That's right, if the band never contacts the drum no heat is generated and our drums will last forever, so the simple solution is not to drive our cars!
Band + Drum = Heat
However, that is too simplistic. Other factors affecting that are the surface finish of the drum and shape of the drum. So just like the crankshaft thing, here we are dealing with well used 100 year old castings and now you're replacing bands that were well seated with unseated bands and what if you have a high spot or ridge on a drum? What if the drum is conical? That little contact area is going to create a point of intense heat concentration. How about drums that are shredding cotton lining so they switch to Kevlar? It’s not the material, it’s that the drum is too rough. That’s akin to running a water pump because the radiator is clogged up. We have seen drums fail underneath all different types of materials, but it seems people tend to find the issue when there is a band change. That's one of the biggest factors that gets ignored here -- they aren't new parts. So now you're taking an old junky drum that hasn't been magnafluxed, and is almost guaranteed not to be true, and blaming it on the first change in the equation…. but one change can affect all the other variables.
Shown below are a set of planetary drums that run in a completely open transmission (no oil bath). I committed the unpardonable sin of regrinding them. They now have about 2,000 miles on them, running Kevlar, and when recently inspected the ground finish is still present. No lines, grooves, scorching, etc. They have the benefit of not running in a bath of potentially abrasive contaminated engine oil, but they also don’t have the benefit of having a bath to cool them.
I post these photos because, despite the fact that they looked decent as they were, you can see all the irregularities. In fact, I have seen drums posted here that look worse than this running in Model T's and people think they are perfectly acceptable. All those high spots would have been taking the brunt of the friction and heat build-up. The rough spots would have shredded the band and contributed to the heat build-up. Nice localized heat build-up instead of it being evenly distributed over the face of the drum. We’ve all heard the hand wringing and gnashing of teeth that “You can’t regrind drums, they can’t take the material loss”. Absolutely, you can’t grind them down to nothing and you can’t start with a drum that has no life left, just like you can’t start with a crankshaft that’s -0.030 under say you want to take off more. In the long run, they are also consumable parts. However, I’d rather run a good drum with 0.020 ground away and making 100% band contact than ones that are scorching the high spots.
There’s always more to this stuff than “lining good” / “lining bad”.
Start with drums with a proper surface finish, then make an assessment.
Amen ! Herm.
Walter and Herm
good stuff. Thank you both.
Couldn't agree more. How many drums out there are going to get the proper surface treatment? My guess would be nearly all drums in operation are not going to be trued on a lathe so why subject them to the over kill of Kevlar?
After reading numerous threads about kevlar the common theme is the love of how long they last or the never need to be changed. If, in fact, kevlar doesn't wear (as many people claim) then this would be a very good reason not to use them.
That's what I always say -- why regrind a drum when front mains are so easy to replace?
"How many drums out there are going to get the proper surface treatment?"
Just like anything else.... as many as there are owners willing to do it.
Again, it's another calculated risk. People make those calculations based on their own economic circumstances and what they perceive to them as being important. If you want to run crappy drums, then go shred cotton or cook with Kevlar. They also don't make a babbitt composition that will hold up on oval crankshaft journals, either. The most obvious problems in mechanics is usually pretty simple once you get right down to the root of it.
Um, Tim, you do realize that Walter now has 2,000 miles on these drums, post repair, "...and when recently inspected the ground finish is still present. No lines, grooves, scorching, etc. " and he is running Kevlar, right?