I'm No Racing Driver... but was wondering about downshifting

[jrainlay]

I don't track my Z4MC... but I do drive it very enthusiastically! The recent discussion about optimal engine revs and acceleration shift points caused me to wonder about deceleration and maximum revs when downshifting. I typically use downshifting to slow the car as much as possible and won't use the brakes unless absolutely necessary. When downshifting, I blip the pedal once or twice to bring revs up so there's a smooth transition to the lower gear. However, I'm generally reluctant to downshift if doing so would force the engine to rev higher than 4000-5000 rpms. Just wondering what forum members think about the pros and cons of downshifting and maximum shift point revs when decelerating. Thanks in advance for your responses.

 

[tomscott]

Your brakes are cheaper than a transmission so use the brakes for every day driving, it can cause extra wear to the transmission but as you say you are single clutching (or double clutching) rev matching and changing this reduces the strain and allows you to turn corners much more efficiently.

But I don't know about anyone else but I feel a bit of an idiot coming to a busy junction and revving up to switch down.. Unless your on a hoon! Always single clutch when giving it some!!!

I was taught to drive 7 years ago and they instructed that if you were coming to a junction leave it in the gear you approach in say 3rd 4th or even 5th and use the brakes to slow then clutch in as your coming to a stop and disengage the gear. My generation weren't taught to switch down through the gears toward a junction apparently to reduce wear on the transmission... still have my driving book rated for 2005 that approves this. Although I disagree with this, the car should never coast regardless of how far should always either be on the power or on the brakes. My cuz learned about 4 years after me and they changed this again but not 100% whether it was shift down through the gears or step changes from 4th to 2nd to 1st.

These cars were designed to rev so as long as your not shifting down and hitting the red line then I wouldn't worry too much. Down shifting and hitting anywhere between 4000-5000 seems ok to me maybe even higher, the curves are pretty linear so these cars pull from 2000 all the way up to red line. 80% or torque is available from 2000 revs.

 

[mmm-five]

I'll heel & toe simply to stop the engine braking the car, as it's too easy to mismatch revs and lock up the rear, sending you into a spin.

I tend to brake consistently, and as I'm starting to come off the brake (although not completely off), I heel & toe to change down.

 

[gannet]

tom - Interesting reading that you were taught to brake then change rather than change down through the box That's what the IAM and roadcraft all say and what I've been doing for longer than I care to remember - works very well for me

I do however rev-match on downshifts but only if selecting a lower gear for a hazard while not necessarily needing slower road speed - for instance approaching a sweeping bend on the open road for which I would like a lower gear for more engine response to drive around it. Gives great satisfaction when you get it right

OP - don't think you need worry about bringing revs up above the 4-5k mark, shouldn't be a problem as long as you don't hit the redline

 

[bcworkz]

Depends on why you're downshifting. To accelerate out of a curve or overtake, there's no point shifting into 6k, you'll just have to shift immediately up again. I try to shift down into peak torque or higher in this case.

For slowing to a stop, depends on the car. Engine braking after a good rev match in throttle by wire (electronics, not cable) cars is entirely unsatisfactory. I don't bother, just use the brakes.

For slowing to time a light or stopping in a car with an throttle cable directly attached to my foot, I go to whatever gear will deliver the results I need. I have no problem doing a huge rev match and hitting 6k+. Scares the crap out of pedestrians though. I'm slowing for you. Really, I am.

Just be sure and not go too low and dump the clutch. Your engine will explode, rev limiter or not.

 

[Gargybloke]

Generally speaking, I'm with Tom on this one. I'm 51 and it was my father's generation who were taught to drive the other way so we are going back some to another time doing it the other way....
Hoon to corner, brake very hard, get in correct gear and get the power on smoothly going round and out..mmm...rear wheel drive and LSD

 

[Incompatible]

Generally speaking, I'm with Tom on this one. I'm 51 and it was my father's generation who were taught to drive the other way so we are going back some to another time doing it the other way....
Hoon to corner, brake very hard, get in correct gear and get the power on smoothly going round and out..mmm...rear wheel drive and LSD

Use your brakes to stop as they are much cheaper then the wear and tear or damage to the transmission. No problem with engine breaking using the gear you're in, but no need to downshift. The purpose of down or up shifting is so you can be in the correct gear for the rpm range.

 

[jrainlay]

Use your brakes to stop as they are much cheaper then the wear and tear or damage to the transmission. No problem with engine breaking using the gear you're in, but no need to downshift. The purpose of down or up shifting is so you can be in the correct gear for the rpm range.

Thanks for all the responses... Obviously there are varying opinions/practices when it comes to deceleration/downshifting/engine braking. I've been driving BMW's since 1970 (yes, I'm an old fart, dating back to the days when BMWs were so rare in the states that owners flashed their headlights at one another), and I've always used downshifting/engine braking as the main method of slowing the car (unless its an emergency stopping situation). That's the way I was taught to drive. Interestingly, I've never experienced any adverse impact from this practice on the drivetrain. Maybe I'm just lucky, but I've never had a serious engine problem, or had to replace a transmission, clutch or brakes on any of the BMWs I've owned. I guess that's why I love the brand!

 

[bcworkz]

Yes brakes are cheaper than clutches. But a proper rev match downshift puts no more wear and tear on the drive line than an upshift, very little. All of the braking energy is provided for free by the engine. (but may be limited as I noted above.) Brakes have to wear to brake, engines much less so. But you MUST do a proper rev match! Some people seem to be incapable of such. People that downshift, then slowly engage the clutch, grinding on it to slow the car, drive me completely bonkers! This is where the brakes are cheaper bit comes from, and if you drive that way, it is completely correct. Don't drive that way!

The other place bad technique can place undue wear on the trans are the synchro cones when shifting in either direction. Syncros are equipped with balk rings to ensure the splines are going close to the same speed before the lever can be placed in the desired position. If you encounter resistance in shifting, it is the balk rings protecting the splines. You can force the issue, the synchro cones will eventually get the splines going the same speed and the balk ring will release. This causes a lot of wear. The proper solution to heavy resistance going into gear is to either change the timing of the shift or possibly double de-clutch so the splines are going the correct speed when you actually shift. Don't force a shift!

Proper technique = low maintenance costs ... Poor technique = high costs. Simples!

 

[BMWZ4MC]

Yes brakes are cheaper than clutches. But a proper rev match downshift puts no more wear and tear on the drive line than an upshift, very little. All of the braking energy is provided for free by the engine. (but may be limited as I noted above.) Brakes have to wear to brake, engines much less so. But you MUST do a proper rev match! Some people seem to be incapable of such. People that downshift, then slowly engage the clutch, grinding on it to slow the car, drive me completely bonkers! This is where the brakes are cheaper bit comes from, and if you drive that way, it is completely correct. Don't drive that way!

The other place bad technique can place undue wear on the trans are the synchro cones when shifting in either direction. Syncros are equipped with balk rings to ensure the splines are going close to the same speed before the lever can be placed in the desired position. If you encounter resistance in shifting, it is the balk rings protecting the splines. You can force the issue, the synchro cones will eventually get the splines going the same speed and the balk ring will release. This causes a lot of wear. The proper solution to heavy resistance going into gear is to either change the timing of the shift or possibly double de-clutch so the splines are going the correct speed when you actually shift. Don't force a shift!

Proper technique = low maintenance costs ... Poor technique = high costs. Simples!

I've found that double declutching has become considerably faster after the CDV delete (really the only benefit of removing this). To double declutch has become so entrenched in my driving style that I struggle not to do so even when driving fairly sedately.

I always rev match, but this is especially important when changing down at speed with any amount of turn on the steering wheel - otherwise the rear wheels will drag and the car may spin out. Actually, I sometime under match the revs to induce a little rear end slip whilst turning into a corner in my 600kg Westfield, then wallop on the power at the apex to keep this drift through the exit. Occasionally this ends in smoke and unplanned doughnuts

With double declutching and accuate rev matching it is possible to perform seriously aggressive down changes. The lower gear can be entered at very high RPMs without the problem of balking that bcworkz has decribed so well, or causing any more stress to the transmission than there would be at that RPM whilst accelerating. The consequent engine braking can be used to supplement the brakes, leading to less heat being created in the brakes, as well as ensuring that the engine is in the desirable rev range for accelerating out of the corner.

I have found that driving like this up to traffic lights does upset the pedestrains on Oxford Street though

Of course if you double declutch down into too low a gear, when you engage the clutch nothing will stop the engine from spinning up to beyond the 8,000 rpm redline and the likely result will be one dead engine +/- one big spin as the rear wheels drag.

 

[ga41]

I'll heel & toe simply to stop the engine braking the car, as it's too easy to mismatch revs and lock up the rear, sending you into a spin.

I tend to brake consistently, and as I'm starting to come off the brake (although not completely off), I heel & toe to change down.

Interesting, i usually heel-toe in the beginning of my braking to let the engine add to the braking more. I usually do short and sharp braking, i think i need to finesse my technique more.

 

[cj_eds]

I tend to brake consistently, and as I'm starting to come off the brake (although not completely off), I heel & toe to change down.

When I drive manuals, I'm with m5 on this one but without the heel & toe. Smooth braking (smooth & aggressive if hooning) then once I'm towards or at the end of the braking do the gear shift. All done before turn-in so that I can power on through the bend. I try to already be on the accelerator by the time I hit an apex.

In the auto, I just stomp the brake then mash the accelerator.

 

[Stromtech]

We have brakes?

I'm in the downshifting crowd. If I wanted to use brakes to slow down I would have bought an automatic transmission.