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MG MGA - Gears Synchronize at what RPM?

I've be goofing around with my double clutching and it occurred to me that there must be a a known engine RPM "ratio" at which 2 gears can mesh without use of the synchronizer's? And without a clutch for that matter.

Does anybody know this little statistic?
T McCarthy

Since on the standard gearbox the ratios are not evenly spaced it would depend on the gear. From the standard gearbox ratios of 1:1, 1.374:1, 2.214:1 and 4.755:1 it would work out this way.

4th to 3rd 1.374
3rd to 2nd 1.611
2nd to 1st 1.308

1st to 2nd 0.728
2nd to 3rd 0.621
3rd to 4th 0.765

Here is an interactive demo of double clutching that lets you picture it. http://www.gearmaster.com/trc/shiftingh.swf
instructions for the demo http://www.gearmaster.com/trc/dc-cb_instructions.pdf

PS the demo is easy because all the gears are double or half of the previous.



Jeff Schultz

With my mind, I really need an edit feature. Wrote down the wrong ratio for first which on the standard gearbox is 3.636:1 so the RPM ratios for 1st and 2nd would be 1.642 and 0.609.
Jeff Schultz

It is a feel sort of thing there is no "numbers" way to do it. What you are synchronizing is the engine rpm with the speed of the car. You don't think your way through it, you feel it. You over rev the engine and as the speed drops through the proper RPM the shifter drops in. Easy to show hard to explain. The explanation is always more complicated than the action.
R J Brown

What Randy said, but:
Actually, it's simple to say but hard to do mentally. Synchronization occurs when the engine speed is at the speed it would be if it were already in the gear you're going to.
And you're right - no synchros or clutch needed - if you do it right! If you get it exactly right, shifts are far smoother than any automajic box. I drove my big truck - 13 speed - without clutch, for about a million miles. Clutch only for dead stops. I figured this out about 3 weeks after I started, on a really hot day, after about 300 miles of thrashing it up and down Pittsburgh area hills on very skinny tight roads. My left leg just hurt too much too use the clutch any more, never used it again. Once when we took the gearbox apart for routine bearing replacement we found a broken synchro - had never noticed it.

FRM
FR Millmore

I did the same thing as Fletcher, but with a 1936 MGPB. Since there were no syncros in any gear, I soon realized that it was just as easy to shift without the clutch as with it. Down shifting was particularly easy - come off the accelerator and pop it out of gear, then with just a bit on the gear shift lever, raise the rpm. when the correct was reached, the gears just slid together without a sound. Cheers - Dave
David DuBois

I agree with the comments above, it is by ear and feel. You learn it by practice. I learned on a '35 Chevrolet back in '59. I could go all the way up and down the gears without a clutch except for a stop. You learn when you have to. The syncros have really spoiled the modern driver.
Jim Ferguson

I agree that changing without the clutch is a matter of experience and a good ear for RPM. My 1922 FIAT had straight cut gear teeth and no synchros. After a bit of practice the gear changes were smooth and silent.
However, for those drivers used to modern, plastic cars I have a spreadsheet to use as a starting point.
This spreadsheet is at:

http://www.angelfire.com/amiga/mga/index.html

You can enter the maximum RPM at which you want to change and it gives the RPM in the next gear which has the same road speed.

A sample is attached.


Mick


M F Anderson

Remember with Microsoft Excel that after you change the numerical value in a cell you have to click the mouse in an area outside the cell before a calculaion will take place.


Mick
M F Anderson

That's perfect!

Thank very much.

Here's a corrollary question.

When double clutching, what purpose does the time during which the gearbox is in neutral and the clutch is disengaged serve?

i.e. can you achieve the same result by blipping the throttle when the clutch is depressed but the gear box still in a gear.

Thanks
T McCarthy

First you need to get the terms right.
When the clutch pedal is depressed the clutch is disengaged. When the clutch pedal is released the clutch is engaged.
Double clutching is not required in synchromesh gearboxes. It was something used on older designed, non-synchromesh truck gearboxes with very high inertial rotating masses in the gearbox.
The clutch pedal was released as the gearlever passed through neutral and the engine RPM raised to raise the rotational speed of the large mass of the input revolving parts inside the gearbox. Just blipping the throttle with the pedal depressed does nothing.
You probably mean single clutching and raising the RPM to match a lower gear to the current road speed. That is someting different, but you do not "blip" the throttle, the higher RPM has to be still held as the pedal is released and the clutch engages.

Mick
M F Anderson

Mick,

That's very helpful.

I'm going to trouble you with one last question and it really the question that I've been working at as I fumble with both double clutching and heel toe'ing.

From your comment can I infer that heel toe'ing in a car that has syncro's does not require a shift to
neutral to blip the throttle? Or am I wrong in this.
T McCarthy

"Double clutching is not required in synchromesh gearboxes."

That is true. But then considering an MG box entirely synchromesh is only true when it is freshly rebuilt. The 3 synch box especially tends to have the synchro effectiveness deteriorate and double clutching when you need to do a downshift that results in a reasonably high RPM in the next gear down calls for double clutching - on the race track, for instance.
Bill Spohn

T McC (I can't recall what the "T" is for)

"can I infer that heel toe'ing in a car that has syncro's does not require a shift to
neutral to blip the throttle? Or am I wrong in this."
The answer would be #2.

"heel toe'ing" refers to a physical method of applying the throttle and brake more or less at the same time, and has nothing to do with gearchanging per se. When you are trying to slow down rapidly in a vehicle with not too robust brakes, engine braking takes some of the load off the brakes, and gets you in the gear you need to accelerate after a turn. It was done in the old days because after a couple of applications the brakes faded away to nothing, like an XK120 from 80 to 20 winds up with no brakes after a couple cycles - why Jaguar introduced discs on the C type! With modern brake materials, even somewhat wimpy brakes are a more efficient and cheaper way to slow down. Nail the binders and downshift to whatever gear you need to get away. That means things like 4-2 or 3-1 downshifts, and you had better know how to do it right.
But, full pattern downshift sequences are a good way to practice, are fun, and sound cool - it's music!

I have to take some issue with Mick on a point, though he's generally right on.
"Double clutching is not required in synchromesh gearboxes. It was something used on older designed, non-synchromesh truck gearboxes with very high inertial rotating masses in the gearbox."

The method is necessary on any non-synchro box or gear, or you will be buying a new one soon. And it is certainly worth learning for any synchro box too, as failed synchros from abuse are the most common reason for modern gearbox death - and they ain't cheap to fix, but it's free to learn how to shift right (unless you have a really bad day!)

The synchro itself is a clutch which gets the two sets of parts to be engaged to the same speed - "synchronized" - and only then lets them be coupled together. Hence the part names "baulk ring" or "blocker ***".

If the bits are not synchronized, it won't couple -0 it's "baulked" or "blocked". A less than proficient driver causes great work for the synchros, and wears them out in short order, especially older weaker designs. That's why MGA 2nd synchros wear out rapidly. They don't wear out if the driver doesn't beat them to death, and a competent driver can shift cleanly when they are dead, and shift into 1st on the fly too.

So, to answer your real question "...require a shift to
neutral to blip the throttle?". The point of the "blip" is to get the input side up to the speed of synchronization, which is done by the engine, which must be coupled to the gearbox input via the clutch. If you don't pause in neutral w/ clutch engaged, then the synchros, if any, have to do the work of speeding up the trans internals, which is better done by the engine. It is a matter of changing the speed of several (15 or 20 in a car, 100+ in a truck) pounds of metal by hundreds of rpm in milliseconds. The engine does this a whole lot easier than the little brass or steel synchro rings.

Go play!
FRM
FR Millmore

My first car was a 1938 Rover 14 Sportsman. Crash box, so had to learn to DD right away. I got so used to it, it became a habit and do it still on my modern junk, up or down.
Art Pearse

Thanks all.

Mick the spread sheet was very helpful and confirmed my driving guesstimate.

Fletcher the T is for Tysen. Thanks also for the descriptions. BTW during your earlier post I was thinking of someone going down Route 30 from Breezewood to Gettysburg in a semi.

Frankly I'd have to clean out my boxers after that run.
T McCarthy

Tysen-
Just after I said I forgot your name I looked at your post re oiling whilst spiffy. Those kids are getting the idea, keep up the good work! Now you just have to teach them the diff between "weird" and "eccentric".

US 30 from Pittsburgh to Chambersburg used to be my idea of a relaxing drive, actually still is, just no truck anymore. Or how about US220 from Cumberland MD to US250 to Staunton VA - can't park the truck in a straight line and have it all in your own lane. 72IH cabover, 400 Cummins, 13 speed, 28ft dump trailer.

FRM
FR Millmore

Tyson, a some very interesting questions you have asked, when i drove a restored 1600 for the first time in almost 30 years i was amazed to realise that i was double de clutching on the downshifts instinctively even though the car was fitted with a 5 speed sierra box which has superb synchromesh on all 5 gears. Then i remembered that my original car had a std 4 speed box with suspect synchro and i had just reverted to downshifting that way!

On the subject of heeling an toeing, this was done mostly in competition driving, especially rallying. The idea is to apply the brakes with the toe of the rt foot at the same time as pushing the gas pedal with the rt heel (or with the rt side of the foot if your feet are too big like mine).
This is usefull under hard accelleration or cornering under power as lightly applying the brakes will prevent the wheel on the inside of the corner from spinning and therefore put more traction down through the non spinning wheel.
This is how a limited slip dif works.

You will have seen this effect at its most extreme if you have ever run your car in gear with one jacked up rear wheel. The jacked up wheel spins whilst the one on the ground puts no power to the road at all.

Rally drivers in the mini developed this technique into left foot braking for even better control of the cars traction.

I have suprised myself by trying left foot braking to get my car up an icy hill that had almost beaten me, and it worked.

It is even easier to prevent wheelspin on ice or mud using the MGAs fly off handbrake but i have to admit that nowadays my car is usually in the garage on days like that.
cheers
colyn
c firth

i think i should have gone to bed earlier and given my previous reply more thought before sending it.
I woke this morning and realised that thinking back i used to use the heel and toe technique mostly during double de clutching and hard braking which is basically what everyone else had said before i decided to poke my nose in!
sorry
colyn
c firth

This thread was discussed between 05/07/2009 and 07/07/2009

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