There has been enough of transmission discussions lately, and I'd like to bring it another step further. I do not know if anybody will find this interesting, but I certainly did.

 

A somewhat relevant story:

When I bought a new car a year ago, the old one got jealous and snapped a piece off the clutch cable right in the dealership's parking lot -- what an irony. Only thanks to rev-matching techniques it was possible to bring her back home the same day.

 

An interesting transmission thread:

http://www.ultimatesubaru.org/usmb/forum/showthread.php?s=&threadid=9498

 

...I was wondering, if I could come up with some mathematical approximation that could be a guideline for practical rev-matching. From what I understand, the main task of it is to match RPMs of the engine with RPMs of the drive train in order to facilitate gear engagement, so my meager calculations are about just that -- mathcing speeds.

(I would not recommend practicing it unless your clutch is disabled for one reason or another and you HAVE to go!)

 

I have set up an Excel sheet, that shows the relation between the speed of the car and engine RPMs in each gear. Anybody who knows their gear ratios, differential ratio and tire size is more than welcome to play with it and see how close my scheme is to their practical experience.

Here is the logic behind my calculations:

Engine RPMs : Gear Ratio : Differential Ratio = Wheel RPMs

Wheel RPMs * Circumference of wheel (in inches) = Distance per minute (in.)

Distance per minute (in.) * 60 min = Inches per hour

Inches per hour : 12 : 5280 = Miles per hour (12 in. in 1 foot; 5280 feet in 1 mile; I found this real cool Converter Tool somewhere on the web, it’s an .exe file and you can easily create a shortcut, in order to launch it from the desktop)

No need to make a science of it really.. You just give it a little gas until you feel the synchros line up.. then it should just slip into gear .. smooth like butter:D

I'm sure there is a science to it, but what i do is i guess what gear is right for the speed, rev it up, and when the rpm's are coming down slip it into gear. A guy at my shop drove his geo 60 miles to work with no clutch, slip shifting it the entire way to fix it in the shop, just something intersting i thought i would add.

if amy one else here has ever driven trucks that is how you shift all the time. if you wanted to use the clutch You have to doube clutch.

 

I have done it in the car just to show people that its possible but I dont recomend doing it all the time. changing a clutch is cheaper than a tranny.

 

there is the version used in racing where uppon a hard down shift (with the clutch) you rev. the motor to match speed and save your synchros so much trouble.

this method saved me once. i snapped a clutch cable on the way to get free parts. the only problem is getting moving. i was at the bottom of a dead end hill so i had to literally slam it into 1st gear. from there, i just did't stop till i pulled in the driveway.

Drove for weeks with a broken clutch cable on an '84 wagon. The hard part is getting going from a complete stop. I always tried to park on a hill, and did rolling stops at all the signs Signals are another story

I have broken 2 clutch cables in my brat and one in my 3dr. Both times with the brat was 20 plus miles from home. No problems shifting. As far as starting goes its best to crank the car in first gear and pump the gas. If you fry the starter, it's still cheaper than a tranny.

My dad did this when the clutch cable on his sunbeam broke:

he tied a piece of string to the clutch lever and used it to start out, then he rev-matched it for the rest of the gears.

Originally posted by calebz

No need to make a science of it really..

I totally agree, -- and when/if my back ever gets better I'll be doing much more useful/fun things... It can't be a science anyway, perhaps a reference point, no more.

Personally, I never tried drastic rev-matching, and I do not exactly feel like trying to wreck transmission on any of my two cars for practice! Renting one is out of question as well, so...

I was simply curious how close the calculations were to the real life. Pretty close -- from what I could feel by paying attention to syncros while shifting regularly.

My wifes Gen I Brat used to eat clutch cables, so I got quite used to driving it without a clutch. I used to have to do it so much that I didn't even have to think about it any more. Had no problems merging onto freeways and stuff. If I encountered a red light, or stop sign, I'd just kill the engine, shift to first, then start it in first. Those starts are tough, and extremely torquey, still have the same on it now, and it works just fine. Heck, if you read your owners manual it tells you that if the vehicle stalls in a dangerous situation, (like railroad tracks), to engage the starter while in gear to move the vehicle!! Don't believe me, look it up. (disclaimer, my vehicles are EA82's or older, newer vehicles may/maynot contain this information).

Re: starting while in gear

Interesting... I couldn't find my ford owner's manual, but I doubt there's something like that in there! They brag about child seats, safety belts, air bags and similar crud, instead of talking about proper shifting and break-in periods.

If you use low range it is a lot easier to get going on the starter and as others have said it is an acquired skill to change without the clutch but you can change out of low just as you would any other gear

Aside from rev-matching to shift when things are broken or you have to drive a lame car, rev matching is used for downshifting when you want to brake hard and not upset the balance of the car, takes heel toeing to get it right.

 

I can do it in my GL hatch without thinking, but in the family Legacy I'm clumsy. Could break the head off a bobble doll.

 

haven't had much time in the RX to get it right, But Oakland Acres is calling. (Rally cross)

 

Jay