Limited Slip Differential

I am not making this up: I drove under a man today as I looked into his eyes . . .

But, that was a little later.

It was, however, before I got close to that trash can up there, to deliver the mail at 9*** SE 70th Ave. I delivered, then put my phone out the window and snapped. I cropped the photo, but did not alter the angle or perspective of it. It’s for any doubters out there about that centimeter . . .

I showed up for work today after having been off yesterday, and did not really know what to expect, only that I would be on Route Salvation. I really liked that, it being after Christmas and all, and I was ready for a day of wonder and amazement. Yesterday must have been brutal at my station, because I was hearing things, mutterings, talk of how they’d gone “off assignment”, and there was standard mail still around, “rolled” from yesterday.

Even though yesterday for me was worthy of another blog post, it had been very difficult for others. And I was thinking of them yesterday, and was again today. And today looked better.

I got it all ready, loaded it, and let’s just say it was a light day. Where during the Christmas rush, I would have 75-90 larger parcels plus all the little ones that will fit in most mailboxes, seven trays of flats (magazines, catalogues, etc.), and 6-9 trays of mail, today I had a record seven parcels, three sparse trays of flats, and one tray of mail.

They wanted everyone back by 4 p.m. so I walked out the door counting on that.
“Hey Timothy, can you take an express package?” (expensive, time-sensitive, and this one international). Sure. So, I got the express package, left the station and turned onto McLoughlin Boulevard. I looked at the package. Oh, the express is not on Salvation, so I definitely need to do it first. It is on . . . God. It is an express package on my God route. Perfect, to deliver an expensive, time-sensitive package to God, and then proceed directly to Salvation, which you may recall is a route, from the beginning of which you can see the end.

But before that, I’d had to travel under that man in the sky. It is true, as I left the station and drove north on McLoughlin there was an electric company truck with the bucket of its “cherry picker” suspended out over my lane. The guy in it was just floating over the traffic, and as I approached he saw me and nodded slightly, saying go ahead. I did, slowly, drive under him, and we smiled as I passed under him toward Salvation. But God first, so I went there and delivered the express and then pulled over. To think about it all, and to adjust one of my left mirrors.

I got out, walked around (remember I am driving on the right side), and tilted the mirror down toward my vision. Just 1 or 2 millimeters. Let’s say 3. A slight angle change can alter the line of sight dramatically, increased by distance. Moving the mirror very slightly in my direction caused me to see much more clearly what I wanted to see which is what I needed to see. And I trust that; I trust my mirrors and that tilt. And I had started with God after going under a man in the sky on my way to Salvation anyway.

I started Salvation from Bell Avenue looking at the end point up there ahead, before turning off. Oh, it is really icy on these side streets!  Ok, slow down and just watch the approach. I was on the route now, and could no longer see the end. It would take all day, on icy streets, but I had seen the end. And I would trust my mirrors and was so thankful for the tilt, as I pulled from box to box on icy streets toward the end that I had already seen.

And I just did not worry about time, because there is a tacit agreement that if you are given more work (like an express to God) after you give an ETA, then you are free of that original deadline. And I did not worry about sliding because I have a limited slip differential in this truck. You see, when you turn a corner or go around a curve, anything but straight, the wheels on the outside of the curve are going farther (making the axle spin faster) than those on the inside. So you have two axles connected by a differential that allows for that. It is very simple geometry, although the engineering is a little more complex. But, it’s been done for us, and it works. Here’s how: when the speed or travel of two complementary wheels is different, power (technically torque) is shifted between them. That’s great on a surface with good traction. But, on ice, well, if one tire spins, it needs no torque, so a normal differential thinks the other doesn’t either. And you are stuck, just spinning your wheels. And in a mail truck, driving all day on the shoulder where there is ice all day, well you’d be stuck too, just spinning, unless . . . your differential is limited slip. Then, you always have torque to at least one wheel. If one spins, then more torque is applied to the other one. And if one needs torque, it can rely on the other sharing some. I could count on traction on the left side, on the pavement, and my right wheels could be mushing along in the snow and ice all day, slipping and spinning which happens occasionally. And I accounted for that when I sorta slid within a centimeter of that trash can, just for kicks.

And, just for kicks, even with the express, or maybe because of it, I was off by 4:00.