Why do buses have such large front wheels?
Nearly all buses I’ve ever seen have a front wheel bay that projects above the level of the floor and forms an obstacle for people boarding the buses with wheelchairs or prams. The wheel arches form a constriction point,
Can buses function just as safely and effectively with smaller diameter front wheels, or are large front wheels a necessity?
party_pants said:
Why do buses have such large front wheels?Nearly all buses I’ve ever seen have a front wheel bay that projects above the level of the floor and forms an obstacle for people boarding the buses with wheelchairs or prams. The wheel arches form a constriction point,
Can buses function just as safely and effectively with smaller diameter front wheels, or are large front wheels a necessity?
A lot of buses “squat” at a bus stop.
party_pants said:
Why do buses have such large front wheels?Nearly all buses I’ve ever seen have a front wheel bay that projects above the level of the floor and forms an obstacle for people boarding the buses with wheelchairs or prams. The wheel arches form a constriction point,
Can buses function just as safely and effectively with smaller diameter front wheels, or are large front wheels a necessity?
They need them for all the obese people about these days, smaller wheels would not be able to climb over.
party_pants said:
Why do buses have such large front wheels?Nearly all buses I’ve ever seen have a front wheel bay that projects above the level of the floor and forms an obstacle for people boarding the buses with wheelchairs or prams. The wheel arches form a constriction point,
Can buses function just as safely and effectively with smaller diameter front wheels, or are large front wheels a necessity?
There’s physical limits to how much weight a wheel can carry, minimum contact ‘footprints’ between the vehicle and the road, physical space required to accommodate brakes, and performance characteristics (in loading/steering/cornering/braking/absorbing bumps) that dictate tyre. If you can’t use bigger wheels you need more wheels, which creates a whole new set of intrusions into the design.
I have no idea of the exact process, but if the people who make these things could make them smaller, without a loss of efficiency, I’m sure they would.
Go round and round.
The smaller the wheel, the harder it hits the bump. It’s to do with the radius. A large wheel rolls over bumps easier, and makes the ride smoother.
Kingy said:
The smaller the wheel, the harder it hits the bump. It’s to do with the radius. A large wheel rolls over bumps easier, and makes the ride smoother.
Tell that to a bloke with 20” wheels on a lowered ute.
;-)
Kingy said:
The smaller the wheel, the harder it hits the bump. It’s to do with the radius. A large wheel rolls over bumps easier, and makes the ride smoother.
I had assumed it was to deal with the weight. But Kingy’s expanation also makes sense.
A bus is a lot heavier than a car when fully loaded but still only has 4 wheels. So each wheel carries a lot more load.
All I can say from engineering experience is that the necessity for not damaging roads sets a maximum legal load permitted on each wheel, which explains why some trucks need to have so many wheels, but I haven’t thought about that in the context of wheel size.
Cars that are not intended for racing could probably get away with much smaller wheels.
Have a look here, National heavy vehicle mass and dimension limits. https://www.nhvr.gov.au/files/201607-0116-mass-and-dimension-limits.pdf
mollwollfumble said:
Kingy said:
The smaller the wheel, the harder it hits the bump. It’s to do with the radius. A large wheel rolls over bumps easier, and makes the ride smoother.
I had assumed it was to deal with the weight. But Kingy’s expanation also makes sense.
A bus is a lot heavier than a car when fully loaded but still only has 4 wheels. So each wheel carries a lot more load.
All I can say from engineering experience is that the necessity for not damaging roads sets a maximum legal load permitted on each wheel, which explains why some trucks need to have so many wheels, but I haven’t thought about that in the context of wheel size.
Cars that are not intended for racing could probably get away with much smaller wheels.
Have a look here, National heavy vehicle mass and dimension limits. https://www.nhvr.gov.au/files/201607-0116-mass-and-dimension-limits.pdf
- Steering axle carries a maximum of 6.0 tonnes
- Non steer axle, tyres less than 375mm carries a maximum of 6.0 tonnes
- Non steer axle, tyres 375mm to 449mm carries a maximum of 6.7 tonnes
- Non steer axle, tyres at least 450mm carries a maximum of 7.0 tonnes.
On the other hand, that 7.0 tonne limit for back axle can be increased to 9.0 tonnes.
This wheel arrangement is the same as for a bus.
Some hydrogen powered buses require more axles. With the extra axle, the maximum permitted load increases from 15 to 22.5 tonnes.
wear rate of both tyre surface and road are in there, given they are steer tyres, probably needs be able negotiate a modest curb also, little accidents
and steering geometry camber-related turning circle radius compensation may be improved with a larger diameter tyre, a wider tyre with larger diameter
the caster is the leaning of the top of the kingpins forward so the steering tends to centre with forward movement (the position the wheel trails in that arrangement), the camber is the flatness against the road, but of tight turning the steering geometry needs compensate and make the inner wheel circle smaller, and the outer wheel turning circle larger, and shift more of the weight to the edges of the tyre
……maybe, my knowledge of steering geometry isn’t great, possibly even worse than I think
a larger tyre may also tend to be more reliable for various reasons, tread thickness (total thickness also), wall thickness, and a larger tyre may not need be inflated to such a high pressure, can have more cushion, them being the first suspension element, and directly on the road
Wheel diameter vs load index, typical values.
12” 83
13” 99
14” 98-105
15” 109
16” 117-128
17.5” 129-143
19.5” 128-148
20” 141-164
22” 154-165
Load index vs load
84 = 500 kg (half a tonne)
100 = 800 kg
130 = 1900 kg
160 = 4500 kg.
ie. you need a big wheel to carry a big load. Such as a busload.
Answer from a bus driver:
The wheels need to be that size for weight/load reasons. The wheel arches protrude into the cabin because the floor needs to be as low to the ground as possible to cater for the elderly.
transition said:
wear rate of both tyre surface and road are in there, given they are steer tyres, probably needs be able negotiate a modest curb also, little accidentsand steering geometry camber-related turning circle radius compensation may be improved with a larger diameter tyre, a wider tyre with larger diameter
the caster is the leaning of the top of the kingpins forward so the steering tends to centre with forward movement (the position the wheel trails in that arrangement), the camber is the flatness against the road, but of tight turning the steering geometry needs compensate and make the inner wheel circle smaller, and the outer wheel turning circle larger, and shift more of the weight to the edges of the tyre
……maybe, my knowledge of steering geometry isn’t great, possibly even worse than I think
a larger tyre may also tend to be more reliable for various reasons, tread thickness (total thickness also), wall thickness, and a larger tyre may not need be inflated to such a high pressure, can have more cushion, them being the first suspension element, and directly on the road
Tamb said:
transition said:
wear rate of both tyre surface and road are in there, given they are steer tyres, probably needs be able negotiate a modest curb also, little accidentsand steering geometry camber-related turning circle radius compensation may be improved with a larger diameter tyre, a wider tyre with larger diameter
the caster is the leaning of the top of the kingpins forward so the steering tends to centre with forward movement (the position the wheel trails in that arrangement), the camber is the flatness against the road, but of tight turning the steering geometry needs compensate and make the inner wheel circle smaller, and the outer wheel turning circle larger, and shift more of the weight to the edges of the tyre
……maybe, my knowledge of steering geometry isn’t great, possibly even worse than I think
a larger tyre may also tend to be more reliable for various reasons, tread thickness (total thickness also), wall thickness, and a larger tyre may not need be inflated to such a high pressure, can have more cushion, them being the first suspension element, and directly on the road
Morning all.
This Wiki answers some of the questions https://en.wikipedia.org/wiki/Ackermann_steering_geometry
https://en.wikipedia.org/wiki/Scrub_radius
all seems a bit complicated, I was up too early, need another coffee before the humiliation of trying to understand all that
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
Spiny Norman said:
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
and they don’t like debris on the runway.
Arts said:
Spiny Norman said:
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
and they don’t like debris on the runway.
Tyres are OK with it. Fuel tanks, not so much.
Spiny Norman said:
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
Well I hope someone knows :)
For comparison, the wheels on the big dirt moving trucks I posted before can carry up to about 100 tonnes/wheel.
The Rev Dodgson said:
Spiny Norman said:
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
Well I hope someone knows :)
For comparison, the wheels on the big dirt moving trucks I posted before can carry up to about 100 tonnes/wheel.
F1 tyres must carry a big load when cornering.
Arts said:
Spiny Norman said:
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
and they don’t like debris on the runway.
This wasn’t from debris, but it was a tyre failing.
It was from a flat tyre on take-off. Because the aeroplane is so big, you just don’t notice a flat tyre or two. During the take-off the flat tyre flew apart, that also blew the tyre next to it, and on landing at the destination the wheels ran on the runway and came apart as well, and you can see the damage it caused.
- The tyre failed on that corner of that landing gear as the idiots that were maintaining the plane fitted the anti-skid sensor incorrectly and it was causing that wheel to lock-up frequently. When the wheel locks, the tyre wears through very quickly ….. and that explains why that corner of that landing gear got 25 flat tyres in one month. Not one of them thought to check why.
Tamb said:
The Rev Dodgson said:
Spiny Norman said:
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
Well I hope someone knows :)
For comparison, the wheels on the big dirt moving trucks I posted before can carry up to about 100 tonnes/wheel.
F1 tyres must carry a big load when cornering.
Up around 5 G’s at high speed. The cars weigh up around 700 kg at the start of the race, so a lateral load of about 3.5 tonnes in high-speed corners, though that’s shared by four tyres.
More useless trivia.
Top Fuel drag cars are a mighty impressive machine.
The tyres are big & soft and they absorb a lot of the power as the car starts to move off the line. They act as a big spring and gradually unwind as the car accelerates. They are pulling up around 6G’s off the line.
Then at the other end of the 1/4 mile run they turn into a very strange shape. No idea why, but you don’t see any other tyre do it.
Spiny Norman said:
Tamb said:
The Rev Dodgson said:Well I hope someone knows :)
For comparison, the wheels on the big dirt moving trucks I posted before can carry up to about 100 tonnes/wheel.
F1 tyres must carry a big load when cornering.
Up around 5 G’s at high speed. The cars weigh up around 700 kg at the start of the race, so a lateral load of about 3.5 tonnes in high-speed corners, though that’s shared by four tyres.
Tamb said:
Spiny Norman said:
Tamb said:F1 tyres must carry a big load when cornering.
Up around 5 G’s at high speed. The cars weigh up around 700 kg at the start of the race, so a lateral load of about 3.5 tonnes in high-speed corners, though that’s shared by four tyres.
The load would be a bit more on the outside wheels wouldn’t it?
Ah yes of course, quite right. If you were able to run fast enough, you’d nearly be able to lift the inside of the car off the track with a good strong arm.
Spiny Norman said:
Tamb said:
Spiny Norman said:Up around 5 G’s at high speed. The cars weigh up around 700 kg at the start of the race, so a lateral load of about 3.5 tonnes in high-speed corners, though that’s shared by four tyres.
The load would be a bit more on the outside wheels wouldn’t it?Ah yes of course, quite right. If you were able to run fast enough, you’d nearly be able to lift the inside of the car off the track with a good strong arm.
I used to run fast but never that fast.
Spiny Norman said:
Tamb said:
Spiny Norman said:Up around 5 G’s at high speed. The cars weigh up around 700 kg at the start of the race, so a lateral load of about 3.5 tonnes in high-speed corners, though that’s shared by four tyres.
The load would be a bit more on the outside wheels wouldn’t it?Ah yes of course, quite right. If you were able to run fast enough, you’d nearly be able to lift the inside of the car off the track with a good strong arm.
Tamb said:
Spiny Norman said:
Tamb said:The load would be a bit more on the outside wheels wouldn’t it?
Ah yes of course, quite right. If you were able to run fast enough, you’d nearly be able to lift the inside of the car off the track with a good strong arm.
I was watching the old time cars racing and on a lot of the cars the inside front wheel came completely off the ground.
Yes, and it helps make the tyres last longer. :)
Spiny Norman said:
Tamb said:
Spiny Norman said:Ah yes of course, quite right. If you were able to run fast enough, you’d nearly be able to lift the inside of the car off the track with a good strong arm.
I was watching the old time cars racing and on a lot of the cars the inside front wheel came completely off the ground.Yes, and it helps make the tyres last longer. :)
Spiny Norman said:
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
> When poorly treated, they’ll only last maybe 20 landings.
Yike, that can be less than 3 days. I never thought of aircraft tyres having that short a lifetime.
mollwollfumble said:
Spiny Norman said:
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
> When poorly treated, they’ll only last maybe 20 landings.
Yike, that can be less than 3 days. I never thought of aircraft tyres having that short a lifetime.
An example is a landing at a Mediterranean island (can’t remember the name sorry) where the end of the runway is at the edge of a cliff. Normally not a problem, but because the 747 is so large, most pilots (me included) would get the cues for landing from the radar altimeter. It’d count down your height above the ground in lots of ten’, from 50’. If you were heavy, you’d start to flare (pitch up a little) at about 30’ then reduce the power and wait for the touchdown. If you were light, you’d pitch for flare at 20’ and cut the throttles at the same time. Anyway, said runway with the end on a cliff cut most of that out and instead of a few seconds between each height callout, eg, “50 …… 40 …. 30 …. 20 …. 10”, you’d get “5040302010” immediately followed by a jarring thump if you hadn’t started to flare.
Anyway on that landing there was also a big crosswind, and because the plane wasn’t flared much it touched-down rather sideways. The landing was far from pleasant, it was said. The Captain said that he really thought that they’d bent the wing landing gear so he went down to have a look – after all the passengers had got off, of course – and found that the gear struts were fine, but there were scrub marks on the tyres literally halfway up the sidewalls.
That’s pretty violent.
Spiny Norman said:
mollwollfumble said:
Spiny Norman said:
Useless trivia – a 747 tyre is about 1.3 metres in diameter, and at maximum weight each of the 16 main tyres takes about 27 tonnes. On a heavy-weight landing, they go from 0 to about 250 km/h in less than a second.
When poorly treated, they’ll only last maybe 20 landings. If treated well …. I’m not sure but it’s a fair bit longer.
They are typically re-treaded many times before the carcass is discarded.
> When poorly treated, they’ll only last maybe 20 landings.
Yike, that can be less than 3 days. I never thought of aircraft tyres having that short a lifetime.
An example is a landing at a Mediterranean island (can’t remember the name sorry) where the end of the runway is at the edge of a cliff. Normally not a problem, but because the 747 is so large, most pilots (me included) would get the cues for landing from the radar altimeter. It’d count down your height above the ground in lots of ten’, from 50’. If you were heavy, you’d start to flare (pitch up a little) at about 30’ then reduce the power and wait for the touchdown. If you were light, you’d pitch for flare at 20’ and cut the throttles at the same time. Anyway, said runway with the end on a cliff cut most of that out and instead of a few seconds between each height callout, eg, “50 …… 40 …. 30 …. 20 …. 10”, you’d get “5040302010” immediately followed by a jarring thump if you hadn’t started to flare.
Anyway on that landing there was also a big crosswind, and because the plane wasn’t flared much it touched-down rather sideways. The landing was far from pleasant, it was said. The Captain said that he really thought that they’d bent the wing landing gear so he went down to have a look – after all the passengers had got off, of course – and found that the gear struts were fine, but there were scrub marks on the tyres literally halfway up the sidewalls.
That’s pretty violent.
I’ll be right up front about this, as far as I can recall, and although I’m getting on a bit my memory isn’t too bad, I don’t think I’ve ever landed a 747 on a Mediterranean island.
Reckon this setup would sort a lot of the issues with front wheels.
Then.. cost…
More along these lines, the low floor bus with ramp has good accessibility for disable and parents with prams, but the front wheel arches takes up a lot of floor space.
