Friday, October 24, 2008

How To Shift a Big Rig

Most people look at shifting a Class 8 truck as some sort of voodoo, best left to the snaggletoothed unshaven ranks that have mastered the dark art. However, it ain't brain surgery, cause there are plenty of idjuts who manage to do it every day.

Probably the closest the average person has driven that can compare to a truck tranny is the old four speed in the pickup at the farm - you know, the one with the "granny" gear that everyone except old Grandpa would grind shifting? The reason for that was those old four speeds had the top three gears synchronized and the lowest gear was not. Synchros keep all the gears spinning when shifting, and the closer a gear gets to engaging the next, the more the synchro will, well, synchronize the gears so they don't grind. If one gear is spinning, and the one it needs to mesh with is not, then the gears will grind until their speeds match and they can mesh. Synchronizers are usually good for about a 100k miles or more. They do wear out. You may have noticed in an older car with a manual that some of the lower gears really catch unless everything is just so - the synchros are wearing out.

In a truck that may put on 130k or more miles in a year - well, synchronizers just can't handle the stress. So, most diesel trucks have transmissions that do not have synchronized gears, period. This means that when the tranny is shifted, the gears must be synchronized and match engine speeds. Think about your car in second gear. It might be running about three thousand rpm. At the same speed, the engine speed in third gear might be seventeen hundred. So, different gears have different engine speeds at the same ground speed. Another difference is the rpm range - diesels usually have about a five hundred rpm drop, while gasoline motors might be as much as two thousand.

So, we "float" the gears and don't use a clutch to shift. Some people use the clutch to help "break" it out of gear, but if you have any torque on the transmission, no amount of prying will let you shift it out of that gear. So, the preferred method to upshift is to ease off the loud pedal, and when there is neutral or no torque, pull it out of that gear and move the selector to the next gear. Then, the rpms must match, or it won't slide in. Usually, when you ease off the accelerator to take it out of gear, you continue the motion to continue to let rpms drop while catching the correct moment to drop it in the next "hole." It is all a matter of timing, and all trucks are a bit different. Some, the rpms seem to take forever to drop, some drop too fast, the tranny might be "loose" or "tight." Another little problem is compression brakes. Most trucks are set up so that if you have the "jake brake" on, releasing the accelerator will switch it on. You don't want to have the jake brake killing your engine rpms unless you can shift very fast. You have to learn to "feather" the loud pedal to keep from turning on the jake, or just get in the habit of shutting it off and remembering to turn it back on when you need it. Most prefer it on all the time, so when they lift it will come on and start to slow the truck.

If a gear is missed, and it happens to me several times a day, then you have to try again to match rpms with the selector poised at the gate, as it were. We "grind 'em in" quite a bit, frankly. The gearshift will vibrate in your hand as the gears start to mesh, and the "notching" slows and stops as the gears finally match.

Shifting down generally requires a reverse strategy. You are generally slowing anyway and need a lower gear to help slow the truck with the said gear, so your foot is usually off the loud pedal. In order to get neutral torque, ya have to ease into the accelerator a bit until the stick slides out, then you have to ease into the pedal a bit more to increase engine rpms so it will slide into the next lower gear. Again, it's all about timing and practice.

This is the shift pattern for the common and lowly nine speed direct transmission. It is basically a five speed pattern with reverse to the top and far left. Low is the first gear, then one through four are the next, giving you five speeds in the low range. See the picture of the lever on the shift knob to the right? That lever is on the front of the shift knob on top of the shifter. It has to be in the lower position to access the low range. Once a driver has run through the "bottom" five, he (or she) will preselect the high range by pulling up on the range lever. The next gear is found in the hole marked 5/1. The next three gears are in the same sequence as before, only the pattern is of a four speed rather than a five. So, five gears in the low range and four in the high gives you nine speeds overall. An interesting thing about these transmissions is that you can shift into the "LO" hole in the high range, but the gearing is exactly the same as the fifth gear in the low range. So, technically speaking, you may have ten gears to select, but only nine are different.

A lot of the fleet trucks out there are equipped with nine speeds. They are fairly simple, rugged and light. In a speed regulated truck that won't be allowed to run over sixty seven or so mph, they are ideal. However, most owner operators and a lot of other company guys (think me here) would like to have a little more performance. You can spec a truck with extremely high rear gears so that a nine speed would run quite fast in the top gear, but then you'd lose flexibility in tough pulls, because the gears would be "spread too far" to accelerate in a decent amount of time. Thus we get overdrive transmissions.


Probably the most popular geared trannies are the thirteen speeds. There are a lot of them that have different torque capabilities and gearing, but they all shift the same. Now, in the bottom picture, you can see the shifter head has a splitter on the side. The top picture of the shift pattern shows that the top four gears can be "split." If you are in the number 5/1 hole in the high range, and your splitter is in "low," the next gear is reached by flipping the lever forward, then just lifting off the "gas." It will drop into the next gear without moving the shift lever. The next gear would be in the 6/2 hole, and the splitter would have to be in the low position. It is a lot easier to shift it leaving it in the high position until the shifter is in the "hole" and then dropping the splitter. Otherwise, if ya miss, you'll be trying to get about three parts of a transmission to mesh at once, and in some cases, you might have to just about stop and start over. At any rate, with the top four gears split gives you eight gears in the upper range, plus the five in the lower range makes thirteen speeds.

An eighteen speed has deep reduction available on the lower five gears, so there are ten lower range and eight higher range gears available. Ten speed directs use the L hole in the high range - there are five high and five low gears. There are ten speed overdrives that have the top two holes switched. If you look at the pattern as a five speed with low as one and four as the five hole - the four and five holes are switched. Going from the three hole normally means over and up, but in a tranny with the inner gears switched you would go over and down, the next gear up. "Lets put 'er to the dash and into the wind" used to be a saying on the CB - a variation of the ten speed overdrive had fifteen gears and it was very popular with owner operators wanting to go fast. The top two gears in the transmission were actually swapped to do this, and the problem with these transmissions was the rpm drops weren't consistent. The three to four shift had very little rpm drop, and the switch to the "big hole" had a large rpm drop. It took a real horse to pull those trannies.

There have been a lot of different transmissions and shift patterns over the years. A lot of older trucks had auxiliary boxes - they actually had two transmissions. One might be a five or six speed with reverse, and the auxiliary usually had four. It took some skill to shift those quickly - it usually required two hands, one through the steering wheel, shifting both shifters at the same time. My old boss picked up an old Peterbilt with a ten speed overdrive with a four speed Browning auxiliary box. I never took it out of the third range on the auxiliary - it had a 318 Detroit, and it would barely pull itself bobtailing in the high range, much less with a trailer with G_d forbid, a load. Another truck I drove once had a fourteen speed Spicer transmission. It was really an air shift 5x4 - like one of the two tranmission types, but it was all in one huge case. You could split each hole four ways, but the usable gears cut it down from twenty to fourteen. In the first "hole," the top two gears were identical to the bottom two in the second hole. So, you only split the bottom two on the first three gears, then you could split the top two holes four ways each. Six plus eight equals fourteen. It was hooked up with a pretty hot (for it's day) Caterpiller motor, and it was a very awesome combination. It would run over 100 mph and outpull plenty of trucks besides. But, those old Spicers were heavy, expensive to repair, and not very long lived.

So, most of the trannies out there are either nine or thirteen speeds. The truck I drive has a thirteen. Most of our other trucks have eighteens - the idea is that we have to "walk in" tanks sometimes - we have to carry the tanks from the trailer to the site for twenty or thirty yards. The slower the speeds, the better, so deep reduction in that kind of scenario is a good thing.

And, I don't start out in low and work my way through each gear each and every time. I usually start my truck out in "third" or "fourth," and skip the splits in the high range until I get to the top gear, or sometimes the top two. My motor has enough torque to handle most of the shifts and the rpm spreads just fine without resorting to hitting every gear. Climbing a mountain? I might split some of the lower gears.

One more thing - in order to get these things into gear while stopped requires a clutch brake. When you depress the clutch all the way to the floor, the clutch brake engages and the transmission is stopped to allow you to drop it into a gear without grinding it to a stop. When you hear someone grinding into gear while stopped, it usually means their clutch brake is worn or they are an idiot. So, if you are a clutch user while shifting, you have to remember to only use the first part of the clutch travel and not engage the brake going down the road. All that does is fry the brake far far sooner.

So, this is probably way more complicated than necessary. It's really just timing. We all miss gears, too. Some claim they don't, but they are the guys with brown eyes as far as I am concerned. I don't believe the other utterances that fall from their lips, either. If for some reason, you have the opportunity to try one out, don't be too scared about shifting the dern thing. That is the easy part. Not running into things is the hard part.

13 comments:

The Local Malcontent said...

"Not running into things is the hard part."

I turned white as a sheet reading all this.

Jeffro said...

Snerk! Glad I could help out!

Old NFO said...

hehehe- I learned on a 57 Mack with the suicide shift :-) "fond" memories! thanks for an excellent tutorial

Earl said...

Now this is why I nominated you for the Superior Scribbler Award, more information than I will ever use, the Army forced me to use a clutch and learn how to shift in three and four gears and you actually think one would be able to do eighteen gears? Luckily I work in a library and my motorcycle only uses five gears, although six wouldn't be beyond me one day. Thanks.

Anonymous said...

And here I thought the old panel truck I once had to drive at a warehouse job -- old enough it had to be double-clutched -- was a challenge.

Anonymous said...

As a long retired 'gear jammer' I enjoyed the reminisce!. Learning to drive a Ford Model without the clutch made the 4x3 a no brainer. 5x4s and 6x4s were mountain gears which the 13s and 18s were highway gears.

Anonymous said...

isn't it interesting that the heaviest vehicles on the road have shifting patterns a bit like the lightest things on the road--bikes. I used to race and tour bicycles and the discussion of "low and high range and "splitting" gears isn't that different from a bike. when I started it was two speeds at the front and five at the back making ten. Now it's as much as 3 and 9. For racing you'd set it up L1,2,3,4 then H2,3,4,5. For touring the gears would be lower and closer together and the pattern might be L1,2,H2,L3,H3,L4,H4,H5.
Never driven anything bigger than a ford van, but actually understood most of that, cos it's like a bike!

Wingnut said...

great article. i learned in a 10 speed, worked with an auto, and will soon be switching to a 13 speed. cant wait! those autos will spoil you fast but i'm excited about learning the 13.

Anonymous said...

I've been using a manual 5 speed dual speed axle for 35 years but this transmission you have in the article was the one I used yesterday on a road test and I failed miserably ...

Marvin said...

I really enjoyed this article. I'm a retired school teacher, but in my youth I drove school buses. The first was a forties era that had, as I recall, only three speeds, but no sincro in one and two -- I can't remember whether it did in three -- but I'd already learned to double clutch. Later I drove a late fifties or early sixties Ford bus with a 4-speed and a split rear, hauling kids between Chicago and Carlinville, IL for a church camp. I loved going through all eight, even though there was never a need, even with a load of kids. The only time I tried to shift without a clutch, though, was in a 1967 Dodge 3/4 ton with a load of hay. The clutch linkage broke early in the trip, and I drove sixty miles home. Of course, in those days you could start in gear without holding the clutch to the floor.
I'm still pretty mystified by twin sticks, but I think I could manage a 9-speed with your help. The thirteen that changes the pattern for the top gears would probably mess me up at least part of the time.

Unknown said...

I admit shifting in a truck is a lot harder than in a small car. The truck just has such a long gear shift. It's almost hard to feel it go into a new gear. I think I have to agree it's some sort of voodoo. http://www.himspares.com.au/ProductBrowse.aspx?Category=SX09

The Shadow said...

The ten speed isn't the only tranny that allows the gears on the right side of the shift pattern to be turned around. I've seen it it done with 9's and 18's, although not every 9 or 18 will work, consult the manufacturer b-4 opening the box.
Placing a quad box or a two speed rear axle behind a regular tranny can provide the extra gear ratios required for hauling oversize and superloads. Regardz.

Unknown said...

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