Go Back   Team-BHP > Under the Hood > Technical Stuff


Reply
 
Thread Tools Search this Thread
Old 1st August 2007, 03:39   #46
Senior - BHPian
 
1100D's Avatar
 
Join Date: Jun 2006
Location: Calcutta
Posts: 4,179
Thanked: 2,360 Times
Default

For some cars, downshifting is the only way to stop! (unless you can open the door, put your foot out and brake)
1100D is offline   Reply With Quote
Old 18th June 2008, 18:00   #47
BHPian
 
sathya_nars's Avatar
 
Join Date: May 2007
Location: Bangalore
Posts: 642
Thanked: 52 Times
Default

I heard that you can *just* apply brake without pressing clutch (while in speed > 60 kmph) to help stop car quickly.

My instructor (while learning driving) told me - press clutch whenever brake is applied.

Which is true?
sathya_nars is offline   Reply With Quote
Old 18th June 2008, 18:06   #48
Team-BHP Support
 
Akshay1234's Avatar
 
Join Date: Dec 2006
Location: Mumbai
Posts: 9,762
Thanked: 6,670 Times
Default

Quote:
Originally Posted by sathya_nars View Post
I heard that you can *just* apply brake without pressing clutch (while in speed > 60 kmph) to help stop car quickly.

My instructor (while learning driving) told me - press clutch whenever brake is applied.

Which is true?
of course you can. you should only press the clutch when the revs drop to below, say 1000rpm otherwise your car might go off.
Akshay1234 is offline   Reply With Quote
Old 18th June 2008, 18:20   #49
Team-BHP Support
 
Vid6639's Avatar
 
Join Date: Jul 2006
Location: Bangalore
Posts: 14,471
Thanked: 19,385 Times
Default

Quote:
Originally Posted by sathya_nars View Post
I heard that you can *just* apply brake without pressing clutch (while in speed > 60 kmph) to help stop car quickly.

My instructor (while learning driving) told me - press clutch whenever brake is applied.

Which is true?
That's what is engine braking. If you press the clutch you have no engine braking and your brakes will wear out quicker. Main thing is without engine braking your braking distance will be longer, with engine braking you can stop quicker but you have to press the clutch at the right time to prevent the car from stalling.

A really good driver will use maximum engine braking by downshifting to a lower gear while braking. I still havent been able to master that completely as I'm too tall and heel and toe is not possible to rev match while braking.
Vid6639 is offline   Reply With Quote
Old 18th June 2008, 23:29   #50
BHPian
 
theEnd's Avatar
 
Join Date: Nov 2005
Location: Bombay
Posts: 534
Thanked: 5 Times
Default

my opinion:

if you have all the time in the world: brake gradually, using only the brake. whenever the rpm drops too close to idle, downshift, making sure to match revs before releasing the clutch in the lower gear (otherwise your car will jerk and your clutch will wear out faster).

if you have no time: squeeze the brake pedal decisively. If you have ABS keep it fully depressed. If you dont have ABS then modulate your pressure to achieve maximum braking without locking up the wheels (might need some practice beforehand).

if you have no time and the brakes don't work: slam the car into a low gear (1st or 2nd), or even reverse (if you use enough force it might work). Try to drive up a hill, or through bushes or scrape along a wall to slow yourself down. Brace for impact.
theEnd is offline   Reply With Quote
Old 19th June 2008, 01:54   #51
Team-BHP Support
 
Rehaan's Avatar
 
Join Date: Feb 2004
Location: Bombay
Posts: 22,381
Thanked: 22,606 Times
Default

Quote:
Originally Posted by sathya_nars View Post
I heard that you can *just* apply brake without pressing clutch (while in speed > 60 kmph) to help stop car quickly.

My instructor (while learning driving) told me - press clutch whenever brake is applied.

Which is true?
You are right.

This has been discussed EXTENSIVELY in this thread > http://www.team-bhp.com/forum/techni...ing-right.html (Shifting to Neutral or Pressing the Clutch when Braking - Is this right?)

cya
R
Rehaan is offline   Reply With Quote
Old 19th June 2008, 02:31   #52
Senior - BHPian
 
DirtyDan's Avatar
 
Join Date: Jul 2007
Location: Dharamsala
Posts: 1,798
Thanked: 745 Times
Default

If you use the clutch more and shift and downshift more you will put extra strain on the clutch and tranny. If you do this mildly it probably will not matter. If you do it wildly you will go thru clutches and maybe a pilot bearing and transmission input shaft, an expensive proposition. Generally brakes are cheaper to replace than clutches and trannys. A lot depends on how you do what you do. Think "smooth". Also, some vehicles have heavy clutches and can handle almost anything you do, most can not.

You post is almost incomprehensible to me. However, this does not prevent me from expressing an opinion.
DirtyDan is offline   Reply With Quote
Old 19th June 2008, 09:52   #53
BHPian
 
Join Date: Oct 2005
Location: @ Driver's Seat @
Posts: 684
Thanked: 57 Times
Default

Quote:
Originally Posted by vid6639 View Post
That's what is engine braking. If you press the clutch you have no engine braking and your brakes will wear out quicker. Main thing is without engine braking your braking distance will be longer, with engine braking you can stop quicker but you have to press the clutch at the right time to prevent the car from stalling.

A really good driver will use maximum engine braking by downshifting to a lower gear while braking. I still havent been able to master that completely as I'm too tall and heel and toe is not possible to rev match while braking.

Vid, Dont you think Engine braking is harmful to the engine life ? Thats what I heard from MASS. I do engine braking sometimes, when when it jerks, I quickly release it and use my toes to rub the brakes
lambuhere1 is offline   Reply With Quote
Old 19th June 2008, 10:06   #54
Team-BHP Support
 
Vid6639's Avatar
 
Join Date: Jul 2006
Location: Bangalore
Posts: 14,471
Thanked: 19,385 Times
Default

Engine braking is harmful only if you don't rev match. You have to rev match so you don't put sudden strain on gearbox and clutch. If you downshift to a lower gear you will notice that the car will jerk. This is what is more harmful than anything else. If you rev up then change down you won't feel the jerk and the engine braking will be as efficient.

I'm still trying to find out how to do heel n toe. My legs are too long and my car's too small. I generally downshift then apply brake for normal braking from a higher speed.
Vid6639 is offline   Reply With Quote
Old 19th June 2008, 11:31   #55
BHPian
 
watashi75's Avatar
 
Join Date: Jan 2008
Location: Hyderabad
Posts: 518
Thanked: 7 Times
Default

The advantage of engine braking is not evident at low speeds and that is the reason some people are skeptical about engine braking. There is a lot of difference in braking from 100 kmph and braking from 120 kmph.

At high speeds it is always advisable to combine regular braking with downshifting. Advantage of downshifting to reduce speed is that there is less chance of the wheels locking up. Besides last minute acceleration to avoid an obstacle is possible because of the lower gear.

Disadvantage is if you fumble or if you are slow in slotting the gears it is even worse than standing on the brakes.

I tend to agree somewhat with BlackPearl that slight rev matching needs to be done. If we drop the clutch quickly there will be a sudden jerk which might affect the steering, which is not good if we need to avoid an obstacle at the last moment like, say, a pot hole. One way to avoid this is to release the clutch a little slowly.

I am not sure of the weight transfer part. For a FWD the engine braking would retard only the front wheels while regular braking would retard all the four wheels. So I would expect more weight to be transferred to front in case of engine braking alone. Can somebody clarify this point?
watashi75 is offline   Reply With Quote
Old 19th June 2008, 12:00   #56
Senior - BHPian
 
Shan2nu's Avatar
 
Join Date: Feb 2004
Location: Hubli - Karnata
Posts: 5,525
Thanked: 87 Times
Default

Quote:
I am not sure of the weight transfer part. For a FWD the engine braking would retard only the front wheels while regular braking would retard all the four wheels. So I would expect more weight to be transferred to front in case of engine braking alone. Can somebody clarify this point?
Yup, engine braking transfers more weight to the front and if theres too much engine braking while you're on the brakes, it can cause the front wheels to lock up even faster than normal braking.

Revv matching is a must if you want the downshifts to be jerk free. It really helps to learn the "heel n toe" technique. You can be smooth and fast at the same time.

Shan2nu
Shan2nu is offline   Reply With Quote
Old 19th June 2008, 15:25   #57
Awaiting Email Confirmation
 
Join Date: Sep 2007
Location: India
Posts: 1,745
Thanked: 19 Times
Default

Quote:
Originally Posted by watashi75 View Post

I am not sure of the weight transfer part. For a FWD the engine braking would retard only the front wheels while regular braking would retard all the four wheels. So I would expect more weight to be transferred to front in case of engine braking alone. Can somebody clarify this point?
I thought downshifting in a front wheel drive car would cause the weight to be transferred to the front, much more than using the brakes, which work on all 4 wheels.

I've had an incident of nasty oversteer in an M800 just by downshifting(I did not touch the brakes)


no doubt, downshifting helps in reducing braking distance and effort. but I'm hesitant to do that because of 2 reasons
1. when the clutch is depressed for me to disengage the higher gear, i lose engine braking and the car leaps forward(I dont like that)
2. when I engage the lower gear, and take my foot off the clutch, the car jerks due to poor rev matching.


I suppose, I 'll get better with practice. can some one donate a car with a super strong tranny for me to practice?

I find that I use engine braking very well in situations like ghat driving and on the track where I know at which point I need the braking and what speed I have to carry through the corner, i.e when everything is predictable and controlled.

but in panic braking situations, I find standing on the brakes the best I can do. downshifting is beyond my capabilities then.
rippergeo is offline   Reply With Quote
Old 19th June 2008, 15:33   #58
Senior - BHPian
 
Surprise's Avatar
 
Join Date: Feb 2005
Location: Chennai
Posts: 2,299
Thanked: 129 Times
Default

Quote:
Originally Posted by vinfriend View Post
The few seconds you press clutch the breaking power will reduce loosing engine breaking and leads to loss of control and skidding also.

Quote:
Originally Posted by Shan2nu View Post
Yup, engine braking transfers more weight to the front and if theres too much engine braking while you're on the brakes, it can cause the front wheels to lock up even faster than normal braking.
Engine braking = loss of contro, skidding, front wheel lock

Now this is confusing.
Surprise is offline   Reply With Quote
Old 19th June 2008, 15:47   #59
Senior - BHPian
 
Surprise's Avatar
 
Join Date: Feb 2005
Location: Chennai
Posts: 2,299
Thanked: 129 Times
Default

Quote:
Originally Posted by rippergeo View Post

no doubt, downshifting helps in reducing braking distance and effort. but I'm hesitant to do that because of 2 reasons
1. when the clutch is depressed for me to disengage the higher gear, i lose engine braking and the car leaps forward(I dont like that)
2. when I engage the lower gear, and take my foot off the clutch, the car jerks due to poor rev matching.


I suppose, I 'll get better with practice. can some one donate a car with a super strong tranny for me to practice?

I find that I use engine braking very well in situations like ghat driving and on the track where I know at which point I need the braking and what speed I have to carry through the corner, i.e when everything is predictable and controlled.

but in panic braking situations, I find standing on the brakes the best I can do. downshifting is beyond my capabilities then.
Exactly my thoughts and had been answered by mclaren earlier in this thread

Been practicing this for a while and encountered a situation where I could have used engine braking, but I came back to the world only when I missed the interrupting object by a minimal margin.

As explained by someone I had only a very very little time to react. Just stand on the brake and steer it away (State Highway)

a. Seeing the side mirror to ensure that I can steer away
b. pressing clutch, down shifting, releasing the clutch to activate engine braking

Nothing worked

I know the answer too -- practice, practice, practice

Last edited by Surprise : 19th June 2008 at 15:50.
Surprise is offline   Reply With Quote
Old 19th June 2008, 19:18   #60
BHPian
 
revvedup's Avatar
 
Join Date: Sep 2006
Location: Delhi
Posts: 251
Thanked: 8 Times
Default

My 2 cents


1.Engine braking happens when your wheels are spinning faster than they can do in that particular gear so if one shifts at about 10 Km/h above the maximum speed in that gear it would lead to retardation in a safe manner while at the same timing insuring that there is not much damage to the
engine and tranny. Case in point is the swift(which does 100 in 2nd) so if you are say at 110 in 5th gear and shift to 2nd it would lead to engine braking.

2. The purpose of heel and toe or blipping the throttle while braking+ downshifting is to ensure that that engine does not run out of air. Petrol engines in general change the amount of air by restricting their flow via the throttle instead of reducing its amount.

Not blipping the throttle would be like restricting the air supply of a person before he starts to run so what one actually does by blipping the throttle is to increase the pressure by increasing the air in the combustion chamber because each time the piston moves downward it creates an area of low pressure (think of it as air pump drawing air, and this is what increases in case of engine braking) until finally it can draw no more air finally resulting in the jerking sound accompanied by the halt. So by blipping the throttle you are actually reducing the effectiveness of engine braking (not completely required if you practice engine braking like how I have mentioned in the first point)

2. Engine braking also causes forward weight transfer and locking of wheels
but at the same time it could lead to faster deceleration as compared to braking alone provided that you do not exceed the limits of traction that your tyres (yes wider soft compound rear tyres help) can provide .

3. Engine braking is primarily used in racing/rally applications( this is not to say it cannot be used on the road) in order to prevent overheating of the braking system ( remember glowing red discs) because engines are typically liquid cooled whereas brakes even in F1 applications are air cooled so they are less prone to overheating than your brakes.

4. From the above, one can conclude that engine braking although advantageous shouldn't be used excessively because it can lead to heating up of the clutch( reducing its bite so as to say) and engine ( especially if you also do your upshifts a little late). However in case of an emergency sit it can save your life/car if you practice it properly.

Last edited by revvedup : 19th June 2008 at 19:25.
revvedup is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
GreenLight: Reduce traffic jams by eliminating 'right turns' at signals D4D Street Experiences 47 6th November 2014 09:40
Can speed cameras reduce road mishaps? ad3952n Street Experiences 22 8th September 2014 14:23
How effective is the auto-cut mechanism of the fuel service stations? Zappo Technical Stuff 35 27th March 2009 14:34
USA - Proposal to reduce Speed limit to 55 mph appuchan The International Automotive Scene 37 1st August 2008 23:30
Hydraulic Bonnet Opening Mechanism.How do i install nikibusa Modifications & Accessories 7 26th April 2007 13:40


All times are GMT +5.5. The time now is 04:45.

Copyright 2000 - 2017, Team-BHP.com
Proudly powered by E2E Networks