(Trans)Mission: Possible

In our second installment of automotive tutelage, you’re going to get the shaft. A couple of them, in fact. The drivetrain of a car is full of ‘em: crankshafts, camshafts, input shafts, output shafts, drive shafts, halfshafts, etc. We’re going to focus on the inner workings of a manual transmission (aka: tranny, gearbox, four-on-the-floor): the Input/Output shafts between the crankshaft and the driveshaft.

Before we dive in, let’s discuss purpose. The purpose of any transmission is to use the limited rpm range of your engine and power band to achieve higher speeds and better efficiency. If you’ve ridden a bicycle with gears, you know that lower gears are used for lower speeds, that higher gears are for higher speeds, and that you have to progressively shift through different gears to maximize your own power. You cannot start off in the highest gear and expect to get up to speed quickly nor can you stay in the lowest gear and expect to get anywhere quickly. Gearing makes use of your momentum and allows you to effectively use your power in a comfortable manner. Simplistically speaking, it takes the same power to go 0-5 as it does from 5-10, 10-15, etc. because you just keep adding to your momentum – just like pumping your legs on a swing. OK?

Okay. The difference between your bike gears and a car’s manual transmission is just how they’re laid out – a bike uses a chain and a derailer to go from gear to gear; a car’s gears mesh continuously and are engaged/disengaged to shift. Your front bike gear with the pedals is akin to the tranny’s Input shaft and the rear sprocket on your Schwinn is comparable to the Output shaft. The size in relation to one another is the gear ratio, expressed as output:input. So, if your front sprocket has 17 teeth (or cogs) on it and the rear has 34, the gear ratio is 34:17, or 2:1. (Figuratively equivalent to 1st gear on a mountain bike.) Here’s where it gets a little tricky: you have to interpret the numbers in the ratio as a gear reduction to come up with something meaningful – a 2:1 gear ratio means that every rotation of the output will require two rotations from the input or every rotation of the input results in 1/2 rotation of output. Think about riding your bike in first gear – your legs are rotating much faster than your tires are and, if you actually look at the gears, you’ll be on the smallest sprocket in the front and the largest in the rear – the lowest gear ratio. Progressing through your set of gears, the highest gear you have will relate to the largest front sprocket and the smallest in the rear. This difference in cog counts is exactly what’s going on in your car’s transmission. Replace your legs with an engine and the back tire with a driveshaft and the analogy is complete.

Lab Exercise: In a three gear situation, you have first at 3:1, second at 2:1, and third at 1:1. At 1000 rpm input, these numbers would relate to 333 rpm, 500 rpm, and 1000 rpm respectively. You can see that at the same input, your output is getting faster and faster as the gear ratios are getting larger and larger. Now, if third gear was 0.99:1 (or anything less than 1), your output will be spinning faster than your input and you are now in ‘overdrive’. Overdrive just means that the diveshaft is spinning faster than your engine – a 0.85:1 overdrive would result in 1176 rpm in the example above.

Advanced Lab: Since the differential has an internal gear ratio associated with it, your transmission’s output does not directly relate to your tires' speed. You may have heard of 411 gears, this just means that there’s an additional gear reduction of 4.11:1 in your differential, so at 1:1 in your transmission, it would still require 4.11 rotations of your engine (and transmission) to move your tires one revolution. Since the tires come in many different sizes, another calculation would be required to translate tire rotation to actual MPH (one of which is 2πr, or the circumference of a circle).

Real-World Lab: (The math portion of the lesson is almost over). What would your speed be in 1st, 2nd, 3rd, and 4th at 2500 engine rpm on a stock 1967 TR4A/IRS? Doing some research, you find out that the gear ratios of the TR4’s gearbox are:
First: 3.14:1
Second: 2.01:1
Third: 1.32:1
Fourth: 1.00:1

This would relate to the following output rmps at 2500 engine rpm:
First: 796 rpm
Second: 1244 rpm
Third: 1894 rpm
Fourth: 2500 rpm

Assuming a 3.70:1 rear (differential), the following would be the corresponding tire rpm:
First: 215 rpm
Second: 336 rpm
Third: 512 rpm
Fourth: 676 rpm

Now, given the original tire size of 5.95-15, this relates to an overall radius of approximately 12.75 inches, or a circumference of 80.11 inches. This means that for every rotation of the tires, they will move a distance of 80.11 inches. Since we have the distance per rotation and the rotations per minute, we can come up with total inches per minute and, with some additional math, miles per minute and, finally, miles per hour. As useless as it seems, the answers to the question are:
First: 16.3 MPH
Second: 25.5 MPH
Third: 38.8 MPH
Fourth: 51.3 MPH

What does it all mean? In theory, using the same amount of work from your engine, you can travel over 50 miles in an hour rather than just 16. Correlating this to when you start in the highest gear on a bicycle, the car would have to work very hard to get up to 50 MPH if it only had 4th gear.

I think the horse is dead so I will stop beating it. What you’re really interested in is how that gearbox works. You know that moving the gearshift around changes the gears but the rest is shrouded in mystery. Well, Sir, knowledge is about to be dropped.

Forgetting about Reverse gear for a moment, and remembering that the gears in a manual transmission have a constant mesh, it’s not a tough jump to imagine four sets of gears with varied sizes, approximately being of size that would allow for the ratios described above.

Now, looking at the same gears on their side (so we can arrange them inline), you see that you can have the gears described above rotating in pairs along two common axi (is that even a word?).

Taking it a step further and replacing each axis with a shaft will give you four geared pairs and constant mesh. Keep in mind that the gears are not fixed to the shaft yet so each pair is free to rotate independently.

We now move the pairs close together and assign labels to the shafts – ‘Input’ for the side connected to the motor and ‘Output’ for the side connected to the rest of the drivetrain. The pairs are still free to rotate on the shafts.

Making it a little more useful, we now affix the upper gears to the Input shaft. This is typically accomplished by having a splined shaft. We haven’t done anything to the Output shaft yet, so if, let’s say, the engine is running at 2500 rpm, the Input shaft will be rotating at the same speed, the Output gears will be rotating at various speeds (see below), but the Output shaft will stay dead.

Adding something to the Ouput shaft, specifically synchros, will give us a way to ‘engage’ the gears one at a time. Shown here, the transmission is figuratively in Neutral. The synchros can slide back and forth and there is a mechanism there on each surface to engage a gear when they meet. So, with no gears engaged, everything is still spinning with the exception of the Output shaft, as shown.

Finally, we shift into first. Neglecting clutching for our example, only the first gear is coupled to the Output shaft through the synchro and it’s spinning at almost 800 rpm.

So, we have the engine at 2500 rpm, the power is going through the Input shaft, meshing with the Output gears, first gear is engaged, thereby powering the Output shaft at 796 rpm, going through the differential to do more math and the car is cruising at 16 or so miles per hour. Let’s switch to second.

Now we’re at 25+ MPH. Third.

Approaching 40 MPH, the tranny’s output shaft is humming at almost 2000 rpm and we switch to fourth.

I think you get it. Wrapping up the lesson, the clutch is a mechanical link between the engine’s crankshaft and the transmission’s Input shaft. Along with mechanisms in the synchros and the gears themselves, the clutch’s job is to disengage power to the Input shaft so that switching from gear to gear doesn’t result in grinding. (By the way, since the gears are in constant mesh, grinding is not actually your gears but is rather from synchro to gear malignment.)

Oh, and as for Reverse, there’s an additional ‘idler’ gear that slides into place, reversing the direction of the Output shaft, seen here:

One last diagram of the actual TR4's gearbox. Looking from the top, the Output shaft is above the Input shaft (so the Input shaft is hidden), the clutch/engine would be to the right and the driveshaft/rear would be to the left. The 'mechanisms' that interact between synchros and gears are also seen here - they're called 'Dog Teeth' of all things.

May I squeeze in a final tidbit of information? If you notice, the Reverse Gear and Idler are straight-geared and the rest are angled. The angled gears reduce noise and mesh better than the straight ones. You've all heard the whine when you go in reverse - that's from the straight gears! Why not use angled for Reverse? That Idler would have a hell of a time sliding in and out if it were angled.

Misses Dash - Interior Part II

By now you have a project on your hands. I’m not sure, in the grand scheme, when, exactly, something becomes a project, but I’m willing to bet that a ’67 sports car that’s missing most of its top side and half its interior qualifies. So now what? To save your project from becoming just another boxed-up bargain listing on eBay, you gotta maintain expectations and be adaptable. Knowing that everything is not going to turn out as planned will keep your sanity in-check and your interests high. So…you find a crack in your differential mount. Big deal. Get a welder, learn to weld, and fix the darn thing. And have fun doing so.

Inspirational words and uplifting sentiment aside, dashboard removal can be quite a task with lots of wires, controls, gauges, and steering columns (er...column) in the way, coupled with intriguing design details that toy with your problem solving skills. Choice time: Do you trust your skills enough to put back together whatever it is that you take apart? If so, read on, Jack.

First things first: Unhook the battery. It’s only 12 volts so you’re not going to get shocked while you’re fiddling around behind the dash, but you could have some nice sparks and arcs if there's power back there. The lead going to the ammeter is live even with the ignition turned off (most of your car’s power goes through here and, consequently, you will not have any power to the rest of the car while the ammeter is disconnected). If you choose to remove the battery entirely, don’t leave it on the floor of the garage or anywhere else near the ground. Mystical forces are at work here and they will drain your battery dead if you let them – a couple of 2x4’s or a shelf will provide adequate insulation.

Before getting down and dirty, there are some prerequisites for dash removal:
- From the engine compartment, free your choke, hood release, and heater control valve cables from their respective mechanisms and feed the cables through the firewall into the car’s interior. You can use a Sharpie to mark the cables as a reminder of position when you reinstall them. Or not.
- Unhook the cable that goes to the heater unit under the dash – the one that controls air flow direction, not that there’s much of a choice there.
- Disconnect the control rod for the vent cowl (one of my favorite features of the TRs is that popup cowl) from the armature under the dash.
- Remove washer fluid lines from under the dash.
- If your vehicle is outfitted with a radio, please unhook it and/or get rid of it now (we’ll be discussing the sound system in a later post).

Now start disconnecting the electrical leads to the steering column under the dash, which consist of wires for the horn, directional switch, light switch, and, in some cases, the overdrive switch. Unless there’s an abnormal amount of deterioration under there, the connectors should separate cleanly with moderate force. Once you think you got all the wires, double check and move back into the engine bay.

Follow the steering column from the firewall toward the rack/pinion and stop at the first joint you come across. Douse it with CRC and loosen the retaining bolt until you have a decent amount of play there. Go back under the dash and unbolt the bolts along the column. Once unshackled, the whole assembly, steering wheel and all, should slide through the firewall and into your pants, setting up a ridiculous scenario in which you squint an eye and say to your buddy, “Arrrg, it’s drivin’ me nuts!”

When playtime is over, you should have a nice amount of room in front of and behind the dash to start removing the gauges, speedometer, and tachometer. Some masking tape and a marker will help since you really should label all the electrical connections before unplugging them.

Although the ammeter, fuel, and temperature gauges are the only ones that work on electricity, all of them have lighting in the form of a pressure-fit bulb. Pop those out, disconnect connections and unhook cables behind the dash, saving the oil pressure for last. Since this gauge is fed directly by the engine’s oiling system, there is a chance residual oil is in either the gauge or the tube or both. Just have a rag handy when you disconnect the oil line. Also inspect the connection for seepage, which could indicate cross-threading or other issues preventing proper sealing. Once the stuff is disconnected, it’s a matter of undoing a couple hand-tightened nuts that secure a U-bracket holding each assembly in place (the ashtray is mounted the same way). The gauge faces are glass, so keep them safe from harm. Don’t worry about whether the gauges are in good working order because we’ll cover how to test all of them in a later post.

So, now you should have an empty dash with wires/cables dangling beneath it. Good progress. When you’re ready, open the glovebox door and remove the glovebox liner – it’s cardboard-ish and should be held in by 6 screws. If you can’t work it completely free, don’t worry; just let it sit in the recess until the dashboard’s been removed. There should be five screws about the dash surface – you remove these and your wooden dashboard will follow.

The dashpad top is next, held in place with glue and a few screws, which are revealed when the wood dash is removed. You also need to unbolt the defrost vents from the underside. What you’ll be left with is the backing plate and the rest of the support structure – don’t be concerned with the surrounding dashpads or the vent ducts – they’ll all come off with the backing plate.

Now you can start tackling the rest of it - move on to the dash support, which is the console-like appendage that holds your radio, envelops the gearshift, and doesn’t really support the dash a whole lot. Undo the four bolts securing it to the floor and then move on up to the plinth. The plinth is the thing directly above the support that holds your controls. Why ‘plinth’? Because Triumph said so. There’s a fastener behind the plinth on the left side – unscrew it and remove the plinth, controls and all, revealing two more bolts on the dash support. Unbolt those and remove the support. Work the fasteners on the backing plate until it can be removed and, Viola! If it wasn’t a project before, it sure is now.

Here's what everything should look like now.