[Tejas Ingle]

Originally Posted by TheARUN

Also, what about Owner training? I think they can have a more comprehensive training program for the Owner and then let the Owner top up on the training programs they give to their Drivers inturn. A trained Owner will be worth more than a bunch of trained Drivers unless Drivers manage the Bus as a whole end-to-end

Excellent point. They are the people more in need of training. That will also take care of proper maintainance of the buses.

[Ashley2]

Originally Posted by Tejas Ingle

...Yes, you cannot apply brakes without a retarder, unless the foot pedal switch is turned off. But retarder takes a bit longer to actuate than the service brakes. Service brakes operate instantaneously.

What is the foot pedal switch?
Retarder does'nt take time to actuate. Even in my earlier post also I mentioned about the engagement of service brakes and retarder. Incase of brake pedal engagement when you press, retarder will decelerate the bus first and later which the service brakes engages.
This is the operating principle and you cannot interfere into this.

The other option is to use retarder stalk to slow the bus.

Originally Posted by Tejas Ingle

They use the service brakes only after the speed comes below 40kmph (Except in emergency or sudden braking). Mostly they use the retarder stick to control the bus. Untrained drivers tend to go very close to the vehicle in front of them and brake at the last moment. So the max work is done by the service brakes.

Infact service brakes comes to service only after say 40 kmph. Till such time its only retarder.

Originally Posted by Tejas Ingle

ABS and Retarder control units are separate in the Merc. Retarder has nothing to do with other braking systems. Engine brake is controlled by the Engine control unit. Exhaust brake is also controlled by the ECU. EBS is not there in Merc. Also cruise control is not present on the Merc 2-Axle, its there on the 3-Axle.

ABS and retarder are separate in all the buses. Retarder has everything to do with other braking especially with service brakes, as explained earlied. Also this needs to get disengaged when there is a input from ABS for wheel spin.

[Tejas Ingle]

Originally Posted by Ashley2

What is the foot pedal switch?
Retarder does'nt take time to actuate. Even in my earlier post also I mentioned about the engagement of service brakes and retarder. Incase of brake pedal engagement when you press, retarder will decelerate the bus first and later which the service brakes engages.
This is the operating principle and you cannot interfere into this.

The other option is to use retarder stalk to slow the bus.



Infact service brakes comes to service only after say 40 kmph. Till such time its only retarder.

ABS and retarder are separate in all the buses. Retarder has everything to do with other braking especially with service brakes, as explained earlied. Also this needs to get disengaged when there is a input from ABS for wheel spin.

I would want to correct you here. This is not the operating principle of retarder but these are the general guide lines for usage of retarder for slowing down.
If a driver is supposed to stop at some distance, he is supposed to slightly touch the brake pedal which actuates only the retarder. This brings down the speed gradually, and the driver presses the pedal furter to actuate the service brakes which can bring the vehicle to an immediate stop. The pedal works in stages.
Retarder is not capable of stopping the vehicle suddenly. All it does is slows down the vehicle gradually.
In emergency braking, the driver would press the pedal completely at once. This will actuate both retarder and service brakes, but max work will be done by service brakes here.

Also, the retarder is completely independent of the service brakes. It is a separate device which sits at the end of the gearbox on its out-put shaft, while service brakes are the friction brakes on each wheel.

A foot retarder switch is supposed to be switched off during rains or in slippery conditions. This is because the retarder operates only on the driven wheel. If the driver touches the pedal slightly, the retarder will start braking the rear wheels only which may cause the bus to skid off the road. Using service brakes along with the retarder stick is recommended in such conditions.

[Ashley2]

Originally Posted by Tejas Ingle

I would want to correct you here. This is not the operating principle of retarder but these are the general guide lines for usage of retarder for slowing down.....

You have not corrected me rather this is the post you have given the complete reply. There was some piece meal replies which I was also replying.

But I was trying to give info on these statements like these only.

But retarder takes a bit longer to actuate than the service brakes. Service brakes operate instantaneously.

ABS and Retarder control units are separate in the Merc. Retarder has nothing to do with other braking systems

Apart from this I am fully aware of the working, integration, location, advantages, disadvantages of retarder in Mercedes. Single axle uses R120 and multi axle uses R115E type retarder and both are from Voith.

[Tejas Ingle]

OK, then I probably got you wrong over these statements.

Originally Posted by Ashley2

Incase of brake pedal engagement when you press, retarder will decelerate the bus first and later which the service brakes engages.
This is the operating principle and you cannot interfere into this.

Infact service brakes comes to service only after say 40 kmph. Till such time its only retarder.

Retarder has everything to do with other braking especially with service brakes, as explained earlied.

But still, Service brakes can work at any speed even above 40kmph if the driver presed the pedal completely? No?

Originally Posted by Ashley2

Also this needs to get disengaged when there is a input from ABS for wheel spin.

This is not there on the Merc, hence the foot pedal ret switch. Is this there on the Volvo?

[Mpower]

Originally Posted by TheARUN

what I concede to Volvo is that they took the first mover advantage and have been able to plan better, they were able to beat the Merc in the market, how good each bus is intrinsically we might never know, but the Volvo is bus lengths ahead of the Volvo at the moment

I doubt if there was any significant first mover advantage for Volvo. If anything it was a disadvantage because it took a lot of effort on their part to convince operators to invest in a product that was 4 times more expensive than what was the norm.

Merc had a ready market available to them that was created by Volvo. They could have analysed all the problems of the Volvo faced in the Indian roads and come up with a better product.

But thanks to their overconfidence, they came in without doing proper homework relying more on their brand power to woo the customer.

[audioholic]

Originally Posted by Ashley2

ABS and retarder are separate in all the buses. Retarder has everything to do with other braking especially with service brakes, as explained earlied. Also this needs to get disengaged when there is a input from ABS for wheel spin.

Why should retarder disengaged when there is a lockout of any wheel? I dont think retarders are strong enough to jam the tyres and cause a skid. Hence, if ABS controls the service brakes, it must be enough IMO.

Moreover application of retarder or service brakes in a Volvo (in automatic setup) is decided by the EBS controller depending upon the pedal behaviour. Not sure how it is done in a mercedes. For a slow and slight pressure on the brakes, retarder gets activated. In case of a sudden press or a complete press of the pedal, the service brakes kick in. Source of this - A volvo bus driver with whom we travelled for a week across North India, mainly in the ghats of Kullu-Manali, Shimla. Here the braking was very gradual yet effective. Service brakes came in to play in the final moments, like a hairpin bend, a high soeed stop etc.

[Tejas Ingle]

Originally Posted by audioholic

Why should retarder disengaged when there is a lockout of any wheel? I dont think retarders are strong enough to jam the tyres and cause a skid. Hence, if ABS controls the service brakes, it must be enough IMO.

Retarders are not strong enough to lock the wheels. But application of the retarder in rains at high speed does cause the bus to skid since the rear wheels start rotating at a speed slower than the speed of the bus.
Service brakes on the other hand apply to all four wheels together and are also governed by ABS which prevents skidding.

[audioholic]

That is true sir, but why does a retarder be controlled by the ABS module? Is it done so anywhere? Be it MB or volvo, I doubt that the retarder control is so effective to either induce a skid or prevent one. The service brakes are much more severe and hence have the power to cause a skid.
Also, given the weight on the rear axle, and two tyres per side, along with an extra set of wheels in case of a three axle bus, I cant practically imagine the rear wheels to skid. Rather I feel front brakes are much prone to skidding. I may not be correct, but from a practical point of view this is what I felt.

[Tejas Ingle]

^^No, there are separate control modules for RET and ABS in MB. It might be integrated in EBS in Volvo, in that case it will only pick up a signal from the ABS that the rear wheels are spinning slower than the front wheels and cut off the retarder.

Your doubt about skidding is valid. But if a bus is moving at about 80kmph the inertia associated with the 18tonne vehicle is huge. The problem with retarder is that it applies only to the rear wheels. If the tyres find good grip, it will slow down the bus. Now you can imagine the torque of retarder. Even though it is much lesser than service brakes, it can effectively slow down an 18 tonne bus at 80 or even 120kmph. Now if the rear tyres dont find grip on a wet concrete surface, the torque applied by the retarder will be wasted, and the same inertia due to 18 tonnes of load will try to keep the bus moving at the same speed it was before. At this point, if the bus is tuned even slightly, the rear will give away and it will be moving side ways.

Now if you throw a hammer in the air, (carefully, you might hit someone) you will notice that the metal end always goes in front while the light wooden handle follows it. Similarly, the rear engined buses have most of the load in the back and hence the bus tends to swing that way. If the rear wheels skid, the bus will tend to spin 180 degrees.
You will find several such cases on the Mumbai-Pune expressway in the rainy season especially in the tunnels. This is because the tunnel floor is more slippery as dripping water keeps it wet 24X7. Most of the cases go unreported as the drivers manage to countersteer and get it back in control.

P.S.: I have experienced skidding of the bus once. I dont know if it was due to retarder usage. I have mentioned the incident here:
http://www.team-bhp.com/forum/street...ml#post2973236

[DWind]

Could it be that EBD module acts based on retarder inputs and consequently is related to ABS Module? As retarder only acts on the powered wheels, EBD might probably distribute more braking to the front wheels (~100% bias?) to compensate for this and then redistribute the bias again when retarder is switched off?
Also, I doubt retarder being active until the powered wheels lock up as retarder's effect is inversely proportional to wheel speed.

[audioholic]

Originally Posted by DWind

Also, I doubt retarder being active until the powered wheels lock up as retarder's effect is inversely proportional to wheel speed.

Probably what even I had in mind. Given presence of EBD, I think it will decide the brake bias taking the effect of retarder into consideration. Also, as these retarders are based on viscous coupling and there is no direct friction based braking effect from them, it need not be under the control of the ABS module. Even if it is, it could be a part of the input to ABS, therefore enabling the ABS module to consider the braking force provided by the retarder for its calculations.

[Tejas Ingle]

I dont think EBD is present on Volvo, not sure. EBS is present which Co-ordinates all the braking activities. So if EBD is not there, modulation of braking force wont be possible. These are interesting concepts, I'll find out if this system is present on any vehicle. It is certainly not there on MB.

Originally Posted by DWind

Could it be that EBD module acts based on retarder inputs and consequently is related to ABS Module? As retarder only acts on the powered wheels, EBD might probably distribute more braking to the front wheels (~100% bias?) to compensate for this and then redistribute the bias again when retarder is switched off?
Also, I doubt retarder being active until the powered wheels lock up as retarder's effect is inversely proportional to wheel speed.

Retarder effect is directly proportional to the speed. More the speed, more is the braking due to retarder.

Originally Posted by audioholic

Probably what even I had in mind. Given presence of EBD, I think it will decide the brake bias taking the effect of retarder into consideration

[DWind]

Originally Posted by Tejas Ingle

Retarder effect is directly proportional to the speed. More the speed, more is the braking due to retarder.

Sorry! Could not correct immediately what I had typed. I did mean directly proportional only.

[Ashley2]

Originally Posted by audioholic

Why should retarder disengaged when there is a lockout of any wheel? I dont think retarders are strong enough to jam the tyres and cause a skid. Hence, if ABS controls the service brakes, it must be enough IMO. ....

Originally Posted by Tejas Ingle

Retarders are not strong enough to lock the wheels. But application of the retarder in rains at high speed does cause the bus to skid since the rear wheels start rotating at a speed slower than the speed of the bus..

Retarders are indeed powerful. In this specific case, R115 offers braking torque as high as 2000Nm. With so much torque its very essential to cut off when braking skid/lock is sensed.
Infact a separate relay unit should take care of this, as a safety measure.