BMW M5 Forum and M6 Forums banner

1 - 17 of 17 Posts

·
Registered
Joined
·
136 Posts
Discussion Starter #1
I watched Gregs driving school and I think it were really good information to know.

It was something that I didn´t understood and that was the dubble clutch downshift. Why do you use that style? Why go two times on the clutch? I remebered that Greg talked something about the synconices too. Can somebody explain to me why you use doubble clutch downshift?
 

·
Registered
Joined
·
1,625 Posts
Double clutch downshifting was a common technique used on older cars, when the clutch/transmission setups were different, or not as robust. In modern cars, a simple rev match using a heal and toe technique achieves the exact same result. I beleive the only reason to double clutch cars these days is for fun, rather than it's affect.

Anybody else want to chime in? Please do correct me if I am wrong.

Travis
 

·
Registered
Joined
·
590 Posts
M5Kid said:
Double clutch downshifting was a common technique used on older cars, when the clutch/transmission setups were different, or not as robust
I believe this is correct. Originally double clutching was done to make old clunky trannys shift smoother. My father drove a semi for years and he taught me how to double clutch upshifting and downshifting. If you didn't do it the thing would grind like crazy especially in the lower gears. The synchro set ups were not as good as they are today. :noSMG:

Joe
 

·
Registered
Joined
·
5,078 Posts
M5Kid said:
Double clutch downshifting was a common technique used on older cars, when the clutch/transmission setups were different, or not as robust. In modern cars, a simple rev match using a heal and toe technique achieves the exact same result. I beleive the only reason to double clutch cars these days is for fun, rather than it's affect.

Anybody else want to chime in? Please do correct me if I am wrong.

Travis
I agree, that sounds correct to me:wroom:
 

·
Registered
Joined
·
3,469 Posts
Hi,

That's a very good question.

In olden days, before the invention of synchro-mesh gears, you'd grind the gears something fierce on down-shifts unless the engine speed and the transmission speeds are substentially similar. When you down-shift, the engine speed tails off (throttle off), but the wheel speed is just a hair less then when you declutch. So if you shift into the lower gear without first raising the engine speed, the speed mis-match will cause a lot of gear grinds and jerky motion. This is exascerbated (sp?) as the lower gear requires an even higher engine speed for the same wheel speed.

In the double-clutch down-shift, you shift into neutral, let the clutch out, blip the throttle to reset the transmission gear speed (they only reset when in neutral and the clutch is out). At the same time, that raises the engine speed such that when you release the cluctch after the down-shift, the engine and the transmission gears are both spinning at similar speed, minimizing gear grind. That's why we call that 'Rev match' -matching the revolutions (engine and wheel).

With the newer manual transmissions, there is something called a synchro gear (or synchro-mesh) in the transmission/clutch assembly. The synchro is a cone-shaped-teeth gear set, so when you let off the clutch, and the engine/transmission come together and re-engage, the cone-shape teeth gearing allows them to slide in place and mesh together (thus synchro-mesh) much easier, with much less grind. So technically, with the newer manual transmissions, you do not have to double clutch, or even rev match, to down-shift, and your clutch/transmission will last a long time.

However, rev match gives you a smooth shift as the engine and transmission are spinning at similar speed, thus no component will drag on the other when they re-engage, producing a jerky clutch engagement.

We enjoy the doube clutch down shifts because the sound of the throttle blip is most heavenly.

Hope this helps a little.

CP
 

·
Registered
Joined
·
1,625 Posts
CP,

Great explanation, and it sets my understanding in stone on the double clutch and it's purpose. Although I'm not quite at the level of double clutching, I do make it a habit to rev-match using the heal and toe technique. When I get it just right, it's quite a pleasing feeling.

Travis

P.S. When are you going to make it up to the Northwest again, man? I know a few of us who would love to get together for lunch and car talk!
 

·
Registered
Joined
·
3,469 Posts
M5Kid said:
CP,

Great explanation, and it sets my understanding in stone on the double clutch and it's purpose. Although I'm not quite at the level of double clutching, I do make it a habit to rev-match using the heal and toe technique. When I get it just right, it's quite a pleasing feeling.

Travis

P.S. When are you going to make it up to the Northwest again, man? I know a few of us who would love to get together for lunch and car talk!

Travis,

Thanks for the kind words. Believe it or not, I rarely heel-toe with my street driving. Here's why. Heel-toe says you're working the brake/throttle almost simultaneousely. You do that when you're scrubbing off speed in a big hurry, and down-shifting while you're slowing down. I'm a rather leisurely driver on the street. I leave plenty of time for engine braking, and actually rarely use my brakes, virtually never on a fast-stop.

As a result, my Lexus SC300 has 125k miles, and I'm still with the original brake/rotors with 50%+ left. :eek: :eek: As such, I virtually never do heel/toe on the streets.

As for a trip north, no plan as of right now. If I do, will give everyone plenty of notice. We can talk car anytime on the board here though.

CP
 

·
Registered
Joined
·
2,564 Posts
As I stated in the "cant drive stick" thread, the other good reason to double clutch is to speed up the downshift process. A few cars I have driven will go into gear faster with a proper double clutch downshift than a simple rev match, due to slower snychros.
As an example, my Jetta or my buds 350Z has a slow gear change on downshifts. Putting the strain on the synchros during the downshift requires quite a bit of spool up time, hence you spend a lot of time with the shifter at the gate going into the lower gear. Double clutch downshift however spins the input shaft of the tranny, rev matching the internals of the tranny to the engine speed. That way when the clutch goes in the second time, the gear is at the right speed, there is no work for the synchro to do, and the gear meshes in with no hesitation. Works really well for 1st gear, and if done right the gear will slot in like the car is off, or the internals have stopped, like when you move the shifter around at a dead stop with the clutch in.
One point of clarification though. I hear around the board the term (to use you Travis as an example if you dont mind) "I rev match using the heel and toe technique..." I know I am probably restating the obvious, but those are two different things. Rev match downshifting is one thing, and then incorporating brake application makes it a heel and toe. Heel and toe by it's very nature implies rev matching. You can rev match downshift without a brake application, and that is why I diagree with using the term heel and toe as the describing technique for rev matching. Heel and toeing is a step further than a rev match as it. Double clutch heel and toeing needs to be pointed out though, as not all heel and toes imply a double clutch.
I double clutch downshift and heel and toe countless times a day. It is not my day to drive tomorrow, otherwise I would count how many double clutch downshifts I do, and how many heel and toes. I will disagree with you Pong, as you dont need to be scrubbing off large amounts of speed to heel and toe. I heel and toe down to stoplights that I know are going to change, I heel and toe from 3rd to second around slow corners in my area. It all comes down to being in the right gear at the right time, and although you absolutely do not need to heel and toe to do that, it works in that application too. Before I started to heel and toe I just rev matched before a turn, and it worked well. Having a car that you need to wring the piss out of to get it to move has made me keep the car in the right gear all the time.
:cheers:
 

·
Registered
Joined
·
5,239 Posts
Couple of clarifications -

1) You can blip to rev-match whether you double- or single-clutch downshift

2) Double clutching IS relatively uneeded in a car with great synchros - however as the gears get lower, the speed differential between gears is greater. A 2-1 downshift, for example, is really hard on the synchros - and you can feel that through the amount of pressure you need on the gearshift lever, and how long it takes. Double-clutching "pre-syncs" the new gear so the synchros have less work to do (or, if you do it perfectly, no work at all). A 3-2 from high revs still seems easier with double-clutching. But a 4-3 or above - I never bother.

I can double-clutch downshift about as fast as I can single-clutch, probably faster for a 2-1- but I do it out of habit. I don't do it all the time. It DOES save wear-and-tear on the synchros, but it adds some wear to the pressure plate springs and other clutch-activation-related bits.
 

·
Registered
Joined
·
2,564 Posts
greg said:
1) You can blip to rev-match whether you double- or single-clutch downshift
Exactly. If you dont blip, then you have done nothing, and you have executed a standard drag the rpms up with the clutch style downshift.
I just was a little concerned with the way in which the technique was stated. To put it in other terms, you dont hammer with a cabinet building technique, rather building cabinets implies the first part.
:cheers:
 

·
Registered
Joined
·
113 Posts
Are there tables to show the relationship between gears and rpm when downshifting? For instance if you are in 3rd gear at 3000 rpm and you down shift to second gear what rpm will you end up at after the shift. I want to practice rev matching so having a table will help me match the revs when shifting between gears.
 

·
Registered
Joined
·
214 Posts
That brings up a question for moi...is it easier..or does it happen more often...to "smoke" a clutch downshifting without proper rev match than to "upshift" and "pop" the clutch?

I ask..humbly...because I downshifted the other day-without proper rev matching-and now it seems that the clutch is slipping. Of course it's been cool here lately and I can't tell if the tires are spinning (DSC off) or if I have to beg Daniel for a coupon..

Thanks all.
 

·
Registered
Joined
·
2,564 Posts
Usually slipping a clutch on an upshift will murder it fast than downshifting. If you arent applying any gas but downshift without bliping, there is a lot smaller load taken up by the clutch than pounding out a powershift. I am yet to see normal downshifting fry a clutch, but with the E39's weak clutch I guess anything is possible.
Tom- I dont know if anyone has made a table for this, but what I recomend to those who I teach is pick a downshift. Say you want to start learning, so start by downshifting from 4th to 3rd. Go 40 mph, and memorize the engine speed in third before you upshift to 4th. Once you are in 4th, clutch in, select the lower gear, blip the rpms to where they were in third, and then let the clutch out. Once you get that down, blip while you are selecting 3rd to speed up the process. Once you are good at getting it from the same speed every time, try different speed and gear combinations. After awhile, the bliping will become instinctive and you can go from car to car and get it right most of the time.
:cheers:
 

·
Registered
Joined
·
3,469 Posts
Also, when matching revs, we are not saying identical RPM. I'd say if you're within 500 RPM, you're close enough.

Bren, on scrubbing speed doing heel/toe, I can't 'feather' the brake AND roll my foot to blip the throttle, as I'm using the ball of my foot as pivot. That means I'm pressing pretty hard on the brake, thus braking hard. With my normal driving style, I rarely brake that hard on the street.

CP
 

·
Registered
Joined
·
2,564 Posts
Fair enough Pong, looks like we have different styles. I pretty much brake with my big toe and the left side of the foot, and rock the right side of my foot onto the gas. I adopted that style so slower speed footwork would be easier. I agree, as I have tried it in the style that you use, and the only way I can make that work is when I am almost in the ABS or further.
:cheers:
 

·
Registered
Joined
·
296 Posts
For some reason, I find it much easier to exactly match the revs in a 3-2 downshift when I double-clutch. So around town, I find myself doing this very often. It just feels nice and smooth.

Best,
Bert
 
1 - 17 of 17 Posts
Top