I've got a 2000 F-150 XLT 4x4 ORP with a 3.55 LS rear end. I'm curious as to how to calculate rev-matching. I think just knowing the gear ratios from 1st to 5th I could figure it out, but I need to know initial rev match into 2nd to get a baseline to go up from....

 

Let me be the first to ask...

...WTF is Rev-Matching?

 

Y2K 7700 4x4

Actually, you are the right one for me to ask, I remember reading you explaining it in concept to somebody who asked, so you know what it is, I guess I just wasn't clear....

In the old days before synchs you had to double clutch...Typically to make a smooth transmission (parden the pun) you had to go in first, put in the clutch, pull into neutral, tap the accelerate to a certain RPM which would get the input shaft and cluster gears spinning up to a certain speed, then put in the clutch, then shift into second gear, which was spinning at a different speed that first gear, thus you had to match to the revs of the gear you were going to, or "rev-matching"....As I'm sure you know, that's what makes clutchless shifting possible, which I would never try. However, rev-matching still is a little useful even with synchs as it makes the job of the synch easier as it doesn't have as much work to do..

Anyway, I was just thinking about how you would compute the required RPM to match, and figured some of the experts here would know...and now that you mention it I recall you describing the process to someone...so what is the answer?

 

If you pay attention to the tach when you shift you'll get an Idea of the difference in RPMs. If you upshift from 1st to 2nd and the tach shows 3500 RPMs before you shift and 2800 after you engage the clutch for second gear, then you have a difference of 700 RPMs between those two gears. It is not usually neccessary to match an upshift as the RPMs drop naturally when the accelerator is released.

Transeversely, if you want to downshift smoothly from 2nd back to 1st you should raise the RPMS 700 revs more than what the tach is showing after depressing the clutch but before you downshift and engage the clutch to effectively match your shift. The order of events for a double clutch downshift would be as follows - observe RPMs, depress clutch, shift into neutral, release clutch, goose accelerator pedal to match RPMs apropriately, depress clutch, shift to next lowest gear, and release clutch. It sounds like a long process when typed out like this but it all happens quickly.

You can still match your shifts without double clutching. You can just goose the throttle as you pass through neutral on the shift. This can sometimes eliminate that notchy, or grinding feeling between shifts without actually double clutching.

This is common practice for operating a non-synchronized transmission (AKA crash-gearbox) to avoid grinding gears. Once an operator becomes familiar with the vehicle they almost always are capable of shifting gears without watching the Tach or using the clutch pedal. That is with the exception of 1st gear when used to move from a standstill.

BTW, in tractor trailers the difference in RPMs between the adjacent gears was usually the same from 1st all the way to 10th or13th depending on what tranny the rig was equipped with. I dont know if that is the case with our trucks and it is quite possible that some or all the gears require a different number of RPMs between each of the shifts to match effectively.

Heeheehee, I've run off at the keyboard long enough. I tried to express myself the best I can but if you still have questions type away and I'll try to answer them the best I can.

Good luck.

 

LE PEW

Thanks, I think I understand. One question, the few things I have read on double clutching state the opposite of what you are saying, i.e. the RPMs need to be higher to get into a higher gear. The reason the RPMs drop during normal shift is because the clutch has been engaged and you step off the gas. On manuals, the input shaft and gears are actually the driven gears, while the cluster shaft has the driving gears. So if you were looking up from the sky down onto a manual, first gear would have first gear driving on the left (a small gear) and first gear driven on the right (a big gear). Second gear driving on the left would have a bigger gear than first, and on the right would have a smaller gear that first. Therefore, when you move the hub or synch from first to second, you are moving from a very big gear that is going slow (being driven by a tiny fast spinning gear) to a smaller gear that is moving faster....

Am I incorrect on this? I'll try to find the article I found, but it definitely described double clutching as hitting the throttle to get into second....maybe I'm just confused...

If you pay attention to the tach when you shift you'll get an Idea of the difference in RPMs. If you upshift from 1st to 2nd and the tach shows 3500 RPMs before you shift and 2800 after you engage the clutch for second gear, then you have a difference of 700 RPMs between those two gears. It is not usually neccessary to match an upshift as the RPMs drop naturally when the accelerator is released.

Transeversely, if you want to downshift smoothly from 2nd back to 1st you should raise the RPMS 700 revs more than what the tach is showing after depressing the clutch but before you downshift and engage the clutch to effectively match your shift. The order of events for a double clutch downshift would be as follows - observe RPMs, depress clutch, shift into neutral, release clutch, goose accelerator pedal to match RPMs apropriately, depress clutch, shift to next lowest gear, and release clutch. It sounds like a long process when typed out like this but it all happens quickly.

You can still match your shifts without double clutching. You can just goose the throttle as you pass through neutral on the shift. This can sometimes eliminate that notchy, or grinding feeling between shifts without actually double clutching.

This is common practice for operating a non-synchronized transmission (AKA crash-gearbox) to avoid grinding gears. Once an operator becomes familiar with the vehicle they almost always are capable of shifting gears without watching the Tach or using the clutch pedal. That is with the exception of 1st gear when used to move from a standstill.

BTW, in tractor trailers the difference in RPMs between the adjacent gears was usually the same from 1st all the way to 10th or13th depending on what tranny the rig was equipped with. I dont know if that is the case with our trucks and it is quite possible that some or all the gears require a different number of RPMs between each of the shifts to match effectively.

Heeheehee, I've run off at the keyboard long enough. I tried to express myself the best I can but if you still have questions type away and I'll try to answer them the best I can.

Good luck. [/QUOTE]

 

If you are driving a synchronized trans and shift to a higher gear RPMs drop lower when you engage the clutch. If you downshift a synchronized trans to a lower gear the RPMs rise when you engage the clutch. So it stands to reason that to match the shift properly, the RPMs should fall for an upshift, and rise for a downshift so long as they rise and fall by the same Rpms required to mesh the gears without grinding the cr@p out of them.

It is possible to raise or drop the RPMs too much which will also grind the gears. The Idea is to match the shift by the exact difference in RPMs needed to mesh the gears without grinding them whether it is up or down.

If you were driving certain tractor-trailers it may be necessary to goose the throttle on an upshift because of the big drop in RPMs due to the high compression some of the big Caterpillar and Cummins engines have. Especially those equipped with a Jake brake or some other type of compression braking system.

Did ya ever break a clutch linkage or cable and drive home without it? On an old Jeep I was able to put it in 1st gear and use the starter to get moving from stoplights then just shift by matching the revs. To come to a complete stop you have to first get into neutral and brake normally. I was able to drive over 90 mile to get home in this manner.

I hope I'm not coming off like some self professed gear jamming genius. What I tell you I know from what I learned in driving school and from several years experience of driving tractor-trailers.

 

LE PEW

I think I've got it, and I think what I've set fits into your description correctly. Let's assume, or let's say we can assume, that 1st gear driven rotates more slowly than 2nd gear driven, and it is the driven gears which the synchro or non-synchro hub must lock onto. If I understand you right, what we are trying to do is match input shaft speed to output shaft speed, as that is the only way the hub will lock into the desired gear. Taking downshifting for example....Let' say we are driving in 2nd, the 1st gear driven is rotating much more slowly than the 2nd gear driven (and the output shaft and road speed). We must therefore "speed up" the input shaft to make the 1st gear driven speed match the output shaft speed....

Now let's say we are driving in 1st and want to go to 2nd...The 2nd gear driven that we want to go to is spinning much faster than the 1st gear driven we are in (which is driving the output shaft and road speed)....Therefore, we must allow it (the input shaft) to slow down to match output shaft speed...

I think I've got it?!!??

 

Funny you, of all people, would give a rip about what I had to say -- considering that, on 3/5/02, you wrote:

"...I wholeheartedly respect (well, not respect) but will amicably agree with you that we will have to disagree. But I definitely do not want to get into a argument back and forth (and you don't seem to be like that type of fellow). Again, you are incorrect, and are using a self-defined term..."

 

I have a chart that does the rpm every 2 mph in all 5 gears. I need tire size and tranny.

Regards

Jean Marc Chartier

PS here is a sample based on 3.55 gears 33 BFGs and 5spd
mph 1st 2nd 3rd 4th 5th
10 1410 814 539 362 289
12 1692 976 647 434 347
14 1974 1139 754 506 405
16 2256 1302 862 579 463
18 2538 1464 970 651 521
20 2820 1627 1078 723 579
22 3103 1790 1185 796 636
24 3385 1953 1293 868 694
26 3667 2115 1401 940 752
28 3949 2278 1509 1012 810
30 4231 2441 1616 1085 868
32 4513 2604 1724 1157 926
34 4795 2766 1832 1229 984
36 5077 2929 1940 1302 1041
38 5359 3092 2047 1374 1099
40 5641 3254 2155 1446 1157
42 5923 3417 2263 1519 1215
44 6205 3580 2371 1591 1273
46 6487 3743 2478 1663 1331
48 xx 3905 2586 1736 1389
50 xx 4068 2694 1808 1446
52 xx 4231 2802 1880 1504
54 xx 4393 2909 1953 1562
56 xx 4556 3017 2025 1620
58 x 4719 3125 2097 1678
60 x 4882 3233 2170 1736[