[tushky]

I saw this Tata Test Truck or may be a Test mule of a New Bus. but i dont understand what type of wheel set up is this. all other axles have Leaf spring suspensions but only last one has Air suspension. why? can anyone tell me?

[silversteed]

Originally Posted by tushky

I saw this Tata Test Truck or may be a Test mule of a New Bus. but i dont understand what type of wheel set up is this. all other axles have Leaf spring suspensions but only last one has Air suspension. why? can anyone tell me?

This is the new 37-Ton truck which was showcased in the Auto-Expo. The rear-most axle has a lift mechanism, which means, the axle will be lowered when the GVW crosses some preset value, and while parking. The air bellows form a part of the lift-axle mechanism.

FYI, most of the long-haul trucks abroad come fitted with air suspension.

EDIT:
More pics of the TM 3723:
http://www.team-bhp.com/forum/commer...ml#post2906240

Here's Ashok Leyland's 37T offering, the 3718il:
http://www.team-bhp.com/forum/commer...ml#post3054723

[tushky]

Thank you so much for the information. nice to know that TML is also offering air suspension on Indian Trucks. hope this Test mule is not for export market

[silversteed]

Originally Posted by tushky

Thank you so much for the information. nice to know that TML is also offering air suspension on Indian Trucks. hope this Test mule is not for export market

I don't know about TML's offering, but I did see a couple of AL 3718il trucks recently. They have the lift-axle ahead of the first rear axle, and uses the same air-suspension setup. FYI, the LPT3118c from TML has the very same lift-axle

[narayan]

I am guessing here --- In a conventional set up where the last axle is not present, the overhang will oscillate substantially under low/no load conditions putting substantial stress on the axle. To avoid this, is a dead axle with air suspension being added ? Because if the intent was to just extend the body to carry more weight, a regular dead axle with leaf spring could have been added

[silversteed]

Originally Posted by narayan

I am guessing here --- In a conventional set up where the last axle is not present, the overhang will oscillate substantially under low/no load conditions putting substantial stress on the axle. To avoid this, is a dead axle with air suspension being added ? Because if the intent was to just extend the body to carry more weight, a regular dead axle with leaf spring could have been added

I'm afraid your guess is totally wrong. There are better, cost-efficient ways to avoid such an oscillation, than adding more weight (the axle weighs quite a lot, and this will have to be reduced from the payload), more complexities (the air suspension itself costs 1-2L, and is not as easily serviceable as a leaf-spring) and additional cost.

The 37 tonner is configured to be used as a 31 tonner with the lift-axle raised, resulting in substantial savings in terms of fuel and tyre-wear.

[tharian]

Originally Posted by narayan

I am guessing here --- In a conventional set up where the last axle is not present, the overhang will oscillate substantially under low/no load conditions putting substantial stress on the axle. To avoid this, is a dead axle with air suspension being added ? Because if the intent was to just extend the body to carry more weight, a regular dead axle with leaf spring could have been added

The main purpose of a lift axle would be to carry more weight when required as the weight is spread out due to the number of axles.
The lift axle is present since not always trucks run fully loaded and it would be a waste to run the lift axle always due to tyre wear, engine efficiency etc. It basically reduces the stress on the chassis on a single or double rear axle rigid truck.

If you have noticed, on the 3118 TATA rigid, the lift axle is in front of the rear twin axle and the last axle is not a lift axle.
Same goes for the AL 5 axle rigid. In fact the AL 4 axle rigid did not have a lift axle at all, but then the 4th axle was right behind the front axle and was steerable as well.

[narayan]

If the lift axle is purely for carrying more payload I wonder why an air suspension is being used ? I have seen regular leaf spring axles being lifted up when load is low

[tharian]

Originally Posted by narayan

If the lift axle is purely for carrying more payload I wonder why an air suspension is being used ? I have seen regular leaf spring axles being lifted up when load is low

Air bellows is easier for lift/drop axles since its just pumping and releasing air and can be done by a press of a button in the cabin.
Leaf suspensions take up more space on the already congested chassis and as far as I know cannot be lifted and if it can be, it would be a task and the leaf would be under stress even when the axle is lifted.

I have seen a lifted leaf suspension axle, but then it was the wheel on one side only that was lifted.

[silversteed]

Originally Posted by narayan

I have seen regular leaf spring axles being lifted up when load is low

That's news to me! Could you please post more details, and some pics?

AFAIK, it's extremely difficult to lift an axle with leaf-spring suspension, considering the stress the leaf would take while in the lifted position. Moreover, it needs some additional mechanism to raise and lower the axle. With an air suspension, the air bellows need to be filled to the right pressure in order to engage the lift-axle. All trucks use air brakes, so there's no need for a separate compressor. All that's needed is an extra air tank for the axle bellows.

[amit_mechengg]

Dont consider it as Air suspension. Its the axle lift mechanism.

[narayan]

Originally Posted by tharian

If you have noticed, on the 3118 TATA rigid, the lift axle is in front of the rear twin axle and the last axle is not a lift axle.
Same goes for the AL 5 axle rigid. In fact the AL 4 axle rigid did not have a lift axle at all, but then the 4th axle was right behind the front axle and was steerable as well.

Originally Posted by silversteed

That's news to me! Could you please post more details, and some pics?

AFAIK, it's extremely difficult to lift an axle with leaf-spring suspension, considering the stress the leaf would take while in the lifted position. Moreover, it needs some additional mechanism to raise and lower the axle. With an air suspension, the air bellows need to be filled to the right pressure in order to engage the lift-axle. All trucks use air brakes, so there's no need for a separate compressor. All that's needed is an extra air tank for the axle bellows.

Was referring to what Tharian has mentioned above. Where the axle is in front of the driven axle. Dont they have a leafspring ?

[tharian]

Originally Posted by narayan

Was referring to what Tharian has mentioned above. Where the axle is in front of the driven axle. Dont they have a leafspring ?

They have air bellows as well. Like Amit_mechengg mentioned, look at it as a lift mechanism as well along with the suspension.

[silversteed]

Originally Posted by narayan

Was referring to what Tharian has mentioned above. Where the axle is in front of the driven axle. Dont they have a leafspring ?

It doesn't make a difference whether the lift-axle is ahead of the live (driven) axle or at the very rear - the lift mechanism and the suspension are integrated.

However, if you are talking about the additional axle in the AL 31T trucks, then you are right, it has leaf springs - but NO lift mechanism. Among the 31T trucks, only TML and Bharat-Benz employ the lift-axle mechanism. AL, AMW and Eicher have a regular steerable axle fitted just behind the front axle.

[Hemicuda]

Originally Posted by narayan

If the lift axle is purely for carrying more payload I wonder why an air suspension is being used ? I have seen regular leaf spring axles being lifted up when load is low

There are no trucks out there with a leaf spring and lift axle.The AL ,Eicher with a streered axle with leaf spring, and the TAta with the Lift axle with air suspension.

Originally Posted by silversteed

It doesn't make a difference whether the lift-axle is ahead of the live (driven) axle or at the very rear - the lift mechanism and the suspension are integrated.

.

The lift Axle here is placed at the rear because that is the only place that can be available.Remember u have a prop shaft to accomodate which provides drive to the third axle in this case or the 8x2( Tata,Al.Eicher)