Some people just clutch-in and don't downshift when approaching a redlight or stop sign, but I love downshifting my car. I always try to rev-match (and never skip gears) so the synchros don't have to strain to raise the engine speed by themselves, but I don't always get it right. What I'm noticing is that if I don't blip hard enough, I can feel the car jerk/buck a little bit when the clutch it let out and the synchros kick in to do their job. Even if I'm short by a small amount such as 200 - 300 rpm, I can feel a little jerk and I know the car is telling me that I was close but no cigar. Nothing violent, but you feel it.

However, if I overshoot the mark even by as much as 1k rpm, I don't feel a buck. The revs just drop very quickly to the correct rpm for the lower gear when I let the clutch out and it feels ok. Am I correct in assuming there's more strain on my synchros and drivetrain when I don't blip the throttle enough as opposed when I do it a little too much? Can anyone explain why? Is it because once the revs reach the peak for that blip, they begin coming down and since the wheels are also slowing, it's easier for synchros to "pull" the rpm down to the correct point once the clutch is engaged? The car certainly seems to react worse when the revs are still too low for the gear I downshift to.

When I downshift, I clutch-in, blip as I'm moving the lever, and let the clutch out at the same speed that I let it out on an average upshift. Should I be engaging the clutch a little slower on a downchange or is this ok? While doing this, I hardly look at my tach--I do it all by ear and just kinda estimate how much to blip and when exactly to let out the clutch. I don't see how you can be exact all the time? You can definitely get close once you're familiar with the car and ratios, but even the best drivers miss the mark sometimes right?

At this point, I almost always downshift into 5th, 4th, and 3rd. I can do all of these pretty smoothly I'd say 9/10 times. 2nd is harder because there's a bigger difference in ratios but I'm practicing. I never downshift into 1st when already moving.

Just want to make sure I'm doing this properly and am being as kind to my drivetrain as I can before I start practicing heel-toe. Any insight appreciated!

 

I will provide my personal opinion on this, keep in mind everyone will have their own particular opinion, so take it for what it's worth.

There will always be a perfect rpm where the gears and syncrhros match up perfect, it will take some experimentation to find out what that rpm is for each gearchange.

What you should be doing is double clutch downshifts to help get a smoother shift. If you do this right it will put less strain on the synchros on downshifts. What you need to do is clutch in put shifter in neutral position, let clutch out, blip throttle with a good jab, then depress clutch and move shifter into lower gear of your choice with a bit of throttle while rev matching a tad. It takes some practice to do it quick enough but if you get it right it is a very smooth downshift.

My personal opinion with respect to downshifting as you approach stop signs and stop lights etc... , is that it is cheaper to replace brake pads than it is to repair a tranny and clutch. I am one of those guys who lets off the gas as I approach a slow down condition, then put it in neutral as I coast to a stop. I don't downshift approaching a stop unless I expect to re-accelerate before I arrive at a complete stop. Just my thoughts. hth.

 

Too much information alert:

JFUSION is basically right, that you need to double-clutch to completely avoid the clunky feeling you are describing. However, it's not the end of the world if you don't. Synchros mostly exist so that you can single-clutch downshift without grinding gears. It's just a matter of whether you think it is necessary to save wear and tear on them. Most people don't.

Here's WHY: The clunk is coming from the input shaft not being "rev-matched" with the output shaft and gear you are trying to select unless you double-clutch.

When you single-clutch downshift, you are rev-matching the ENGINE, but between the clutch and the gearbox is a shaft. Because you have the clutch in and the transmission in neutral, that shaft isn't connected to anything and is slowing down due to friction.

For upshifts, this would be totally fine because you want the input shaft to slow down for the next gear anyway. That's why it doesn't usually clunk, because everything stays pretty well matched up in terms of speed.

But for downshifts, you want the input shaft to SPEED UP. Since it needs to speed up but is in fact slowing down, when you engage the lower gear, you are asking the sychros to do a lot of work! They have to accelerate that input shaft a lot to get the gear to engage. That's way more work than they do on a regular upshift, and that's why you get the clunk as they engage. On a regular upshift, the synchros might adjust a few hundred RPM or so as you engage. On a downshift, the sychros might need to adjust the shaft by 3000 RPM!

Double-clutching makes things better because when you rev-match, you do it with engine and the input shaft connected (clutch pedal out, gear in neutral), so they BOTH get rev-matched to the higher speed. Then you can clutch in and engage the gear easily. The synchros only have to adjust a few hundred RPM again, and you don't get a clunk.

This is the same thing that happens when people clutch in and move the gear into neutral while still approaching a stop, then get a clunk when they put it in first. They left that input shaft spinning around like crazy and then tried to connect it to a gear that wasn't moving at all (because they're stopped). The synchros have to do a lot of work to grind that shaft to a stop before the gear can engage. That's why it is smart to leave the car in gear until you reach a complete stop, then move to neutral and first.

 

Wow this topic is very informational, I have the same concerns as the OP. So for double clutching you have to blip in neutral a bit higher or a lot more higher? then put it into gear and gas a little bit more?

 

great info thomsbrain, good reading on your reply.

Yeah actually what I do is keep the car in gear as I decelerate and then put it in neutral as I come close to stopping but not so late that it bogs. After I have completely stopped I will engage first gear. If I have to grab first before stopping to accelerate again I will double clutch to make it a smoother engagement.

Bakalaq, there are two procedures involved one is rev matching and the second is double clutching. The blip that occurs in neutral is different for each vehicle, so you have to experiment a bit to see if requires a little throttle or a lot of throttle. I've only owned my car for one day so I have yet to find the sweet spot on double clutch downshifts.

As thomasbrain has stated double clutch downshifts are not necessary on a modern transmission, I prefer to do it to get a smoother downshift based on my road racing experience. It just feels better to me.

 

These guys aren't double clutching down. 2:41 for example
[media]http://www.youtube.com/watch?v=EwOSeRfYzAI [/media]

 

^damn what happened at the end? i hate how these guys make everything look so easy...but when you try it yourself you're like !@#@!%

 

Those guys dont have to DC downshift since they dont care about wearing out synchros. They most likely keep rebuilding their transmissions.

Without DC downshifting, your synchros WILL wear out faster. Is it bad for the trans? No, as long as you dont grind gears...you just have to get the synchros swapped after you burn em out.

 

If your rev matching is essentially exact and the whole transition is smooth, is there really much accelerated wear on the syncros? Or is it negligible?

 

^If youre not double clutching, the synchros are whats making the input/gear shaft match revs. Basically, the synchros are still taking wear no matter how nicely you "match" the engine to tire speed if you dont DC.

If you DC perfectly, which is highly rare, the synchros can theoretically take no wear.

Dont worry too much though, unless youre totally messing up the RPMs like going 9K, then shifting to a super high gear and dropping the RPM to 2K, the synchros do what they were meant to do - synchronize the input shaft and gear (That you select) so they dont grind and so they mesh properly.