Tuesday, May 13, 2008

Hollow Pointless

Have you ever had a chain snap on you? It's not fun. Usually it happens when you're really stepping into it, which makes the results that much worse. Knee to the stem, thigh to the crossbar, usually you go over the bars. If you're lucky, you land on your hands instead of your head. If you're not lucky, it happens when you're trying to sprint through a busy intersection and you get run over by a bus. Either way, it's not much fun. But hey, chains break. It happens.

That said, isn't the chain the last thing you'd want to lighten up on purpose? There was a time not so long ago when BMX chains were judged the same way hip-hop chains were: Heavier was better. There was the KMC 415h (or Kink chain) and the massive Sharp 420—which was better suited for a motorcycle or a garage door opener. Had things continued in that direction, we'd all be riding chains that look something like this (minus the feet):


They didn't continue like that, of course. Gearing dropped, so you didn't need to worry about your 45 bashing into everything anymore. And with chainrings out of harm's way, chains were free to get lighter. People went with traditional KMCs, like 410s, 510s and 710s. But that wasn't good enough. Oh no. Road chains had long used drilled-out pins and hollow plates. Izumi made a hollow-plate BMX/track chain back in the day. Why not do the same with new BMX chains? So KMC introduced the 710sl:


It came in at 365 grams as compared to 420 (!!!) grams for the regular 710 (according to Fat). Not much of a difference—less than two ounces. Of course it comes with enough links to run 48/16, so the weight difference is presumably less if you're running 28/10 or lower. And it, um, looks cool.

There are differences between road and BMX, however. Small as BMX drivetrains are, the chains still take hits every once in a while. And running it on a eight-, nine- or 10-tooth driver places a lot of stress on the chain, even if it doesn't have drilled-out plates and hollow pins. KMC can give you numbers for "pin power" and "breakload" all they want, but those don't take into account bashing a drilled-out link on a rail or ledge and then cranking full-out towards something else.

Look, I appreciate that plenty of people run drilled-out chains, and a majority of them are still very much alive. But of all the things to drill out, the part on your bike that arguably takes the most stress? What's next? Steerer tubes? Pedal spindles? Heck, even the Grim Reaper makes more sense to me than this. There are certain things I'd rather not see drilled out. Chains. Stem bolts. Forks. Anything that, if it snaps, will probably send me to the hospital or the dentist. And even barring injury, I'm not a big fan of liamfahyhamptoning around. So I like my chains like I like my teeth and my Alaskan wildlife preserves—undrilled.

(Of course, seeing that half-link chains have seemed to have more problems with snapping than regular chains, there's no way anyone would come out with a drilled-out halflink chain. Well, no one except KHE. Lovely.)





P.S. Sean Burns only rides Wipperman chains. Their 1G8 is apparently the strongest chain on the market, according to—um, them. This video has nothing to do with chains, but a minute of Sean Burns never hurt anybody. Well, except him.

11 comments:

Brady said...

I thought that eastern already did a fork with a drilled out steerer.

James said...

My chain breaking is always in the back of my mind when I'm riding through the city,that and car doors opening.I would never run a chain with drilled out holes,someone is going to fuck themselves up with this.

brien said...

dick chains: http://assblasters.org/?p=412

Russ said...

Totally meant to link to that Tunney post and forgot. Thanks.

And I wasn't sure whether Eastern actually did the slit steerer or if it was just a bad dream. I kind of prefer to think all of Eastern's products are a bad dream.

Anonymous said...

The kmc hollow chain is the best chain i have ever had it is so smooth and looks good/differnt compared to others also i have had it for quite along time now with no problems.

VxD said...

"my chain looks good" has to be the worst defense for a chain.

Wouldn't the absurd gear ratios road bikes can achieve, coupled with the penchant for sprinting many road bike riders have, actually cause more stress in regular riding (I understand that bashing the chain is the real problem)? I agree with the principal of the argument, that if there's any chance of your chain taking a hit, don't skimp on material.

Russ said...

I'm certainly no expert when it comes to the physics of it all (where's George French when you need him?), but road chains are a LOT longer, so one would think the load would be dispersed over a greater area. And even the smallest cog on a road cassette is something like an 11.

Also, at least when it comes to the pro circuit, I would assume chains are replaced on a semi-regular basis. BMX has always taken more of a "oh, I'll replace it when it breaks" approach. Not the best idea when you're dealing with double-butted this and drilled-out that.

g. edward jones, jr. said...

Russ,

The short version of the physics of the situation is that the primary load is carried by the links on the teeth of the drive sprocket. On a 44 tooth sprocket that meant that, maybe, 8-9 links were sharing the load of each crank. On a 25 tooth sprocket that force is split amongst like four teeth and is actually increased (the smaller sprocket means that your pedal is farther away which means more leverage). There is an article about this on gsport.co.uk, and it brought back freshman engineering courses I'd rather forget.

All of this is to say that as drive sprockets get smaller, the force on the chain gets higher, but we - paradoxically - use more fragile chains.

In comparison, AFAIK, most road and mountain bikes don't go too far below 35teeth for their front sprockets because of the inherit gearing disadvantages of 26" wheels.

ssnnakebite said...

510 ftw

g. edward jones said...

http://www.gsportbmx.com/tech/super_small_gears.php

Eh, I got the numbers wrong, but still, here's the meat.

"So let's compare a 44 tooth sprocket a 36 tooth sprocket and a 25 tooth sprocket.

A 44 has a working radius of about 89mm so on a 180mm crank that’s almost bang on 1/2 way out. Therefore if you can press down with a force of 1000 Newtons (which you might well be capable of) the chain will have to take a load of around 2000 N.

A 36 has a radius of about 72mm so on a 180mm crank the chain tension will be 180/72 x 1000 which is about 2500 Newtons.

A 25 has a radius of only 50mm. With our 180mm crank we get a chain load of 3600 Newtons!

So on a 25-9 gear set up the chain has to take nearly twice the tension of an old school 44-16.

But there's more. On any sprocket the chain wraps round approximately half the teeth, so on a 44 tooth sprocket that 2000 Newtons is spread over 22 teeth. Theoretically each tooth only has to take a load of just over 90N. With the 25 we have to spread 3600 Newtons between just 12.5 teeth so each tooth has to take nearly 290 Newtons over three times the load! Infact the chain can't share the load out evenly over all the teeth so it isn't quite this bad but it is a massive increase in contact force.

Similarly the rear sprocket is massively loaded. At the back end that same 3600 Newtons has to be shared between just 4 teeth! Nearly 900 Newtons per tooth (compared to 400 for a 13 tooth rear sprocket). This load also has to be carried by the individual rollers of the chain.

So the forces involved are much much higher than in a normal drive train but the question is are they TOO high?

Well industrial chains of this size are typically quoted as having a tensile strength of just over 8000 Newtons, but this is not a working load this is a failure load! And this is a failure load under normal industrial operating conditions well lubed and looked after and new. The chains we use are cheap foreign ones that we treat like shit…."

And years of this stuff is why I changed my major

ted said...

z510hx for life.