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Old 3rd February 2010, 13:16   #46
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Quote:
Originally Posted by tsk1979 View Post
Many offroaders are going by there "offroad experience" of lower gear.
However, on a 10-20 foot slope, you can't tell which has more braking.
Based on my experience of driving the Indica/Safari/Santro and Getz down long slopes(many kms) I have seen that even in the old indica, in first gear the engine would slowly start accelerating close to redline in 1st gear if the descent was very steep.
Ditto with the safari. I have to keep tapping the brakes.
However on similar mountain roads, the getz and the santro could do very well in 2nd gear, with engine in midrange, and less frequent tapping of brakes required.
Exactly this is what i tried to convey. And no one is considering the involvement of 4WD here.

Thanks
--Sree--
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Old 3rd February 2010, 13:37   #47
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This Link would provide a little more information. Read till the end and it throws up another interesting variable "Turbo Diesel"

Engine Braking

It didn't give me the answer, but instead made me more confused.
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Old 3rd February 2010, 13:49   #48
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@Arka taking your third example if the crawl ratio is 52 and the maximum torque of the engine is 12 kgm @ 2000 rpm then max torque at the wheels will become= 12kgmx52=624 kgm do you think so much amount of torque goes to the wheel?

Spike
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Old 3rd February 2010, 14:58   #49
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Default Empirically.

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Originally Posted by SPIKE ARRESTOR View Post
@Arka taking your third example if the crawl ratio is 52 and the maximum torque of the engine is 12 kgm @ 2000 rpm then max torque at the wheels will become= 12kgmx52=624 kgm do you think so much amount of torque goes to the wheel?

Spike
Hi Spike,

About 25% of 624Kgm will reach each wheel as,

1) At the T-Case there is a 50:50 Split
2) That 50% is further split to 50:50 i,e 25% will reach the wheel.

The Empirically proper calculations will be

at T-Case 3.986 X 2.46 X 12kgm = 117Kgm (50:50 Front and Back Diffs)

At Rear Diff 117/2 X 5.38 = 316.5Kgm

At each wheel 316.5/2 = 158.2 Kgm.

In an open diff if one wheel catches air, then that wheel will get 316.5Kgm.

This is the reason why 2WD Lo is disabled in most vehicles.

Also the Dana 44 30 Spline 1.31" Axles can take 6000ft.lbs i.e 957Kgm of Torque per Axle.

Arka

Last edited by ex670c : 3rd February 2010 at 15:02.
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Old 3rd February 2010, 15:45   #50
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Arka that was a good info on the Dana 44 axle with 30 spline, i didnt know that. One more question, what is the torque split in old gen Jeeps in 4wd high and low modes?

Spike
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Old 3rd February 2010, 16:10   #51
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Quote:
Originally Posted by ex670c View Post
Hi Spike,

This is the reason why 2WD Lo is disabled in most vehicles.

Arka
We can fool it by installing fw hubs. Any one having plans to pull a heavy trailer on 2wd LOW on tarmac please go for full floating axles LOL
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Old 3rd February 2010, 16:13   #52
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Torque split between the front and rear axle is 50/50 be it old or new gen jeeps unless it has a electronic torque split as far as i know.
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Old 3rd February 2010, 16:28   #53
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Default Torque Split

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Originally Posted by SPIKE ARRESTOR View Post
Arka that was a good info on the Dana 44 axle with 30 spline, i didnt know that. One more question, what is the torque split in old gen Jeeps in 4wd high and low modes?

Spike
Hi Spike,

The Torque Split is 50:50 in Hi & Lo Modes.

Regards,

Arka
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Old 4th February 2010, 13:34   #54
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Quote:
Originally Posted by vinod_nookala View Post
The problem with present engines are they are rev friendly and attain higher rpms easily.
Quote:
Originally Posted by tsk1979 View Post
Diesels do not have good engine braking ability. Therefore on steep slope, you need to keep tapping brakes lightly otherwise it will revv away to glory.
I have seen that while descending very steep slopes on 1st low, if I do not keep tapping brakes, the engine happily revvs close to 4500rpm

Quote:
Originally Posted by Samurai View Post
Guys, Common rail diesel engines don't give engine braking like the old school engines. Something about ECU trying to act smart. So diesel Jeeps give very good engine braking while common rail Safari/Scorpio won't.
Not True - Depends on the ECU tuning and the sensors it maps and reacts!

Quote:
Originally Posted by vinod_nookala View Post
Guys,
Revability of the engine and the ability to get to redline depends on the charactrstics of the engine. Thats why may be safari revs up to 4500 rpm in 1st low in steep downhill. Depends upon how the engine is tuned. This would never happen in a old school diesel engines.
Which is that characterstic which makes it rev friendly and also hold engine braking iam clueless! May be its pump design, may be some thing to do with block design, may be flywheel weight etc etc

But in loose terms a petrol engine cannot match a diesel engine in terms of engine braking due to differences in the compression ratio. Petrols can rev higher than the highest reving diesel engine. Hence the resistance to rev without fuel supply will be higher on a diesel engine.
Not true again Vinod - Engine design is complicated than we imagine it to be!

Quote:
Originally Posted by star_aqua View Post
its all the pump cotrol that matters here.

The Pump is the Heart of a diesel engine just like the carb or MPFi fuel rails are in petrol.

I've done steep descents in a CJ3B & MM540 [2112cc 4.90 DP] as well as a MG413W [Petrol Carb]

Each of the vehicles react differently but eventually all of them CRAWL. I'm 100% sure even the Tata Safari will crawl.

Mr. Tanveer, if your TS does not crawl in 4wd - 1, i believe there is something wrong in one of its sensors - maybe the accelarator position sensor or the common rail pump.. I understand you have been having problems with your vehicle since you purchased it - I have read your extensive thread on that here!

Please get it rectified.
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Old 4th February 2010, 14:28   #55
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Quote:
Originally Posted by headers View Post
Mr. Tanveer, if your TS does not crawl in 4wd - 1, i believe there is something wrong in one of its sensors - maybe the accelarator position sensor or the common rail pump.. I understand you have been having problems with your vehicle since you purchased it - I have read your extensive thread on that here!

Please get it rectified.
My safari crawls fine in 1st low. On slopes 30-40 feets down, steep ones too, I do not have any crawl speed issues.
I am talking about engine braking. For example, a down slope couple of kms long. Steep descent, like from Wari La Pass on smooth road.
There in 2WD more in 1st gear in the safari, the engine rpm gradually keeps on increasing, all the way to 4000rpm.
I have to keep tapping brakes to control speed.
Same thing in indica.
But on similar slopes, in the getz, even in second gear the engine kept in midrange, and I could go downhill without tapping brakes. similar in santro.

Try this with the jeep also. Put it in first gear(Not low ratio but normal 1st gear), find a steep downhill section(for example 36 hairpin bends to ooty, and let her cost, initially speed will be slow, and it will slowly start increasing, eventually you will need to tap brakes now and then to control the speed
In a petrol engined car lesser tapping is required(mostly none at all) because the engine braking effect is much more.

Remember we are not talking about gear ratios. We are talking about the rpm engine reaches just under the power of gravity.
In petrols you will rarely cross midrange, but in diesels the engine rpm will start creeping close to redline.
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Old 4th February 2010, 15:02   #56
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Default Down Hill

Quote:
Originally Posted by tsk1979 View Post
Try this with the jeep also. Put it in first gear(Not low ratio but normal 1st gear), find a steep downhill section(for example 36 hairpin bends to ooty, and let her cost, initially speed will be slow, and it will slowly start increasing, eventually you will need to tap brakes now and then to control the speed
In a petrol engined car lesser tapping is required(mostly none at all) because the engine braking effect is much more.

Remember we are not talking about gear ratios. We are talking about the rpm engine reaches just under the power of gravity.
In petrols you will rarely cross midrange, but in diesels the engine rpm will start creeping close to redline.
Hi TSK,

On protracted down hill sections it is advisable to tap the brakes because,

While Driving on Flat and Inclined surfaces the the Drive is
Engine>Fly Wheel>Clutch>Gear Box>T-Case> Differential> Wheels.

While Driving down slope, due to gravity and the weight of the vehicle, the wheels tend to drive the vehicle
Wheels>Differential>T-Case>Gear Box>Clutch>Flywheel>Engine

But since the engine is running there is a clash of the Titans at the Clutch Plate and Cover Assembly, since the engine wants to turn the wheels, but on a down slope the wheels are effectively forced by gravity and the weight of the vehicle, to rotate the transmission in the opposite direction, this makes the engine race up, to generate enough torque and power to prevent gravity from rotating the engine in the opposite direction.

Petrol Cars seem to have better engine braking because of Lower Gears.

Try this experiment in a CRDe/DICOR/CRDI/MPFI vehicle.
Select a Flyover and put the vehicle in 1st gear and gently release the clutch, without pressing the accelerator allow the vehicle to climb, and note the engine RPM, till the engine stalls or vehicle fully climbs up; mark this spot.

Descend the same flyover section/spot on 1st Gear without accelerator, and notice the engine RPM, will it be higher than what it was while climbing?

If a Car has a reduction of 10:1
i.e 10 RPM on the Fly wheel = 1 Wheel Rpm,

Down Slope (Due to Weight and Gravity)
the reduction will be 10:1, 1 RPM of the wheel = 10 RPM on the Fly Wheel

This why it is not advisable to run prolonged down slopes on idle speed (600-1100 RPM).

1) Gently Tap the accelerator to get the engine to over come the resistance, at the flywheel

2) Cadence Braking to Further slow the vehicle down.

Regards,

Arka
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Old 4th February 2010, 16:44   #57
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Quote:
Originally Posted by ex670c View Post
But since the engine is running there is a clash of the Titans at the Clutch Plate and Cover Assembly, since the engine wants to turn the wheels, but on a down slope the wheels are effectively forced by gravity and the weight of the vehicle, to rotate the transmission in the opposite direction, this makes the engine race up, to generate enough torque and power to prevent gravity from rotating the engine in the opposite direction.
Transmission will not rotate in the opposite direction but in the same direction but will rotate faster than the engine RPM hence the wheel tries to drive the engine which in-turn results in engine braking.
Quote:
Originally Posted by ex670c View Post
This why it is not advisable to run prolonged down slopes on idle speed (600-1100 RPM).

1) Gently Tap the accelerator to get the engine to over come the resistance, at the flywheel

2) Cadence Braking to Further slow the vehicle down.

Regards,
Arka
I am not convinced as to why one needs to tap the accelerator while coming down the slope. Totally against all known principles of driving down the hill. Can someone explain in layman terms?
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Old 4th February 2010, 16:49   #58
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1. I am talking about steep slopes. If I just release clutch slowly, the vehicle(indica or safari) will stall. Same for MPFI vehicle
2. Secondly, in first gear, I am hurtling down rpm at 2000 and slowly increasing, if I tap accelerator, won't the speed start increasing.
Remember, we are talking about hard tar slopes, where engine speed will slowly go on increasing as we go down.
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Old 4th February 2010, 18:55   #59
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Quote:
Originally Posted by ex670c View Post
Try this experiment in a CRDe/DICOR/CRDI/MPFI vehicle.
Select a Flyover and put the vehicle in 1st gear and gently release the clutch, without pressing the accelerator allow the vehicle to climb, and note the engine RPM, till the engine stalls or vehicle fully climbs up; mark this spot.

Descend the same flyover section/spot on 1st Gear without accelerator, and notice the engine RPM, will it be higher than what it was while climbing?

This is a fantastic experiment to understand your vehicle. Had done this sometime ago!!

http://www.team-bhp.com/forum/test-d...ml#post1564116 (97BHP, 25.5 Kgm, Common Rail - The Bolero CRDe Storm)

Cheers
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Old 4th February 2010, 23:01   #60
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Diesels have less engine braking exactly because of their higher compression. The higher compression of a diesel provides far more force to the DOWNSTROKE of the piston than a petrol piston of the same displacement and therefore much more force turns the crankshaft and then the drive shaft etc. than is the case with a petrol engine. What a jake break does is let the air out of the balloon, let the air out of the DOWNSTROKE by venting it out the exhaust valves. This takes force away from the crankshaft and hence the rest of the vehicle. It is the DOWNSTROKE that is the real difference maker in engine braking between diesel and petrol along with the fact that the intake is open on the diesel on the upstroke and closed on the petrol.

I think it is the gearing that makes off-roaders think their diesels brake better than petrols.

Jake brakes were invented to save brakes on heavy vehicles. They might be useful in slowing the free fall of rogue planetoids like the Tata Safari.

Last edited by DirtyDan : 4th February 2010 at 23:20.
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