rev match practise help

[yli]

Hello, everyone,

I have searched previous posts concerning this topic but still cannot find a satisfactory explanation that can clear up all my doubts. So I have to bother the experts here again and hope you can help me out of the trouble.

Over the past few days I practiced rev match when downshifting by double declutch or another way: clutch in during the whole downshift (to make things convenient I would call it single declutch). In overall the results were not satisfactory. Two or three out of ten time I got it right and the downshift was smooth without any jerk or lurch of my car. But the rest of the time I just could not do it properly. Sometimes I blipped the throttle too much and moved the gear level towards the lower gear too quickly with a result of too high revs for the lower gear. And sometimes things just went into the contrary in which case the rev fell too much.

I know I need more practice to do it properly and I will seek your advice on it later. But before I do that I want to discuss this double declutch and single declutch thing that have been bugging me for quite a while.

1, double declutch. Inevitably it is directly connected to the synchro-mesh gears (I only talk about the synchro-mesh gear box. And for those who like me is a newbie to the advanced driving world, here is a good depict of how it works: www.howstuffworks.com/transmission3.htm). I have read numerous online posts suggesting that double declutch help to reduce the synchro-mesh gears wearing by manually matching input shafts speed with the output shaft speed, which seems sensible to me. However some indicate that double declutch actually increase synchro-mesh gears wearing, like this one: www.singster.co.uk/mascot%20mar-apr%202003%20(4).html, which I don’t quite understand. Also some say that double declutch increase the risk of mismatching the input shaft and the output shaft speed because you declutch twice for which I cannot see any connection between declutch and mismatch the input and out shaft speed. So what is exactly going on with this double declutch in relation to mechanical sympathy with synchro- mesh gears?

2, single declutch. Yesterday when I was reading a previous post someone suggested the sequence of doing it: clutch in, into neutral, engage the lower gear, rev match by blipping the throttle and clutch out. I had always thought it should be done by blipping the throttle first and engaging the lower gear after. So which one is correct? Or does it make any difference at all no matter which way I do it? My own thought is that probably the former one is more mechanical sympathetic for the synchro mesh gears. By engaging the lower gear first actually reduces the speed difference between the input shaft of gear box and the output shaft. For instance, in third gear, the input shaft might rotate at 1500rpm and the output shaft speed might be 1200rpm. Now planning to downshift into second gear. The input shaft might need to rotate at 2200rpm to adapt to the output shaft speed. If I engage second gear first while the clutch is in(at this point the input shaft speed might drop to 900rpm because of the disconnection with the engine), the input shaft only needs to increase 1300rpm. However if I blip first, the input shaft might already drop to 800rpm (since clutch is in, the input shaft disconnect with the engine and therefore its speed drops further because of the time consumed by blipping and the disconnection with the engine), which means the input shaft has to be increased by 1400rpm(compare to 1300rpm in the former method) to match the output shaft once the gear shift is completed. This, I think, cause more wearing on the synchro mesh gears. But the conventional way is to blip the throttle first, I think. It must have its own advantages. And I am just a newbie to this advanced driving world, correct me please if I got things wrong.

3, practice the rev match. I found that delaying the blip can make things easier. So I guess timing the blip and engage the lower gear properly might be the key part of it. I am wondering how the experts here think of it. And any advice on how to practice is appreciated.

Thanks for reading such a long post. I hope I do not drive you crazy partly because of the technical staff and partly because of my English. And have a nice day.

Li

[thegman]

Interesting. i have wondered the same things.

To me the following method:

- Clutch in
- Into gear
- Rev match
- Clutch out

Seems redundant - the whole point of rev matching is to remove the requirement to use the clutch??? I have been trying to following combo:

- clutch in
- blip throttle
- Into gear
- clutch out

obvious problem is that this requires a very swift change before your revs begin to fall. my car has a very small flywheel which makes this harder.

any help much appreciated.

[R_U_LOCAL]

To avoid repeating myself, most of your questions should be covered in my post on gearchanging...

www.pistonheads.com/gassing/topic.asp?h=0&t=361550

Have a read through it and if there's anything about it you don't understand, or that you feel needs clarifying, then ask away.

[bor]

If you want to select a lower gear at a constant road speed (ie not when you are braking), then there is a far easier way:

1.Keep accelerator pedal where it is - ie do not release it or blip it.
2.Depress clutch as normal
3.Move gear lever to lower gear. You need to experiment with how fast you do this, not too fast, not too slow.
4.Engage clutch as normal.

If you are decelerating on a closed throttle, then you need a blip, but forget about double de-clutching until you can get the blip right.

[GreenV8S]

My £0.02:

Double declutching is completely irrelevant if you have a modern car in good working order.

Matching revs on downshifts is far more important on rwd cars than fwd cars, and more important on more powerful cars. On powerful rwd cars it can be extremely important. On weedy fwd cars it is a matter of elegance rather than necessity. Do you really need to do it at all? As an alternative you can use the clutch to pull the revs up smoothly and progressively without upsetting the car (although of course it's possible to upset the car if you do it roughly), if you do it right then neither you nor your passengers will know exactly when you started engaging the clutch, or when you finished.

If you don't overlap braking with the gear change then matching revs is just a matter of experience to judge what revs you need, and practice to learn what you need to do to the throttle pedal to get those revs. The slower you do this i.e. the more time you spend in the gear change the easier it is. In general it's not a good idea to spend a long time in the gear change but while you're practising matching revs you will find it helps to allow a lot more time than normal. If you do overlap braking/gear change then the same applies, but the mechanics of applying the throttle and brakes simultaneously make it a lot harder to learn.

Edited by GreenV8S on Saturday 5th May 12:24

[WeirdNeville]

yli said:

3, practice the rev match. I found that delaying the blip can make things easier. So I guess timing the blip and engage the lower gear properly might be the key part of it. I am wondering how the experts here think of it. And any advice on how to practice is appreciated.

You've hit the nail on the head there I think. Rev matching is practice, practice, practice and it depends what car you're in as to what works.

It sounds like you might be over-analysisng it a bit whilst you're practicing. Whenever I start to think too much I tend to screw stuff up. Relax and go with it. What feels right probably is.

For reference the way to do it is (for a downshift):
Foot on clutch, Gearstick to neutral, Now lift the revs + hold them where you think they should be, Gearstick to new gear, Release clutch smoothly.

Find a nice straight bit of road, perhaps one where your car can be in 2nd, 3rd or 4th and practice changing gear but keeping speed constant rev-matching every change. Try 4th->2nd and use a big rev!

The 1.4 focus I used to drive at work needed loads of warning to change revs, I thnk because it had a heavy flywheel to mask it's lack of torque. On a spirited change from ~5000 rpm in 1st you had to wait nearly 3 seconds to let the revs die to a point where you could engage 2nd without a jolt. By comparison my own car revs much more freely, and you can almost 'blip' the throttle on sequential downshifts. 5th->2nd still needs a good boot on the throttle though!

I've also found diesels very good for practicing rev-matching, because they're very harsh when you get it wrong and you need to send the engine an email as warning to change speed it slows the whole process down.

Read RUL's link above, practice a bit and you'll soon get the hang of it. Then change cars and you'll have to learn again!

[yli]

Thank all of you for your input. If I understand all the replies correctly it seems that a sustained rev match method should work better. I will put the method into practice to see what happens.

R_U_LOCAL, that is a very good post. I first read it before I practised the rev match on my car and thought nothing really special at that time. But after I have some practise experience and re-read it again make me able to appreciate the points made by you and other fellow posters. Now I can compare my own practice with other people's successful method in detail and improve my own skill.

But I still feel some questions were not fully addressed regarding the synchromesh gearbox. The following is quoted from your post:

R_U_LOCAL said:

[quote=AL666]Thanks for the great post(s) Reg, I was just wondering if whilst changing gears smoothly, the revs should be raised/lowered whilst the gearstick is in the neutral position, when the clutch is down, or if it can be done in either gear when the clutch is down?

Cheers,

AL.

The revs should be matched whilst you're changing gear. There isn't really one exact point where you should do it (although this isn't the case with non-synchro gearboxes, where the revs should be matched when they're in neutral as part of the double de-clutching, but you'd only find those in very old cars or some competition cars, so it's not very relevant).

Also GreenV8S suggested that,as I quote," Double declutching is completely irrelevant if you have a modern car in good working order".

Just out of curiosity, I am wondering if 1) double declutch does little or no good to the synchromesh;2)revs matched when gear lever is in neutral gives any advantage over gear lever is engaged with respect to the synchromesh wear. I would not care about it when I am driving as long as I can shift gear smoothly. But I do hope I could clarify it theoretically.

GreenV8S, my car is front wheel drive. I try to learn rev match in order to avoid excessive clutch wear as suggested by lots of people here. I don't quite understand your "As an alternative you can use the clutch to pull the revs up smoothly and progressively without upsetting the car". Isn't this going to cause excessive clutch wear? or I just get you wrong? A little bit confused.

Again thank everyone for your advice. I will apply them into my practice and hope things could get better.

Li

[GreenV8S]

yli said:

GreenV8S, my car is front wheel drive. I try to learn rev match in order to avoid excessive clutch wear as suggested by lots of people here. I don't quite understand your "As an alternative you can use the clutch to pull the revs up smoothly and progressively without upsetting the car". Isn't this going to cause excessive clutch wear? or I just get you wrong? A little bit confused.

It rather depends what you consider 'excessive'. The wear incurred by using the clutch to pull the revs up during a normal downshift is miniscule compared to the wear incurred by ordinary driving such as pulling away from standstill; the torque required is small and the speed difference is also small. I think you might as well worry whether blipping the throttle is going to cause excessive wear to the throttle cable. The level of wear in either case is so small imo to be negligeable in the scheme of things. We don't blip the throttle to save clutch wear, we do it because it reduces the disturbance to the driven wheels. But in circumstances where that disturbance is also negligible, why bother?