double clutch thoughts

krd · 06:45 PM

Are there any benefits to double clutching the G50?

TimT · 05-08-2009, 06:27 PM

The 915 is a syncro gearbox..

yelcab1 · 05-08-2009, 07:41 PM

Quote:

Originally Posted by krd

Are there any benefits to double clutching the G50?

Yes, it saves the synchros

Nine17 · Nine17's Garage

Quote:

Originally Posted by krd

Are there any benefits to double clutching the G50?

Always.

SteveinTO · 05-08-2009, 08:59 PM

I think it's faster than waiting for the synchros if you do it right.

rnln · 05-08-2009, 09:04 PM

SteveinTO,
Can you show us how to make doublec clutch faster than waiting for the synchro?
Thanks.

911pcars · 05-08-2009, 09:39 PM

Quote:

Originally Posted by rnln

SteveinTO,
Can you show us how to make doublec clutch faster than waiting for the synchro?
Thanks.

You may have to wait awhile until the synchros wear out to see the difference. If you don't double clutch, you'll won't have to wait as long to compare the difference. However, if you double clutch, synchro life will be extended.

Sherwood

rnln · 05-08-2009, 11:38 PM

Sherwood,
Specially when it's cold, shifting between 1st and 2nd gear will grind or at least hard (my car). I usually pause at nuetral until the rpm drop to 1.5k or 2k before I shift it in. I tried double clutch before and it works too. I mean both ways work but of course double clutching is not as easy as pause to me. According to Steve, he said double clutch is faster if you do it right. If that is the case, I would like to learn whatever to make it faster.

slodave · slodave's Garage

It took me a little time to learn to double shift, but now I can't stop. It's become second nature. I really try not to down shift into 1st, but when I need to and I'm rolling, double clutch makes it slip right in. I occasionally will double clutch on an upshift as well.

Noah930 · Noah930's Garage

Quote:

Originally Posted by krd

Are there any benefits to double clutching the G50?

Synchros or not, isn't there some sort of satisfaction you gain from knowing how to drive a crashbox properly?

rnln · 05-09-2009, 02:39 AM

huh? double clutch and pause at nuetral are to help the problem of grinding and hardshift when upshift only. Our cars don't have problem with downshifting, worn synchro, as far as I know. Am I correct?

Quote:

Originally Posted by slodave

It took me a little time to learn to double shift, but now I can't stop. It's become second nature. I really try not to down shift into 1st, but when I need to and I'm rolling, double clutch makes it slip right in. I occasionally will double clutch on an upshift as well.

javadog · 05-09-2009, 03:22 AM

No. Double (de-) clutching shifts are more useful when downshifting. On an upshift, a pause in neutral, or more accurately a limit to the pressure exerted on the synchro when engaging the next gear, is all that is needed.

It's easier to demonstrate then to explain.

JR

Quicksilver · 05-09-2009, 05:17 AM

The only easy way to show it to someone is to get them in a car that has a trans with complete toast for syncros. On an up shift getting the gear is a matter of timing and/or listening to engine speed. The speed of most people's clutch foot and how fast the engine's RPMs drop combine to make double clutching a pretty useless ritual. If you have a massively heavy flywheel then that makes for a different story but it still isn't required.

When downshifting with the same 'toasty' syncro combination you will never get it to downshift without double clutching and a very accurate throttle blip.

The real problem is double clutching will never work right if you have to think about it. Get a crappy car (preferably with a box full of marbles for a tranny) and practice until your brain is no longer involved.
(Here is where being so cheap has paid off!!!)

yelcab1 · 05-09-2009, 05:17 AM

It is even more satisfying when double clutching goes with heel and toe shifting on the down shift before the corner comes up

alniki · 07:56 AM

I presume double-clutching is meant to match the engine rpm to the rpm of the transmission gears after you change gear, no matter up- or downshift.

When upshifting, with the accelerator decompressed and the clutch disengaged, the rpm of the transmission gears decrease as you shift into a higher gear with the mph hardly changing while the rpm of the engine/clutch/trans input shaft also decrease. So there usually is little or no noticeable jerk when you engage the clutch to connect the engine and transmission.

When downshifting, the situation is quite different because while the rpm of the engine/clutch/trans input shaft will still decrease as mentioned above, the the rpm of the transmission will increase as you shift into a lower gear with the mph hardly changing. This would make a heavy jerk when you engage the clutch to connect a low-rpm engine flywheel/clutch/input shaft with high-rpm transmission gears.

To prevent the downshift jerk, you cant just wait for the rpm of the transmission gears to decrease as the car slow down at neutral or with clutch disengaged, usually with the throttle closed. Or if you don't want to lose speed, as in racing, you can pause at neutral, release the clutch, blip the accelerator to increase the rpm of the engine flywheel/clutch/input shaft, then quickly de-clutch to shift from neutral to a lower gear and release the clutch to engage before the rpm of the engine flywheel/clutch/input shaft drop again.

Correct me if I'm wrong.

Quicksilver put it just right: The real problem is double clutching will never work right if you have to think about it. .....and practice until your brain is no longer involved.

javadog · 05-09-2009, 08:22 AM

Quote:

Originally Posted by alniki

I presume double-clutching is meant to match the engine rpm to the rpm of the transmission gears after you change gear, no matter up- or downshift.Correct me if I'm wrong.

You are trying to match the speeds of some of the parts in the gear clusters on the input and output shaft of the transmission. In a broader sense, on a downshift, you are raising the engine speed to the level it will be when the next lower gear has been engaged. On an upshift, this isn't necessary, as the parts in question are naturally wanting to slow down anyway when the clutch disengages.

JR

dentist90 · dentist90's Garage

On an upshift, not only is a throttle blip unnecessary it is counter productive. You want the input shaft to spin down to match the rotation of the output shaft in the next higher gear. Double clutching is only purposeful on a downshift, and it takes some practice to get it down right, especially when you're braking into a corner. Much easier to practice it when you're slowing down in a straight line , say as you approach an upcoming stop sign.

911pcars · 05-09-2009, 09:16 AM

Quote:

Originally Posted by javadog

You are trying to match the speeds of some of the parts in the gear clusters on the input and output shaft of the transmission. In a broader sense, on a downshift, you are raising the engine speed to the level it will be when the next lower gear has been engaged. On an upshift, this isn't necessary, as the parts in question are naturally wanting to slow down anyway when the clutch disengages.

JR

Partially exact. Double clutching is the only way to also bring the gear-to-select assembly up to the same speed as the engine to match the rotational speed of the output shaft (differential/drive axles). And that is most needed for downshifting to reduce wear on the synchros.

I couldn't find a suitable video demo on YouTube. The double-clutch demo videos with race cars were performed at race speeds and thus difficult to follow. Slo-mo would have been better. The other videos don't provide a clear view of the shift lever position during the double clutch maneuver nor the sound of the engine when the throttle is blipped momentarily.

Sherwood

javadog · 05-09-2009, 10:33 AM

Quote:

Originally Posted by 911pcars

Partially exact.

Sherwood

Partially exact? You know, I've answered this question so many times over the years that each time I answer it now, I use less and less detail, as I type with just a couple fingers and I'm not willing to type long paragraphs anymore. Id be happy to name the parts involved but few people would recognize the names.

Transmissions are very poorly understood by many people. Someone needs to do a decent animation of how a modern-ish gearbox works.

JR

SteveinTO · 05-09-2009, 08:35 PM

I doubt I have anything new to offer in the explanation of double-clutch down-shifting. It just always has seemed to me if you have a big jump, like 5th to 3rd, and you're whistling into a corner, the synchros take forever to match things up.

Back in my Formula Fords days a 4th to 1st downshift was SOP at Riverside Turn 6, generally while turning and braking. The Hewland 4-speed had no synchros because they were slow and complex (the synchros). Simple dog rings engaged the gears and you didn't use the clutch. As you brake with the ball of your foot, you bumped the lever into neutral, whang the throttle and push it into 1st. No clutch. Kind of like throwing a light switch. No drama either, when done correctly, the engine just changes pitch. That feeling (from 40 years ago) is the standard by which I judge, unfortunately.

Our Porsches are a little different, but I think in most cases you can out shift the synchros. 'Course I could be a "cult of one" here, I've never driven one of our cars when it was new.