That's a good basic understanding.

You're right - The clutch connects & disconnects the tengine's crankshaft from the mainshaft of the transmission.

The other shaft with the mating gears is called the countershaft or layshaft. It'll be spinning along with the axles & wheels, whenever the car is moving down the road.

There's 2 ways to think about the action of a synchro. It's a little clutch that gets the gears spinning together so they can engage. But it's only strong enough to do that job if the clutch is disengaged (pedal down). And... if the mainshaft & countershaft aren't way WAY off in speed.

The other way to look at it is like this... If the synchro can't get the gearset to match, it'll resist you forcing it into gear.

 

right on

you guys are the bomb

thanks for the lesson

much appriciated

 

Correct, except for the bit about "kinda feel like your running out of gas", it's more like you jumped on the brake.

It has to do with the rotational speed of the 'cluster gear' relative to the rotational speed of the main shaft, the ideal relative speeds of each being dictated by the ratio of the lower gear we are trying to select.The cluster gear is directly attached to the first motion shaft, being the shaft attached to the clutch driven plate that passes into the box and is splined 'solidly' to the cluster gear.

If the cluster gear is rotating too slowly (because of engine rpm while in the previous gear) then the difference in rotational speed between thelower gearin mesh with the cluster gear (that we are attempting to select) and the rotational speed of the mainshaft will be too great for a quick and fast shift, but we should still be able to select the gear if we wait long enough in neutral (with clutch disengaged) before we actually make the shift into the lower gear. This would be evident when attempting to shift from say 5th to 3rd (or 2nd), or from say 2nd to 1st (which is likely to be a biggish gap between ratios).

If we try to do this too fast then the synchromesh may baulk the shift, but we can force it (might get a gear crunch though, and doing this a lot will wear out the synchro ring). Keep in mind that for the synchro to 'drag' the cluster gear up to the appropriate rotational speed for the lower gear we are trying to select, that the synchro ring must overcome the inertia of the cluster gear, the first motion shaft and the clutch driven plate (+ any frictional drag in bearings andoil).

To acheive a smooth / fast shift we need to bring the rpm of the cluster gear up to a higher speed by 'blipping' the throttle as we pass through neutral in the shift gate, but it's not quite that simple since with the clutch fully disengaged the cluster gear is not connected to the crankshaft. To bring the cluster gear up to speed quickly to achieve a fast / smooth change we need to only partially depress the clutch pedal so we only partially disengage the clutch itself. This causes some drag from the crankshaft through the clutch to the cluster gear, enough to bring the cluster gear rpm up to speed but not so much as to baulk the shift (when done correctly).

Really fast shifts / smooth are achieved with 'rev matching' using only partial clutch disengagement. If the clutch is fully disengaged then the cluster gear will have to be 'dragged' up to speed by the synchro ring causing baulking and / or aslower shift. In this case all the rev matching has achieved is correct engine rpm when the clutch is fully re-engaged, which should still give a smoother shift but not necessarily a faster one. The degree to which the clutch pedal should be partially depressed is something you find by practice. Practice enough and it should become second nature, but some people never really 'get it'.

 

so for fast shifting you hit the gas lightly with the clutch slightly pressed to speed match and shift?

it sounds like this would burn the clutch up fatser? but its not like id be doing it all the time

gotta get a little practice it though

 

More or less. With theclutch partially depressed, lightly 'blip' the throttle pedal as the gear lever passes through neutral on the way to the lower gear. Initially you might depress the pedal a bit more to ease gear disengagement, but then oncein neutral let it back out a little to slightly increase drag to speed up the cluster gear etc. With practice it becomes a smooth motion at the pedal. It's all about timing and finnesse.

It takes practice to do it properly, and you should never need to force it (don't force it if you get it wrong, just abort the attempt and try again next time). Sometimes when you get it slightly wrong (i.e. release the clutch pedal slightly too soon) you can crunch the shift. Practice...

When you get good at it you can graduate to 'heel and toeing', whereby you 'rev match' while also braking into a corner, using all three pedals simultaneously. When you can do this consistently well without having to think about it then you can lay claim to being able to use a manual gearbox properly...

This is what you can hear when a race driver revs the engine as they are braking into a corner, i.e they are rev matching in order to be in the correct gear at corner exit, but also to create engine braking into the corner (helps keep some heat out of the brakes since you don't have to use the brakes quite so hard). To create engine braking into the corner without over-revving the engine the driver will typically drop sequentially down though the ratios as they brake, but in some instances will skip a gear.

In road driving keeping heat out of the brakes isn't such a big deal, and I often skip a whole bunch of gears, e.g. brake in 5th up to corner entry, then as I'm progressively lifting offthe brake during entry I heel / toe straight into 2nd, and thenaccelerate from the apex out of the corner. Get this absolutely right and it's pure shifting joy! (get it wrong and it's embarrasing).

In theory there is someclutch wear as we are talking about clutch slippage (which ipso facto always causes wear), but it's a matter of whether it's more or less wear than doing it otherwise. Keep in mind that during the shift the clutch is only lightly engaged just enough to allow some relativelylight degree of'drag' to allow the rising engine rpm to bring the cluster gear etc up to speed (i.e. overcome the rotational inertia of the cluster gear, first motion shaft and clutch driven plate). This doesn't create much clutch friction / heat / wear, certainly less abusive on the clutch than allowing it to slipfor longer with enough friction to allow chassis inertia to progressivly bring the engine itself up to match road speed, or suddenly engaging the clutch in the lower gear (in which case the slippage will be short lasting but heavily loaded).

The cluster gear (etc) does have inertia that results in friction in the clutch during a 'rev matched' gear shift using partial clutch disengagement, but the engine and flywheel has vastly more, and also compression resistance, so using the clutch to increase engine rpm during a shift is far more abusive on the clutch. The aim is to depress the clutch only very briefly, but to achieve a very smooth / fast shift with no major forces being passed through the clutch (until you get back on the throttle once the clutch is again fully engaged), which will always cause less wear than passing a heavy force through a slipping clutch.

 

so when i was driving this weekend i was trying the "speed matching" and from your explination it seems like thats what i have been doing (unless im not really trying to shift quick) I didnt notice untill i was in my car trying to do it how you were explaining it, and thought to myself... self this is how youhave beenshifting :P

i guess it already is second nature, since when you were explaining it i didnt realize thats how i have been doing it, cause i guess i dont really think much about it, i just do it

as far as crunching the gear in, i try to never force it in. like you said when i feel that, i let up, and try again next time

and as far as downshifting to reduce the stress on the brakes, thats how my dad tought me to drive, so thats always kinda been second nature to me

 

oh and i gotcha, duh the whole idea to speed match is to get them both spinning the same speed, therefore, even if the clutch is rubbing while thepedal is slightly pressed there spinning close to the same speed, so the wear isnt that great, and a lot easier on the clutch then if you were to cause them to match speeds by releasing the clutch when the rpms are drastically different

 

Even if the rotational speed difference is quite substantial (say when skipping a gear ortwo on the downshift)the wear caused by rev matching with a partially engaged clutch will be minimal due to the lightness ofloadings on the clutch friction surfaces. It's heavier loading + slipping that causes substantial wear and tear. The most wear to clutchesis caused when pulling away from rest, which is why city cars tend to go through clutches much faster than country cars.

Note that when upshifting that you should fully depress the clutch as this will usually (unless the engine is fitted with a lightweight flywheel) allow a faster shift, even if the engine rpm may not yet have dropped to ideal rpm when the clutch pedal is released (in which case the shift should be done a bit more slowly unless you're not bothered by some increase in clutch wear and a somewhat less smooth shift when the pedal is released with rpm too high, thus 'forcing' a faster rpm drop through the box and clutch).