kiwi216 said:
Could not change down into second.
The 3rd gear syncro had failed.
umm.. the synchros are only used when going into gear though.. are you sure you meant synchro on third had failed? i'd have expected the synchro on second to be failed
They said it was usually caused by too much vigorous driving/ vigorous gear changes.
it is.. i'll explain a little, in the hopes that more people will be nicer to their gearboxes
your gearbox is effectively a drive train formed of 2 cogs. there are many cogs ina gearbox, and changing gear selects different cogs to be pushed together.. the sizes of the cogs vary and this is what gives you gear ratios.
now, in a car the drivetrain is formed of the engine, linked to the clutch, linked to the gearbox, linked to the wheels. the clutch's job is to disconnect the engine from the box so that gears can be changed and the amount of driveing torque can be varied (setting off, for example)
you need to appreciate that 2 cogs whirring round need to be doing the same speed before you can push them together.. if they arent, then the teeth of one will whizz past quickly over the other one, generating a crunching sound and wearing the cogs out.
the synchro's job is to equalise the speed of the 2 cogs.
when you push a gearbox out of gear and into neutral, some parts of it, icluding bits that are linked up to the clutch (the friction plate) begin slowing down. the synchro has to speed these up too, so it's quite a bit of work; we arent jsut spinning one cog up to speed of another.. we have to spin an entire driveshaft and part of the clutch up to speed too. the cog that is attached to the wheels is turning constantly.. and youre going down the road at 30mph in fourth.. the engine, clutch side of the gearboxetc, is probably doing about 1500rpm
if you wanted to shift down into 3rd, well thats about 2200rpm to do 30mph in third.. so we press the clutch in, and shift into neutral, then hold the stick against 3rd gear. at this point, the synchros start spinning up the components on the clutch side of the gearbox, so that the cog for 3rd gear can mate properly with the cog that is fixed to the wheels. this means the friction plate for the clutch and associated gearing (was doing 1500, now slowing down cos we are in neutral) must be sped up to 2200 rpm to engage third. when the cogs are at speed, in goes third gear.. clunk
we can then release the clutch and drive along at 2200 rpm in third, doing 30mph
this takes time.. and like anything, you can force it hard into third, and provide more friction to the synchro so it can spin the components up quicker.. but like anything that wears out, it wears out faster if youre impatient with it.. wheelspins and clutch-burning drag races at the lights wear your tyres and clutch out, and the synchros are no different.
we didnt always have synchros.. in the old days, double-declutching was a technique used when shifting from a higher gear to a lower gear. instead of using the synchros (that we didnt have back then) to speed the clutch plate etc up to speed, you use the engine. the process is:
depress clutch, move from 4th to neutral, release clutch, rev engine up, pass necessary speed for next gear down (i.e. if we are going into 3rd, and out example requires 2200rpm to engage the cogs, we rev to 3000), depress clutch, hold gear stick against gear.. components in the gearbox will begin slowing.. when they reach 2200rpm, third gear will engage
driving was an art, back then
note that double declutching wasnt necessary when you were shifting up, if you could move fast enough to be holding the gear stick against the gear before the system had slowed..
so in our example, youre in 3rd, doing 30mph at 2200rpm. if you could depress the clutch, and get to 4th gear by the time the gearbox components had dropped to 1500 rpm.. in it would go
it is worth noting that if you learn this technique, then you dont necessarily have to have your synchros repaired; you can just double declutch every time you want to shift into second from a high gear.
examine your driving habits though; frequently people downshift too early, causing the gearbox unnecessary stress. if it wont go into gear, dont rive on it! the old time saying of "if it's hard, youre doing it wrong" is never more true than in this day and age, where the average motorsit has nearly no clue about the mechanical device he is piloting
