I was reading the “How Stuff Works” article on a manual transmission, but was kinda stuck on how the dog teeth on the collar mesh with the gears and it was hard to tell from the diagram. Do the dog teeth engage with the same teeth that the layshaft is engaging or on some sort of inner ring? If the gears are spinning (since they are always engaged by the layshaft) why don’t you hear a grinding sound as the teeth attempt to engage with the new gear?
the 'dogteeth" (actually part of the synchros)in a manual trans have high lead-in angle so they slide in pretty easily. Also, because the layshaft is (in normal use) spinning at an appropriate speed already, the teeth tend to be almost spinning at the same speed, so engaging them isn’t so hard.
b.
Each gear in the tranny has two sets of teeth: the main ones (big) that mesh with the countergear (laygear) and the the dog teeth that engage with the shifting hub & collar assembly (sychronizer). Grinding is avoided by the use of synchronizer rings (balking rings), usually made of brass, which grab on to a ramp (on the gear next to its dog teeth) and bring the gear’s speed up or down to match that of the shifting hub assembly. Since reverse gear is never to be engaged unless the vehicle is sitting still, it does not have a synchronizer ring.
If the synchro rings are significantly worn, or if you try to get reverse while the car is moving, you will hear grinding.
Great explanation guys, thanks. One more question. Why is it that I can’t upshift If I’m going to fast. It makes sense that I shouldn’t be allowed, but what is the mechanism preventing me from putting the stick into first when Im goin 40?
Assuming you’re in a higher gear going 40, shifting to 1st is a downshift. There’s a great discrepancy in the internal speeds of 1st gear and 1-2 shifting hub assembly, making it a bit tough to get engagement. It is possible to make the shift with the clutch in, but it’s hard on the synchro rings and when you let the clutch out the engine can overrev and be destroyed. There’s less of a fight shifting into appropriate gears at appropriate speeds. Being aware and trying to do things smoothly pays off.
To clarify, there is no mechanism that prevents shifting into 1st at high speed (or any speed). It doesn’t go as easily as an appropriate shift, and that’s all the warning you get. If you try hard, you can make the shift, and almost certainly do some damage. Those who routinely move the shift lever with great vigor are more at risk for said damage than those who move it smoothly and gently.
To clarify, there is no mechanism that prevents shifting into 1st at high speed (or any speed). It doesn’t go as easily as an appropriate shift, and that’s all the warning you get. If you try hard, you can make the shift, and almost certainly do some damage. Those who routinely move the shift lever with great vigor are more at risk for said damage than those who move it smoothly and gently.
Gary T,
Is that all it is? Are you sure? I’m still not understanding why, as KidCharlemagne asks, I can’t push the shift lever into the position of 1st gear when I have the clutch in.
There is no specific device that prevents shifting into any gear at any time. (The shift collar assembly is restrained by spring loaded detent balls or other detent devices so that vibration will not cause it to drift toward the gears and cause accidental engagement.) However the shift collar, on which there is a set of dog teeth, is connected to the output shaft by means of splines so those teeth are rotating at the transmission output speed, say 1800 rpm at 60 mph. The first gear with its attached dog teeth is rotating at, say, 300 rpm. It is virtually impossible to get the dog teeth on the shift collar to mesh with their mating teeth on the first gear under those conditions. If you do manage to get engagement you are amost certain to break off the dog teeth.
The designers count on the operator not to be a total idiot, which might be overoptimistic in some cases.
Ditto.
I’ve driven several cars with manual transmissions, and this is something that seems to be consistent among all makes and models (IME). Even travelling at a relatively low speed (less than 5 mph), I cannot for the life of me get the damned thing into first gear. I have to practically brake to a complete stop before I can downshift to first (especially annoying when stuck behind an SUV that slows to 2 mph to make right turn).
On preview, I see that David Simmons has answered Eonwe’s question. But I still say that it is impossible to downshift to first while the car is moving at any appreciable rate. Whether this is by design is my question.
To clarify, I have inadvertently shifted from fifth to second (not good), and have “ground teeth” shifting from fifth, and hitting reverse rather than fourth.
But I’ve never been able to even attempt downshifting to first while the car is moving.
Eonwe, it’s because you’re not brutal enough.
My understanding is that we’re talking about driving along at some road speed (30-50 mph?) in, say, 4th or 5th gear, and then trying to select 1st gear. The shift lever doesn’t readily go there, which is good.
Easy shifting is greatly enhanced by having a correlation or synchronization between the rotational speed of the stuff on the input side (engine, clutch, input shaft, countergear) and the stuff on the output side (specific gear, output shaft, driveshaft/drive axle). To illustrate, I’ll use some arbitrary numbers.
Let’s say that with an engine speed of 1000 rpm in 1st gear you’d be going 8 mph, and thus at 2000 rpm you’d be going 16 mph. In 2nd gear at 1000 rpm you’d be going 16 mph, and at 2000 rpm, 32 mph. In normal upshifting, you accelerate in 1st until you reach 16/2000 and shift to 2nd. Your road speed doesn’t change (momentum). To be going 16 in 2nd, the engine speed needs to be 1000. You declutch, get off the accelerator, pull the lever out of 1st and try to get 2nd. The engine speed comes down quickly because you took your foot off the gas. When the engine speed hits 1000 with your road speed of 16, the lever slips easily into 2nd. In a like manner, the engine speed comes down at the moment of shifting into each higher gear.
You can get a feel for this by upshifting without using the clutch. When it’s time to shift, put a finger or two on the shift lever and pull toward neutral, not quite hard enough to move it. Then let up on the gas. The instant the car is neither accelerating nor coasting, the lever should slip right into neutral. Continue to pull toward the next gear position. (Do this whole out of gear/into gear in one continous motion, just like when you do use the clutch.) The instant the engine speed hits the correlation point, it should slip right into gear. (This technique is a bit harder on the tranny than using the clutch, so don’t make a habit of it.) This illustrates the difference between the innards being coordinated or not to the gear you’re selecting. When they are not coordinated, it doesn’t go into gear readily.
And that is all that physically deters you from engaging 1st gear at higher speeds. If you rev the engine high enough (not recommended), it can go into 1st easily. If you pull/push hard enough on the shift lever (not recommended), it can go into 1st (not so easily).
Many 5-speeds where 5th and reverse are adjacent do have a lockout that makes it impossible to move the lever directly from 5th to reverse (which could be disastrous). From 5th, the lever has to move sideways in the neutral zone first.
Adddenda after previewing, seeing some added posts:
Every now and then I hear about a race car driver missing a shift, accidentally putting the lever into a lower gear instead of a higher one. Usually when the clutch is let out, the drivetrain forces the engine speed up too high and it thrashes the engine. In racing, one can get carried away with doing things quickly and vigorously. If they aren’t also done precisely, things go wrong.
Moving cars can be downshifted, even into 1st. Fortunately it’s pretty tough to do.
It is just the nature of the beast. It would be like trying to step onto a moving sidewalk that was going 40 mph.
So basically, even though the clutch is depressed, there’s still some physical “meshing” that has to happen when you position the gears, before you actually engage them? Ok, that makes sense.
A few weeks ago my clutch was completely gone, and I had the pleasure of driving without it for a day or two. Not fun.
My understanding is that before the advent of synchronization, one could not, or should not, downshift to 1st. However, the manual for my 94 Honda recommends shifting down to 1st anytime I’m driving 5 mph or slower. So, I often downshift to 1st, but at very low speeds.
While we’re on the general topic of shifting, I often have problems shifting into reverse. I don’t know if this is generally true, but I’ve had this problem with the cars I’ve had. I find that if I shift to 1st and go slow for an inch or two, then stop, I can shift more easily into reverse. What’s the reason for this?
barbitu8, on most manual transmissions reverse is engaged differently from the forward gears. In the design we discussed above, each gear’s main teeth are always in mesh but the gears spin on the mainshaft until the shifting hub assembly engages the dogteeth. For reverse, the gears are not in mesh until you shift, which involves sliding the reverse idler gear into mesh with both the reverse gear and the countergear. Usually the teeth are lined up right (tooth on one gear aligned with space between teeth on another gear), but sometimes they’re not and the teeth hit, stopping movement. The procedure you described rotates some gears a bit and gives them a chance to line up right. Another way to accomplish the same thing is to let the clutch out in neutral for an instant, which will rotate the countergear.
I think that before synchros, you could still downshift into first, but you had to double-clutch in order to do it. (In fact, without synchros, my understanding is that you have to double-clutch in order to make any downshift at all.)
What you said is absolutely correct, Arcite.
I understand all the concepts, I just don’t have a visual so I’m having a hard time understanding how the dogteeth engage the gear’s with my not feeling any “catching” at all. I think I’ll have to go to my local garage and ask to see one. Curiousity can be such a curse.
GaryT
When you were explaining the stuff about rev matching the input and output sides you spoke about pulling the car out of gear with the clutch still in and that if the revs were matched it should be effortless. I had an asshole friend in college who always pulled my car out of gear while I was accelerating, decelerating, braking, sliding, bumping, hitting him for taking my car out of gear etc. He said it didn’t hurt the car (and it didn’t ever make a weird noise.). Was he right?
The dog teeth do not engage the gears. All of the gears in a manual transmission, with the exception of the reverse idler, are engaged at all times. Each gear has machined on it a set of dog teeth. These teeth are rectangular cross section with a tapered front end where they will engage their mating set of dog teeth. When the shift collar is slid toward the gear the first thing that contacts is the synchronizer. This is a set of brass cones having a shallow taper, one cone attached to the shift collar and one to the gear. When the cones come together the friction between them stops the gear with respect to the shift collar and the dog teeth slide into mesh with any misalignment being taken care of by the tapered ends.
The dog teeth do not engage the gears. All of the gears in a manual transmission, with the exception of the reverse idler, are engaged at all times.
No argument with the thought here, but this is not how we have been using the term “engage” in this thread. I would say the all the forward gears are in mesh with the countergear at all times, and the selected gear is engaged when the shifting collar interlocks with it.
I had an asshole friend in college who always pulled my car out of gear while I was accelerating, decelerating, braking, sliding, bumping, hitting him for taking my car out of gear etc. He said it didn’t hurt the car (and it didn’t ever make a weird noise.). Was he right?
It probably didn’t hurt it to speak of, and it’s nowhere near as stressful as trying to shift into gear at inappropriate times (e.g. not declutched). Still, other than that moment when there is no tension on the gears and it pops out readily, doing that would tend to cause a little (maybe tiny) bit of wear on the dog teeth, shifting hub assembly, internal shift forks, and shift linkage. If it were done constantly over a fair period of time, it could result in noticeable wear. If he was yanking hard on the shift lever when the drivetrain was under strain, I’d say it was doing some harm.