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Old 27th February 2013, 17:31   #1
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Default How do Multi-axle Trucks Turn?

I have studied during my engineering days that when a four wheeled vehicle is taking a turn the line extended along the front wheel’s axis of rotation must meet the line extended along that of the rear wheels at one point. Thus all four wheels can roll along without any one slipping or sliding thus reducing tyre wear.
How is this taken care of in a truck having two axels at the rear?
Please see the attached diagram.
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Old 27th February 2013, 20:21   #2
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Default Re: How do Multi-axle Trucks Turn?

I am not sure, if I have understood your question.

I always used to wonder that how does a 4 wheeler take a turn when both the wheels are attached to the engine! How is it possible for the two wheels attached to the engine to take a turn, as while doing that, the inner wheels will need to turn slow while the outer ones will turn faster comparatively.
One day, while searching on youtube, i happened to find a video, that explained my doubt to the core. This cleared the concept of differential from the base.

May be this is of help to you or someone else.

You can skip to around 1:30 directly.
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Old 27th February 2013, 21:21   #3
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Default Re: How do Multi-axle Trucks Turn?

Quote:
Originally Posted by carzone View Post
I am not sure, if I have understood your question.

I always used to wonder that how does a 4 wheeler take a turn when both the wheels are attached to the engine! How is it possible for the two wheels attached to the engine to take a turn, as while doing that, the inner wheels will need to turn slow while the outer ones will turn faster comparatively.
One day, while searching on youtube, i happened to find a video, that explained my doubt to the core. This cleared the concept of differential from the base.
Thank you Mr Carzone for that link. My question is not about distribution of turning force or torque when wheels driven by one engine has to turn at different rpms during turning, which is what a differential does. Mine is about steering geometry, the angles the front tyres need to be turned on a multi axel vehicle.. Please see the attached link about ackermann steering geometry.http://en.wikipedia.org/wiki/Ackerma...ering_geometry.

Last edited by moralfibre : 3rd March 2013 at 08:29. Reason: Do not quote an entire long post for a relatively shorter reply.
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Old 27th February 2013, 21:36   #4
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Default Re: How do Multi-axle Trucks Turn?

Quote:
Originally Posted by norhog View Post
How is this taken care of in a truck having two axels at the rear?
There is no special consideration. The two rear axles are rigid. Hence there is a lot of skid while turning at the rear axles. Providing steering at rear is not a good option for ruggedness. Tire life is sacrificed !
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Old 27th February 2013, 22:36   #5
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Default Re: How do Multi-axle Trucks Turn?

The differential is the mechanism which allows different wheels to rotate at different speeds while transmitting motion and this is required while negotiating curves. There is a differential in front wheel driven vehicles too.

Last edited by rajeev k : 27th February 2013 at 22:38.
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Old 27th February 2013, 22:52   #6
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Default

Phew! You took me 13 years back in time to my final year project on the Ackerman steering

Here are a few pointers:
http://www.eng-tips.com/viewthread.cfm?qid=54516
http://www.google.com/patents/US6059056

There is a pretty complicated mechanism of sensors, actuators, steer/contra-steer, in some cases independent suspensions to keep things simple - if I could sum it all in one line.
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Old 27th February 2013, 22:53   #7
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Default Re: How do Multi-axle Trucks Turn?

Pardon me but i have seen central wheels (Which are on the rear) turn too. So if the driver is turning right the rear wheels also slightly turn right (May not be to the same degree as the front ones).

I must add i have seen this mostly in trucks and do not remember seeing this in buses.
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Old 27th February 2013, 22:56   #8
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Default Re: How do Multi-axle Trucks Turn?

Quote:
Originally Posted by Ravindra M View Post
There is no special consideration. The two rear axles are rigid. Hence there is a lot of skid while turning at the rear axles. Providing steering at rear is not a good option for ruggedness. Tire life is sacrificed !
Thank you Mr Ravindra,I think thats the way it is. Some tyre life is sacrificed.
I guess these trucks and now multi axel volvo buses will be laying down some rubber on our ghats.
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Old 28th February 2013, 10:52   #9
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Default Re: How do Multi-axle Trucks Turn?

Trucks travel in straight lines (or very soft arcs) for most of their lives. hence It does not make sense to implement all wheel steering in multi axle trucks as that would make the truck expensive, with more parts to fail. Sacrificing some tyre life while the truck takes sharp turns is economically cheaper as it turns out.

Also, You must have noticed Multi axle Tata trucks which have 4 axles in total. The second one- right behind the front axle is supported on air springs and can be raised or lowered depending on the vehicle load. Guess how this one steers?

It has no direct connection with the steering wheel, and is supported on follower wheels- much like a wheel-chair's front wheels - so they turn to follow the trajectory of the front wheels. Some trucks in which this axle cannot be raised, have the second axle connected to the steering system as well.

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While searching for an image, i came across this tata truck which has the follower wheel arrangement in the rear most axle. As you can see the axle is raised above ground and seems to be turned slightly- that is because its not connected to the steering mechanism and will follow the axle in front of it by design when it is lowered. Also notice that the steerable last axle is only a helper axle. the main heavy duty axles still dont have steering to keep costs and reliability in check.
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Old 1st March 2013, 05:49   #10
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Default Re: How do Multi-axle Trucks Turn?

The rear wheels turn at a sharper angle than the frotn wheel, they act as pivots and turn around their own axis.

If you notice a Bus/Lorry taking a turn, the driver will take the turn in such a way that the rear wheels stay closest to the kerb, the whole vehicle pivots on the rear wheel similar to a pendulum

Last edited by TheARUN : 1st March 2013 at 05:50. Reason: Spelling mistake
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Old 1st March 2013, 09:59   #11
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Default Re: How do Multi-axle Trucks Turn?

Quote:
Originally Posted by dhawcash View Post
It has no direct connection with the steering wheel, and is supported on follower wheels...
...that is because its not connected to the steering mechanism and will follow the axle in front of it by design...
Non-steered axles but follower wheels! Very interesting point indeed!
Will these wheels always provide crab-steer (since they just follow, and nothing else) or is there a contra-steer possible at low speeds?
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Old 1st March 2013, 11:20   #12
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Default Re: How do Multi-axle Trucks Turn?

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Originally Posted by voodoochild View Post
Non-steered axles but follower wheels! Very interesting point indeed!
Will these wheels always provide crab-steer (since they just follow, and nothing else) or is there a contra-steer possible at low speeds?
No. Theses follower wheels provide only normal steering. They would not alter the way the vehicle is turned in anyway, i.e. no changes in the effective turning radius or anything.

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See attached image- crab steer is when all wheels turn in the same direction to provide a crab like motion . This is used in forklifts and backhoe loaders.
Counter steer would help reduce the turning radius.

But in this configuration, the vehicle would turn the same way irrespective of whether the follower wheels are raised or not. follower wheels are just a way to reduce load on other wheels. and also increase the total contact patch to reduce the pressure exerted on the roads so as not to damage them. I guess all of us have witnessed pimple like distortion on the highways caused by overloaded trucks.
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Old 1st March 2013, 18:22   #13
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Default Re: How do Multi-axle Trucks Turn?

Quote:
Originally Posted by dhawcash View Post
Trucks travel in straight lines (or very soft arcs) for most of their lives. hence It does not make sense to implement all wheel steering in multi axle trucks as that would make the truck expensive, with more parts to fail. Sacrificing some tyre life while the truck takes sharp turns is economically cheaper as it turns out.
...
While searching for an image, i came across this tata truck which has the follower wheel arrangement in the rear most axle. As you can see the axle is raised above ground and seems to be turned slightly- that is because its not connected to the steering mechanism and will follow the axle in front of it by design when it is lowered. Also notice that the steerable last axle is only a helper axle. the main heavy duty axles still dont have steering to keep costs and reliability in check.
Thanks a ton Mr Dhawcash. Your answer really helps me understsnd. By the way the TATA system seems to be a follower system but I am sure I saw some kind of hydraulic Ram connected to the second axel from front on a Ashok Leyland truck. Also the Ashok Leyland system has leaf springs and not air bellows as in case with TATA. Another point here since the second axel being a follower, during reversing it needs to be lifted up or some other thing is at play here??

Last edited by moralfibre : 3rd March 2013 at 08:30. Reason: Do not quote an entire long post. Only quote relative references. Thanks!
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Old 2nd March 2013, 01:09   #14
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Default Re: How do Multi-axle Trucks Turn?

Quote:
Originally Posted by norhog View Post
.. By the way the TATA system seems to be a follower system but I am sure I saw some kind of hydraulic Ram connected to the second axel from front on a Ashok Leyland truck. Also the Ashok Leyland system has leaf springs and not air bellows as in case with TATA. Another point here since the second axel being a follower, during reversing it needs to be lifted up or some other thing is at play here??
Im not too sure about the hydraulic ram thing in ashok leyland- maybe they implemented hydraulics to steer the axle. But in tata trucks with the follower wheel arrangement, there is a spring and damper attached to the wheels to keep them from dangling here and there while raised.

And i had a chat with a truck wallah a while back, and asked him the same question regarding reversing. he said they have to lift the axle while reversing mostly, but if they have to reverse in a straight line for only a few meters, not raising the axle also works fine.
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Old 2nd March 2013, 12:32   #15
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Default Re: How do Multi-axle Trucks Turn?

Quote:
Originally Posted by norhog View Post
Thanks a ton Mr Dhawcash. Your answer really helps me understsnd. By the way the TATA system seems to be a follower system but I am sure I saw some kind of hydraulic Ram connected to the second axel from front on a Ashok Leyland truck. Also the Ashok Leyland system has leaf springs and not air bellows as in case with TATA. Another point here since the second axel being a follower, during reversing it needs to be lifted up or some other thing is at play here??
norhog, multi-axle trucks come with 3,4 or 5 axles. The 3-axle ones have a 25-T GVW. There will be two rear axles, both are not steered. The haulage trucks usually do not have the rear-most axles powered. But tippers have both rear axles live. In that case, you will find differentials fitted on both the rear axles.

The 4-axle trucks have a tonnage of 31-T. Ashok Leyland provides an additional steerable axle right after the front axle. The wheels are steered according to the steering angle. Since trucks come equipped with power steering, it's nothing too expensive or more complex, to provide steerability in the second axle. The second axle cannot be lifted. Leaf springs are inexpensive and more rugged compared to air bellows, so from that point of view, the AL setup is better.

Not only AL, but Eicher and AMW also provides this same setup.

Tata provides a different setup. The additional axle is provided closer to the first rear axle. This axle can be lifted, hence fitted with air bellows in order to facilitate lifting and lowering. AFAIK, the lift axle is lowered automatically when the GVW exceeds some limit (not sure if it was 19T or 21T). While reversing, this axle has to be lifted, or else they drag and make it difficult. Since haulage trucks don't have to reverse very often, truckers live with this handicap to enjoy the savings in terms of tyre wear when the truck is not loaded.

In either case, if you are wondering about the rear-most axles being able to be steered, the answer is no. The rear-most axles are free-wheeling, and hence you won't find differentials.

The 5-axle trucks are 37-tonners. I'm yet to find one in operation, but AL ,AMW and TM have launched their 37 ton trucks. The AL setup uses a lift axle just before the first rear axle, the TM setup is what you find in the pic in a post above mine. The first two axles are steered in either case.

Quote:
Originally Posted by dhawcash View Post
Im not too sure about the hydraulic ram thing in ashok leyland- maybe they implemented hydraulics to steer the axle...
All trucks use hydraulic power steering, hence what you are referring to might belong to the PS system.

Last edited by silversteed : 2nd March 2013 at 12:39. Reason: added info
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