Downshifting to slow down.

[jammyozzy]

I don't have any experience with cars, but I always "finger-palm" (or is it wrist?), as Retsmah put it, when riding a motorbike. I guess it's especially important on a light bike, I've nearly propelled myself off of a Fizzy by downshifting badly and locking the rear wheel.

I also find it important to keep within the bikes powerband, especially on such a low powered bike. Getting out of the way of blind Volvo drivers, mums in SUVs and the like is very reliant on having some torque to accellerate with.

 

[Deathclown66]

I have a '92 Jetta, and heel-and-toeing is nearly impossible for me, what with extremely close gas and brake pedals, and size 12 boots.
I rather simply rotate between braking and very light (2-2.5k rpm) rev-matched downshifts, easier on the trans, and the brakes.

If you use your engine for braking, it will surely break.
downshifting is nearly the worst thing you can do to your transmission, even when rev-matched, it is still harder on it then upshifting with revs matched.

for clutch, and transmission wear and reliability, it's best to minimize and avoid when possible, any downshifts. Obviously, driving situations will regularly mandate downshifts, but there is no need or reason to dowshift so early.
Not to mention all the extra wear and tear on engine internals from the extra rpm's that come with frequent hard downshifts.

 

[Scaff]

I have a '92 Jetta, and heel-and-toeing is nearly impossible for me, what with extremely close gas and brake pedals, and size 12 boots.

I have to agree that its not always possible to cleanly heel and toe in every car, I can remember getting the chance to drive an RS6 and despite being a wonderful car in many, many ways; the brake pedal was set about and inch higher than the clutch and throttle pedals. It made heel and toe totally imposable and quite honestly did put me off the car.

I don't think I could now run a car that didn't allow me to heel and toe.

Regards

Scaff

 

[YSSMAN]

I have a '92 Jetta, and heel-and-toeing is nearly impossible for me, what with extremely close gas and brake pedals, and size 12 boots.
I rather simply rotate between braking and very light (2-2.5k rpm) rev-matched downshifts, easier on the trans, and the brakes.

I completely agree here. My '96 Jetta is a wonderful machine, but the pedals are a bit screwed up for seriously fast shifting. It was quite funny watching my brother try to drive the Jetta coming from his Sentra, as the clutch is positioned a lot higher and closer together with the other pedals.

 

[Keef]

I rev-match all my dowshifts, wether downshifting to accelerate or downshifting to slow down. I usually combine engine braking with my brakes while driving normally, but since I can't heel-and-toe downshift I simply use the brakes to slow when driving quickly. I'm rarely on the limit and always find time to let off of the brake pedal before or during the corner to rev-match downshift, then accelerate out of the corner.

I'm positive my pedal placement is the cause of my heel-toe problems. I've extensively watched videos and seen the motions race drivers do, and I've spent quite a bit of time in my Del Sol trying to get it right, but it comes to me by chance, and not very often. It's a Civic basically, so obviously the pedals aren't set up for performance driving. I just can't do it repetatively, on demand, and under pressure in my Sol. So I gave up trying. I'll wait 'til I get my S2000.

EDIT: Also, I never downshift into first gear from speed inless I'm entering a parking lot and going under about 10 mph. My car slows down like a dog in first gear. Whiplash material, really. I learned not to abuptly let off the gas in first pretty quickly.

 

[Duke]

I don't think I could now run a car that didn't allow me to heel and toe.

Wait 'til you get the Bim. The pedals are positively made for it. 👍

 

[Wolfe]

Scaff - I believe what Wolfe is saying is this:

If you're not supposed to use 'compression braking' on track, why don't racers just keep the clutch in and downshift, say 4~>2 in one go before rev matching and reengaging? Why go 4~>3~>2 and engage the clutch between each shift?

My quick answer is that it's easier to do accurately in multiple smaller steps rather than one big one, with less chance of disaster from a mistake. I'd also posit that it allows the driver to make a quick decision to stay in the intermediate gear if opportunity allows.

Your interpretation of my post was mostly correct, but your answer still doesn't clarify my confusion over the definition(s) of engine/compression braking.

Scaff's source says compression braking = bad, and compression braking + normal braking = really bad...yet on the very next page, heel-toe braking, which consists of both compression braking and normal braking, is recommended.

Furthering this contradiction is the notion that compression braking = bad for your engine, when in fact it is just as natural as acceleration, and is, again, a component of heel-toe downshifting. If compression braking was so horrible, then coasting off-throttle while in gear would ruin engines.

Everything would be clear if the usage of the term "compression braking" in the context of all of this meant downshifting without rev-matching, but Scaff replied that the term also applied equally to simply being off-throttle while in gear, which is harmless and part of the braking effect of heel-toe.


None of this affects my appreciation for the heel-toe technique, nor does it cause me to believe that coasting is the worst thing you can do to a car, but it would be nice if I knew exactly how these terms (compression braking and engine braking) are most often used, and whether there is an alternative term that I should be using instead to describe letting off of the throttle in gear and rev-matched downshifts (with or without heel-toe).

 

[Boz Mon]

Wow I didnt expect this thread to get this big. I kinda want to post it on the Prelude forum because you guys go so much more into it then them. But I am not going to do that because I dont want a bunch of people coming over here and talking in incoherent sentences. I'm not ending the thread or anything like that either, I am soaking up all of this info. Thanks again guys 👍

 

[Scaff]

Scaff's source says compression braking = bad, and compression braking + normal braking = really bad...yet on the very next page, heel-toe braking, which consists of both compression braking and normal braking, is recommended.

OK I can understand why it gets a bit messy. You are right that compression braking is bad and that compression braking when down-shifting and braking is really bad. So lets look at them in turn.

Compression braking only - Caused by a straight lift of the throttle without using the clutch or brakes. On the road this will generally cause only minor problems as you are unlikely to be on the limit of the cars ability, as such it can be used to slow the car down. Its less effective that using the brakes does put some minor strain on the drivetrain. The level of compression braking experienced will also vary from car to car and from gear to gear, as a rule of thumb the lower the gear the greater the degree of compression braking.

On track (or if driving hard on the road) the use of compression braking can unbalance the car, if in the middle of a corner it could result in sudden traction loss from the rear of the car.

Note - This is not the same as a 'lift' of the throttle, which is usually used to describe reducing the throttle position, but not removing your foot all together.


Compression braking + Normal Braking
The normal state of play for straight line braking, now while it would be nice to try and remove compression braking from the situation (and Colin Chapman among others has tried) a degree of it is inevitable. The degree of compression braking in this situation is generally low in comparison to the brakes themselves and will rarely interfere, brake modulation and control is determined by the dominant braking system, allowing the driver as much control over the braking as they have ability.


Compression braking + Downshift
The worst possible combination, changing down without a heel and toe action and letting the clutch out quickly will result in sudden and strong compression braking. The reason its stronger is when the clutch was depressed the car was running in a higher gear and when its released the car is now in a much lower gear and the miss-match of engine to road speed is far greater.

This sudden and violent compression braking can be felt as a sudden 'jerk' during braking in a road car and is uncomfortable and puts a large strain on the drive-train. If the tyres are close to the grip limit, then you have the possibility that the additional braking will lock the tyres and braking distances will increase and a loss of control could happen.


It is also important to remember that compression braking will only effect the driven wheels of a car, as such it will upset the cars balance much more than a well balanced braking system.


You say that compression braking is a component of heel and toe, and while that is true, its only because its unavoidable. Heel and toe (done correctly) removes the most problematic form of compression braking, that which occurs during the downshift.

Furthering this contradiction is the notion that compression braking = bad for your engine, when in fact it is just as natural as acceleration, and is, again, a component of heel-toe downshifting. If compression braking was so horrible, then coasting off-throttle while in gear would ruin engines.

Nothing here contradicts the notion that compression braking is bad for your engine, anything that resists the engines normally operation is bad for the engine. Its simply a case of to what degree, if you don't believe me you could always put your notion to the test. Find a stretch of empty road and at about 60mph slap the car in first and see what happens!!! I am of course joking (don't do this), the result will not only be a lock up of the driven wheels, but also massive and sudden strain in the engine, which will, without any doubt cause damage to a greater or lesser degree.

Coasting off-throttle will damage an engine, if done often enough, its putting additional strain in the engine. Yes the strain is very slight when compared to compression braking combined with a down-shift, but its still present.

Everything would be clear if the usage of the term "compression braking" in the context of all of this meant downshifting without rev-matching, but Scaff replied that the term also applied equally to simply being off-throttle while in gear, which is harmless and part of the braking effect of heel-toe.

The most common usage is that of down-shifts without heel and toe, but as with most things its never just that simply. Compression braking is simply using the engine to slow the car, something engines are not designed to do, how much strain its places on the engine and drive-train will depend on the nature of the compression braking.

None of this affects my appreciation for the heel-toe technique, nor does it cause me to believe that coasting is the worst thing you can do to a car, but it would be nice if I knew exactly how these terms (compression braking and engine braking) are most often used, and whether there is an alternative term that I should be using instead to describe letting off of the throttle in gear and rev-matched downshifts (with or without heel-toe).

Sorry but its all called compression or engine braking, but you are right, without context it can get confusing. I hope that the above has helped to explain it a bit better.

Regards

Scaff

 

[Slick Rick]

ignore

 

[zdenko]

Ok now lets look at this my way. You have just bough a brand new car. You are gonne keep it for 3 years. Say you do 30 to 40000kms on it. If u use compression braking you may not have to change your pads in that time. Where as if u use your brakes you may. Plus you will save a bit in fuel. And i'm preety sure the car is not gonna break its clutch in 40000k's if you drive normally. So in this case you would save money. And the next suker who buys the car will be up for pads soon and will probably not break the clucth. It will probably last another 100000k's.

 

[TheCracker]

I think that compression braking combined with acurate down shifts, done at the correct moment to avoid sudden changes of rev levels, is the most appropriate braking method on track. Whilst compression braking can upset a car's (already compromissed) balance if done heavy handedly. If combined with heal and toe changes i believe you get the optimum trade-off between maximising your braking ability, keeping the car balanced and not placing too much strain on any particular mechanical componant.

 

[///M-Spec]

^ I hope you also mean to combine the above with threshold braking, right? Or do you not touch the brake pedal at the track?


M

 

[TheCracker]

^ I hope you also mean to combine the above with threshold braking, right? Or do you not touch the brake pedal at the track?


M

You're aiming that question at someone who broke their crank last time out

Sorry, i did mean to say when combined with traditional braking too!

 

[Scaff]

Ok now lets look at this my way. You have just bough a brand new car. You are gonne keep it for 3 years. Say you do 30 to 40000kms on it. If u use compression braking you may not have to change your pads in that time. Where as if u use your brakes you may. Plus you will save a bit in fuel. And i'm preety sure the car is not gonna break its clutch in 40000k's if you drive normally. So in this case you would save money. And the next suker who buys the car will be up for pads soon and will probably not break the clucth. It will probably last another 100000k's.

First off can I ask that you stop with the text speak, the AUP (which you agreed to follow when you joined) states that I should not be used. So its 'you' not 'u', thanks.

Now as to your post, are you suggesting that you don't use the brakes on your car at all? That's how it reads and lets be honest that just not practical.

If you are suggesting that you use compression braking regularly during downshifts I can assure you that not only will you most likely need to replace your clutch in the time period involved, but you may end up causing damage to the engine itself. Damage that may be classed as driver abuse and not covered under warranty.

Brakes stop a car, not the engine, brake pads are cheap & easy to replace, engines and clutches are significantly more expensive and difficult to replace.

Sorry but you're idea here just does not hold water in the real world.

Regards

Scaff

 

[zdenko]

Jeez take it easy. Of course you would use your brakes as well. Otherwise you would be running into the back of every cars rear end and going through every red light. Sorry for the u instead of you, bad habbit.
I'm more talking you see the lights turn yellow and then you do it and obviously you use the brakes to slow down as well otherwise you won't obviously stop(unless its an uphill section) I do tend to give it a bit of revs to do this so it's not so jerky because jerkyness usually means more wear. And obviously i wouldn't do it from 2nd to 1st. First gear is just there to make it easy to take off not drive in. As long as you don't just drop the clutch and let it out easy it shouldn't wear it out to much.

My dad hasn't broken a clutch or had engine damage yet and he kept one car for 120000km's. He also only ever changed the front pads once at about 90000'ks and never changed the rear drums.

Edit: Just to add. If i were doing a quick stop i wouldn't do it. JUst maybe from 4th to 3rd then brakes to slow wn the rest of the way and put it in neutarl when u start getting really slow. Approximately 10km/h. I just do it when you have a big long gap to slow down in.

 

[Scaff]

My dad hasn't broken a clutch or had engine damage yet and he kept one car for 120000km's. He also only ever changed the front pads once at about 90000'ks and never changed the rear drums.

Basing an opinion of this nature on such limited info is rather dangerous, as isolated cases are not always representative. Mine is based on over a decade working in the motor industry and experience of a wide range of vehicle's.

I have to say I would be rather interested to see a car that has done 90,000kms /56,000 miles on the same brake pads, as while possible if the car had mainly motorway/highway mileage on it, its quite unusual.

120,000km/75,000miles on a single clutch is quite possible, the clutch on my Celica has covered 85,000miles/137,000km, so that's more than possible. As far as engine damage goes, well unless you strip an engine its difficult to start what additional wear or damage may have occurred.

The main point still stands, brakes are designed to stop the car, are cheaper and significantly easier to replace than drive-train components.

Regards

Scaff

 

[Wolfe]

You say that compression braking is a component of heel and toe, and while that is true, its only because its unavoidable. Heel and toe (done correctly) removes the most problematic form of compression braking, that which occurs during the downshift.

Right. I didn't mean to imply that it was important in the slowing action of heel-and-toe -- I just meant that it was obviously deemed harmless enough to not be cut out of the technique.

Coasting off-throttle will damage an engine, if done often enough, its putting additional strain in the engine. Yes the strain is very slight when compared to compression braking combined with a down-shift, but its still present.

Anything you do in a car strains it. As I've told other people before, if you don't want to have to replace your car's clutch/brake pads/tires/etc. there's an easy solution: don't drive it.

Sorry but its all called compression or engine braking, but you are right, without context it can get confusing. I hope that the above has helped to explain it a bit better.

Regards

Scaff

It's crystal-clear now.

 

[zdenko]

have to say I would be rather interested to see a car that has done 90,000kms /56,000 miles on the same brake pads, as while possible if the car had mainly motorway/highway mileage on it, its quite unusual.

Actually it probably helped that the car was a daihatsu charade 98 model. It only weighed about 850 to 900 kilos. And it was about 20% heavy city 50% suburban town driving and 30% motor way/highway.

I do agree that you do more wear by doing compression braking but i would not personally keep a car for over 100000km's so it's the next person who buys it off me that will cop the reapir costs later down the line.
I like to get rid of a car before the timing belt needs doing. In some cases though thats 50000km's.

 

[YSSMAN]

One odd thing I noticed today while driving to school is that heel-toe driving is practically impossible in my car because of the way the pedals are laid out. Spacing is alright, however the gas pedal sits for what feels like about an inch to an inch and a half below the brake pedal. Too low to use the two sides of your foot to blip the throttle with heel-toe driving, although using the heel of your foot to brake and the tip of your foot to feather the throttle may work, but I'm not going to test it on busy streets.

 

[Race Idiot]

although using the heel of your foot to brake and the tip of your foot to feather the throttle may work, but I'm not going to test it on busy streets.

Thats how I used to do it in my old polo which also had a simmilar pedal setup, actually it's the same with my mums golf as well. Also both cars are a pita to revmatch, probably due to having a fairly heavy flywheel.

Although when I started out driving I used to dowshift without rev-matching while braking because I didn't know any better. Nowadays I can downshift and passengers can't even tell, although it's allot easier in my current car than my old polo. I dunno vw's are just kinda annoying in that regard.

 

[Deathclown66]

compression braking is bad for your engine, anything that resists the engines normally operation is bad for the engine.

Are you saying letting off throttle at 70mph in 5th gear is bad for the engine? Or are you saying constant high-rpm downshifts are bad for the engine?

 

[Scaff]

Are you saying letting off throttle at 70mph in 5th gear is bad for the engine? Or are you saying constant high-rpm downshifts are bad for the engine?

It depends on just how deep you want to look at it.

Technically letting off the throttle at 70mph in 5th is adding additional strain to the engine, so could be described as 'bad' for the engine. We are however talking about a small amount of wear, so a minimal amount of 'bad'.

Obviously repeated down-shifts without rev-matching will cause far more wear and as significantly more 'bad' for the engine.

Regards

Scaff

 

[kennythebomb]

It depends on just how deep you want to look at it.

Technically letting off the throttle at 70mph in 5th is adding additional strain to the engine, so could be described as 'bad' for the engine. We are however talking about a small amount of wear, so a minimal amount of 'bad'.

Obviously repeated down-shifts without rev-matching will cause far more wear and as significantly more 'bad' for the engine.

Regards

Scaff

So what should you do then? Suppose you are on the highway cruising in 5th at 70mph, and up in the distance you see cars slowing down, or you know you will be slowing down soon (off-ramp). Surely starting to brake now would mean you are unnecessarily dropping speed at this point, so I just let off the throttle and then let it wind down some, and since I've learned the technique in the last few days I've been rev-matching when I go down to fourth. And then by the time it winds down in fourth, I'm on the brakes and eventually I pop it in neutral and brake to a complete stop.

If I wanted to completely avoid engine wear, I would just slow down (using the brakes) before my normal braking point?

 

[Scaff]

So what should you do then? Suppose you are on the highway cruising in 5th at 70mph, and up in the distance you see cars slowing down, or you know you will be slowing down soon (off-ramp). Surely starting to brake now would mean you are unnecessarily dropping speed at this point, so I just let off the throttle and then let it wind down some, and since I've learned the technique in the last few days I've been rev-matching when I go down to fourth. And then by the time it winds down in fourth, I'm on the brakes and eventually I pop it in neutral and brake to a complete stop.

If I wanted to completely avoid engine wear, I would just slow down (using the brakes) before my normal braking point?

I didn't say that you should not do it, nor that it could be avoided at all, the question I was asked was in regard to if it is bad for the engine, and technically yes it is, but the additional strain and wear is minimal and also unavoidable.

Stop worrying so much about every form of compression braking, most can't be avoided and some can be useful (steep incline descents, particularly in a 4x4 off road for example) and keep focused on heel and toe and how it can avoid the worst form of compression braking. That which occurs during a downshift.

Im putting together some info in regard how you can find your 'target' rpm for a given speed in a given gear, I hope to have it posted later today/night.

Regards

Scaff

Edited to add

OK the following may be of interest/use to anyone who currently uses heel & toe downshifts or is starting to. One of the biggest problem to start with is knowing how much to blip the throttle to rev match a particular speed and gear. Now while nothing replaces practice and getting a good feel for it (trust me you will know exactly when you get it right), this can help out.

First off we need to calculate a ‘mph per 1,000rpm’ figure for each gear in our car, this does require a bit of research on your chosen car and a small amount of quite straightforward maths.

For example in this I will be using a Toyota Celica (as I happen to currently drive one).

Calculating Overall Gear Ratio
First we need to find the gear ratios and final drive ratios, these can be used to calculate the overall gear ratio. The overall gear ratio for each gear is a rather easy one to work out, simply take the individual gear ratio and multiply it by the final drive ratio (so if 1st gear in a Celica has a ratio of 3.17 and the final drive is 4.31, then 3.17 * 4.31 = 13.66), do this for each gear and you have a full set of overall gear ratios.


Final Drive 4.31:1
1st Gear Ratio 3.17:1 * 4.31 = 13.66
2nd Gear Ratio 1.9:1 * 4.31 = 7.66
3rd Gear Ratio 1.31:1 * 4.31 = 5.65
4th Gear Ratio 1.03:1 * 4.31 = 4.44
5th Gear Ratio 0.86:1 * 4.31 = 3.71
6th Gear Ratio 0.73:1* 4.31 = 3.15


Calculating Tyre Diameter
A little more complicated to calculate than overall gear ratio, but not too difficult at all. You will need the full tyre size of the driven wheels of the car, and it is important to ensure you use the driven wheels.

So the tyre sizes for the Celica are:

Tyres
F 205/50R-16
R 205/50R-16

So using the figures for the front tyres we use the following formula to calculate the tyre diameter.

Tyre Diameter = width(mm) * sidewall height(%) / 25.4 * 2 + wheel diameter

So for the Celica’s 205/50R-16’s that would be

205 * .50 / 25.4 * 2 + 16 = 24.07


Calculating MPH per 1,000rpm
Now that we have the above information we can use the following formula to calculate the MPH per 1,000rpm for each gear.

MPH per 1,000rpm = Tyre Diameter / 336 * 1,000 / Gear Ratio


So for 1st gear in the Celica that would be:

24.07 / 336 * 1,000 / 13.66 = 5.24 mph per 1,000rpm

Taking the rest of the gears we end up with:

1st Gear = 5.24 mph per 1,000rpm
2nd Gear = 9.35 mph per 1,000rpm
3rd Gear = 12.68 mph per 1,000rpm
4th Gear = 16.13 mph per 1,000rpm
5th Gear = 19.31 mph per 1,000rpm
6th Gear = 22.71 mph per 1,000rpm


OK so let’s have a look at how we can use this, on my drive back from work is a nice tight 90degree left hand bend, on approach to the corner I will generally be doing around 60mph and the corner can be taken at around 25mph. So how can the above help us with this.

Well on the approach if I’m at 60mph in 3rd gear the engine will be at around 4,700rpm, I would then brake. Slowing the car to just above 30mph in 3rd (2,400rpm approx); aiming to finish the change at approx 25mph in 2nd gear (3,200rpm). So I depress the clutch and start to change down, blipping the throttle as I do and aiming for around 3,200rpm. Once in 2nd gear I release the clutch, while maintaining steady brake pressure I continue to brake down to the 25mph for the corner.

Now the reason why I don’t aim to finish the braking and gear change at the same time (25mph) is quite simply because I don’t want to be changing gear an trying to turn into a corner at the same time.

A number of reasons exist for this, firstly it gives you an opportunity to use brake modulation to balance the car and ensure you are at the right speed to enter the corner. Ideally you want both hands on the wheel when you enter a corner, having one on the way back from the gear lever is rarely a good idea. Finally it gives you a buffer in case you mess up the heel and toe; a boat load of additional braking is very bad news if you are turning into a corner. Now obviously the amount of braking you need to do after the change will depend on the corner and speeds involved the same goes for the amount of braking required prior to the change. The only way to get used to these is through trial and error.


I do hope that the above is of use or at least interest.

Regards

Scaff

 

[kennythebomb]

I didn't say that you should not do it, nor that it could be avoided at all, the question I was asked was in regard to if it is bad for the engine, and technically yes it is, but the additional strain and wear is minimal and also unavoidable.

Stop worrying so much about every form of compression braking, most can't be avoided and some can be useful (steep incline descents, particularly in a 4x4 off road for example) and keep focused on heel and toe and how it can avoid the worst form of compression braking. That which occurs during a downshift.

Im putting together some info in regard how you can find your 'target' rpm for a given speed in a given gear, I hope to have it posted later today/night.

Regards

Scaff

Oh, okay. Yeah I'm sort of new at the whole manual thing so when someone says 'engine wear' my ears sort of perk up. Thanks for the rational explanation