute it even more. This is enforced with weighing stations and the like.
CalTrans http://www.dot.ca.gov/
GENERAL RULE
35550. (a) The gross weight on any one axle shall not exceed 20,000 pounds,
and the gross weight upon any one wheel, or wheels, supporting one end of an
axle, shall not exceed 10,500 pounds.
I believe the streets and freeways can only take so much localized pressure before they start to get damaged.
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Most busses in the U.S. use a rear engine/rear drive layout, with the engine located behind the rear axle for ease of maintenance. There is no drive shaft per se, because the engine/transmission/rear axle are integrated.
