[a4anurag]

Originally Posted by Ashley2

Dear, dead axle has only 2 tyres in buses.

I am confused

If looked at the bus from rear end:

Engine -> axle (2 tyres) -> Axle (4 tyres).

Please explain how the power is transferred from the engine to the tyres.

Anurag.

[Ashley2]

Originally Posted by MAS

From the last photo it seems that the axle with four tyres is hooked up to the engine/gear box. The dead axle seems to be the last one, closest to the engine. Can anyone help with right information?

The one near engine is dummy and infront of that is live axle. Propeller shaft passes over last axle to axle in front. Check last pic.

[MAS]

Originally Posted by a4anurag

I am confused

If looked at the bus from rear end:

Engine -> axle (2 tyres) -> Axle (4 tyres).

Please explain how the power is transferred from the engine to the tyres.

Anurag.

The last photo has the answer. The propeller shaft can be clearly seen.

[a4anurag]

Originally Posted by Ashley2

The one near engine is dummy and infront of that is live axle. Propeller shaft passes over last axle to axle in front. Check last pic.

Originally Posted by MAS

The last photo has the answer. The propeller shaft can be clearly seen.

So this would be the same way of construction in the Volvo 9400 XL and Mercedes Benz?

If the propeller shaft goes above the dead axle and drives the one with two wheels won't it get spoilt due to the high torque produced by the engine and also the articulation that the bus would undergo during usage?!

I don't understand the reason to pass it over. What if the axles are exchanged and the one near the engine is driven it would save fuel as transmission losses would be lower and lesser maintenance due to lower number of moving parts.

Anurag.

[Wagon R Rocks!]

Originally Posted by a4anurag

I don't understand the reason to pass it over. What if the axles are exchanged and the one near the engine is driven it would save fuel as transmission losses would be lower and lesser maintenance due to lower number of moving parts.

Anurag.

Hello,
The purpose of providing drive axle away from the engine is to reduce the effective turning radius of bus. Bus, being a longer vehicle is desired to have shortest turning radius. By basic principles of Ackermann steering mechanism, the point where imaginary line from steered wheel centre (front wheels) joins the line passing through centre of live axle (centre of rear wheels in case of bus) is that point about perfect steering condition occurs (pure rolling motion). Always, steered rear axle has very short turning angle (about 10-12 deg.) whereas front wheel steering angle can be upto 45 deg. Hence, if we put the rear steered wheel ahead of the drive wheel, it will increase the wheelbase of the bus thereby increasing turning radius. I hope this explanation will help you visualise this mechanism.
Thanks.

Sanket

[silversteed]

Originally Posted by a4anurag

So this would be the same way of construction in the Volvo 9400 XL and Mercedes Benz?

If the propeller shaft goes above the dead axle and drives the one with two wheels won't it get spoilt due to the high torque produced by the engine and also the articulation that the bus would undergo during usage?!

I think you are confused.

In the Volvo 9400XL based on the B9R, the last but one axle is the dead axle. It has one tyre at either end, ie., two tyres in all. It is not powered.

The last one, closest to the engine, is the powered, or live axle. It has 2 tyres on either side, a total of 4 tyres. It has a differential just like any front-engined bus' rearmost axle..

The Scania chassis (and the Merc O500RSD and Volvo B11R) in the pic has this configuration:
==(Front axle)=====((Live axle))(Dead axle)^Engine^
Front axle -> single tyre on either side
Live axle -> double tyres on either side
Dead axle -> single tyre on either side

See the pic below:


The live axle always has 4 tyres, for better traction. The only exception in full length buses would be the airport tarmac coaches, where the front axle is driven but has only one tyre on either side.

Now to answer your question further:

I don't understand the reason to pass it over. What if the axles are exchanged and the one near the engine is driven it would save fuel as transmission losses would be lower and lesser maintenance due to lower number of moving parts.

The reason to pass the prop shaft over the dead axle is to reduce the wear and tear of the dead axle. If the config used were the same as that in the Volvo B9R, the lateral forces afflicted during acceleration and deceleration would cause additional wear and tear. The rearmost axle, which is the live axle would be pushing the dead axle in this configuration. Ashley2 has explained it very well above

In Scania's config, the dead axle is trailing the live axle. It merely follows the live axle. There's no question of lesser moving parts, as the same number of moving parts is needed whether the live axle is placed at the end or ahead of the dead axle. The only difference would be in the length of the prop shaft, which, is not very significant here.

If your question is regarding rolling resistance caused by 4 tyres instead of 2 tyres, then the argument in favour of using 4 tyres for the live axle is better traction which translates to improved braking (remember, retarders act on the prop shaft, so improved traction helps).

[Nanometer]

There is a small shelf right next to the engine on the right in pic 5 , think this is for holding the AC compressor ,correct me if I am wrong. Also the fuel tank looks to be located in between the front wheels , can you think for better location in case of a frontal impact , I do understand its for balancing the load on the chassis. Also i always wondered see the 3 + 2 seating combination on buses , how is the load balanced ?

[Ashley2]

Originally Posted by Nanometer

There is a small shelf right next to the engine on the right in pic 5 , think this is for holding the AC compressor ,correct me if I am wrong. Also the fuel tank looks to be located in between the front wheels , can you think for better location in case of a frontal impact , I do understand its for balancing the load on the chassis. Also i always wondered see the 3 + 2 seating combination on buses , how is the load balanced ?

That's for Ac compressor mtg.
With respect to fuel tank, there is always a mix of locations - front and back in Volvo and Mercedes also. IMHO fuel tank near front axle is always a danger zone. Also the centre one is little lowered and hitting the road is always a propability (though there are fire walls and structure), and can be case in last fire accident.

[Nanometer]

I would prefer a center engine and a fuel tank at the back , we need to go through some historical data on all kinds of vehicle and see the accident data and decide on the best option for the fuel tank . Also personally experienced the rear dead axle or the multi-axle setup does add to the comfort of the passengers when driven on bad roads. Also since the chassis can be extended , does it mean you can extend the controls or the wiring from the driver to the engine the way you want ?

[a4anurag]

Originally Posted by Ashley2

With respect to fuel tank, there is always a mix of locations - front and back in Volvo and Mercedes also. Also the centre one is little lowered and hitting the road is always a probability (though there are fire walls and structure), and can be case in last fire accident.

This is the set up in a Volvo multi axle where the fuel tanks are split by the sides and one in the centre. It gives good vehicle balance and keeps the centre of gravity lower.

Originally Posted by Nanometer

I would prefer a center engine and a fuel tank at the back.

Won't it become tail heavy and have a dragging kind of feel?

Originally Posted by Nanometer

Also personally experienced the rear dead axle or the multi-axle set up does add to the comfort of the passengers when driven on bad roads.

Very true as the two axles compensate each other for the movements from the road and that is not in-filtered into the cabin.

Originally Posted by Nanometer

Also since the chassis can be extended, does it mean you can extend the controls or the wiring from the driver to the engine the way you want?

I don't think so much extension can be done. May be the front and rear overhang can be increased not the wheelbase part. Correct me if I am wrong.

Anurag.

[Nanometer]

Thanks Anurag and Ashley2, The Volvo already has a middle engine bus in there line up , may not in India but outside, I think that would be same as having a rear engine bus and passenger luggage.

The Box in the middle looks like has the battery inside and other guards and tools required as per govt norms to be provided with a new vehicle.

[silversteed]

Originally Posted by a4anurag

I don't think so much extension can be done. May be the front and rear overhang can be increased not the wheelbase part. Correct me if I am wrong.

The wheelbase can be changed, but not the overhangs.

[Nanometer]

How is the driver controls connected to the engine at the back , is it drive by wire or mechanical or hydraulic ?

[dhawcash]

Greetings.
I used to work at azad coach builders, jaipur, and have seen numerous rear engined bus chassis ( not multi axle ones though) i remember vivdly staring at a mahindra rear engine chassis and observing how it all works. It had a manual gear box and air suspension for all three axles
Ill try and answer a few queries here:

1. Controls and linkages?
In general, most modern diesel engines are drive by wire. So a set of electric wires connect the engine to the hot pedal.
Brakes are pneumatic, like in any bus, so the usual system. Clutches are also pneumatically controlled( it was in the mahindra chassi) so again flexible air hoses are used.
The gear shift mechanism employes cables, just like in cars like skoda octavia. Only these are much thicker and longer.

2. Chassis extension?
I think its easily do-able. Just need to extend a set of wires, pneumatic hoses, and gear shifer cables. As the chassis are transported by truck, it might make sense to do that as shorter chassis would be easier to transport by a truck ( mass to volume ratio is better)

3. Rear chassis sag?
Thats air suspension for you. Once the engine starts and the systrm has pressure, it returns to a balanced and normal ride height. Notice any parked bus with air suspension- rear end would always seem to be lower than usual.

4. Power to 2nd axle - prop shaft going over last axle?
Most buses have a suspension travel of around 3 inches ( compression only). Body builders are instructed to maintain wheel- mudguard clearance of 4 inches in general, to allow unhindered suspension movement. So basically, prop shaft movement is already known and the dead axle is designed with adequate clearances to prevent metal to metal contact as the suspension works.
The scania set up means a longer prop shaft. This has advantages regarding wear and tear, as the universal joints at both ends of the prop shaft face lower angular movement as the suspension moves.
Another benefit is that the driven axle is located nearer to the centre of gravity. The same setup is used in front engined multiaxle trucks for this reason.

5. Rear dead axle tyre wear?
This axle is there only to support the higher loads, which would be too much for the 4 tyres on the live axle. But the contact patch of 2 tyres is lower than 4, so less traction. Also, no steering mechanism. So what happens when the bus takes a sharp turn? The dead axle is forced to drag sideways over asphalt. Result? higher than usual tyre wear!

[a4anurag]

Thanks for the help buddy!

Originally Posted by dhawcash

1. Controls and linkages?
In general, most modern diesel engines are drive by wire. So a set of electric wires connect the engine to the hot pedal. Brakes are pneumatic, like in any bus, so the usual system. Clutches are also pneumatically controlled( it was in the mahindra chassis) so again flexible air hoses are used. The gear shift mechanism employes cables, just like in cars like skoda octavia. Only these are much thicker and longer.

So there will a decent loss in power transmission (longer cables, air lines) and more parts that are susceptible to damage.

Originally Posted by dhawcash

2. Chassis extension?
I think its easily do-able. Just need to extend a set of wires, pneumatic hoses, and gear shifer cables. As the chassis are transported by truck, it might make sense to do that as shorter chassis would be easier to transport by a truck ( mass to volume ratio is better)

So all the chassis are shortened and then transported after which at the body building shop it is extended to it's usual size? Is this what they do or it is sent in full length? I guess Volvo build their buses themselves, right? Earlier it was Azad and Sutlej for Mercedes! (I love the Volvo 9400 XL - I go weak on my knees)

Originally Posted by dhawcash

3. Rear chassis sag?
Thats air suspension for you. Once the engine starts and the systrm has pressure, it returns to a balanced and normal ride height. Notice any parked bus with air suspension- rear end would always seem to be lower than usual.

Why is this done? So you mean to say every time the engine is switched OFF the pressure is released from the system and it builds once the engine starts? I have never seen this behaviour in a Volvo multi-axle bus.

Isn't this risky? What if the pressure build up low then required? Any malfunction indicators?

Originally Posted by dhawcash

4. Power to 2nd axle - prop shaft going over last axle? Most buses have a suspension travel of around 3 inches ( compression only). Body builders are instructed to maintain wheel- mudguard clearance of 4 inches in general, to allow unhindered suspension movement.

Is 3 inches sufficient for the size and weight of the buses? Thanks for the information here.

Originally Posted by dhawcash

5. Rear dead axle tyre wear?
This axle is there only to support the higher loads, which would be too much for the 4 tyres on the live axle. But the contact patch of 2 tyres is lower than 4, so less traction. Also, no steering mechanism. So what happens when the bus takes a sharp turn? The dead axle is forced to drag sideways over asphalt. Result? higher than usual tyre wear!

If that is the case why aren't both the axles having four tyres each rather than 4X2 set up?

Cost saving? Can you enlighten me on the suspension part of the two axles also? Working and set up!

Thanks.

Cheers,
Anurag.