chain slack

[JimK]

I was zooming down a nice long hill with a few bumps along the way... when my chain fell off! I could put it back on quite easily without removing the rear wheel. Must be a bit loose!

I just checked - T1-T2 is about 75mm! So indeed, the instructions in the Thorn owner's manual look accurate! My idea right now is to put on a new chain... to rotate through a few chains & thereby minimize the wear on the sprocket and chainring. I saw somebody recommending five chains to match the rate of wear on the chains and sprockets. I have an extra new chain at hand, so that will be number two. After that.... we shall see!

I've only had my bike five months & I can't say I ride it very much - a bit surprised to have so much chain "stretch" already... but maybe it's too much slush and grit and not enough oil. I imagine I will be learning through experience!

Jim

[JimK]

Truth is, I never measured the chain slack when I got the bike from the shop! Anyway, I just put on a fresh chain, and the slack was already T1-T2=50mm. So I took the plunge and tweaked the EBB - my first time fiddling with that.

Wow, just the slightest angular rotation and the chain tension changes hugely! It's very easy to pull the chain fully taut - a mistake, of course! So then I back it off a touch - oops, too much, I'm back to 50mm! Play a bit... I think it's about 25mm slack now - still pretty tight, but it turns freely, so that should be okay I think.

Here I am, plunging ahead and learning! I just hope I don't screw things up too badly!

Jim

[JimK]

Since I have put on a nice new lubricated chain, I thought I would try lubricating my brain a bit too. How close will the little dents be in the EBB shell?

Let me try to estimate. I will zoom on on a section of chain from the measuring point to the chainring:

Now to attempt a bit of math:

The chain length stays constant as I reduce the slack by increasing the distance between the chainring and the sprocket. If I want to reduce the slack by say 24 mm, when the distance from the measuring point to the chainring is about 225 mm, then I need to move the chainring from the measuring point about 0.25 mm. The measuring point also moves from the sprocket that same 0.25 mm, so the total movement of the chainring from the sprocket is 0.5 mm.

The bottom bracket axle is about 5mm eccentric. The shell is about 20mm in radius. So at the position where the axle is at about 12 o'clock, a 0.5 mm movement of the axle should translate to an 2 mm movement of the shell.

So the dimples in the shell should be about 2 mm apart at their closest.

Anybody want to check my math?

[Relayer]

I was zooming down a nice long hill with a few bumps along the way... when my chain fell off! I could put it back on quite easily without removing the rear wheel. Must be a bit loose!

I just checked - T1-T2 is about 75mm! So indeed, the instructions in the Thorn owner's manual look accurate! My idea right now is to put on a new chain... to rotate through a few chains & thereby minimize the wear on the sprocket and chainring. I saw somebody recommending five chains to match the rate of wear on the chains and sprockets. I have an extra new chain at hand, so that will be number two. After that.... we shall see!

I've only had my bike five months & I can't say I ride it very much - a bit surprised to have so much chain "stretch" already... but maybe it's too much slush and grit and not enough oil. I imagine I will be learning through experience!

Jim

Funnily enough my chain came off a few weeks ago, this surprised me a lot since my chain doesn't seem to be particularly slack, in fact it's positively taught compared to some pictured on this forum!  However at the time I was also zooming down a hill and changing up into 14th gear and I think I hit the bump at exactly the same time - no recurrence so I won't fiddle with the EBB just yet, but in future I will be aware when hitting bumps at speed.

P.S.  I am sure you math is good Jim   

[julk]

JimK,
make sure the dimples do have a gap between them, otherwise they just run together in a groove and the eccentric can move easily.

I am sure the eccentric dimples on my stolen Thorn were further apart than 2mm, but I waited until the chain was just about dropping off before adjusting the eccentric.

I still have to adjust the new bike chain, not slack enough yet...

Impressive maths!
Great photos of your rides (previous posts)!
Keep up the posting.
Julian.

[Andre Jute]

+1. I too like JimK's posts and photos.

Andre Jute

[JimK]

My chain fell off a few days ago. That was chain number two, a SRAM PC-830 that my Local Bike Shop sold me. My idea is to rotate chains, maybe three chains, so the wear on the chains more closely matches the wear on the chainring and sprocket. I already had a KMC Z610HX ready to go, so that was today's rainy day project.

Plus some time back I had purchased a Pedro's Demi Torque Wrench, hoping to get myself moving toward more precision work on my big investment. All I got with the wrench, though, was a little set of hex keys to work with it. Today I happened to be near the new Harbour Freight tool store in Poughkeepsie. My very handy friend Dan says that's a great place for good deals. Sure enough, I got a nice socket wrench set for $5. Ah, but when I get home, it turns out the smaller sockets are 1/4 inch drive but the larger are 3/8 inch drive. The wrench itself is 3/8 inch drive, and the kit has a female 3/8 to male 1/4 converter. The Pedro's wrench is 1/4 drive, so now I need a female 1/4 to male 3/8 converter. Well, the local hardware store had the converter - for $5! Anyway, now I am all equipped.

To start with, I measured T1-T2: sure enough, 70 mm! Chains do fall off when they're that slack!

Problem one: how to get the old chain off.

http://www.sheldonbrown.com/brandt/power-links.html

Thanks, Sheldon! (RIP)

Putting the new chain on - it's just a hair too short. All chains are not created equal! I will have to dink with the EBB. Out comes the torque wrench! I pulled the eccentric screws all the way out - sure enough, those dimples are easy to see down the holes! Makes it easy to avoid blurring hole boundaries and ruining the EBB. I dinked with the EBB position to get the chain first connected and then just a little slack. Looking down the screw holes again, no dimples in sight. No idea where the old ones went.

Still pouring rain, and now it's dark. Plus there is a film festival in town, bringing lots of visitors who don't know our roads. I'll wait for the test ride for better conditions!

[JimK]

My chain slack was up to 60 mm, so I decided to stick with my current chain and just tighten up the bottom bracket. This is chain #3, a KMC Z610HX DropBuster. When this gets loose again then I plan to rotate back to chain #2. This way I hope to minimize wear on my chainring and sprocket.

My technique of the moment: I remove one eccentric screw entirely. This way I can look down its screw hole to see the indents on the eccentric shell, to avoid getting them spaced too closely.

And a good thing, too! I have two indents quite near each other, and would have tried putting one right in between had I not been looking and just trying to get the chain right. One existing indent was just too tight, so I backed off to the other indent. This makes my chain a bit loose, but still a solid bit snugger than the 60mm where I started. I didn't measure the final slack... I'm obsessive enough already!

Once I line up the shell with an existing indent square under the screw hole that I'm looking down, I can finger tighten the other screw which will land it right in its corresponding indent. With the shell thus secured, I can spin the pedals a bit, measure slack, or check the system as I please. Once I have decided that I like my choice of indent, then I can put back the other screw and torque both screws down to roadworthiness.

[Danneaux]

JIm,

I believe you're onto something wrt your method of squinting down the holes.  I use this method to adjust the timing chain and set-screw eccentric on my tandem.  It has allen grub screws that enter the BB shell at about the 10 o'clock point, so it is easy to see with the bike upright.

I don't know if this will help you at some future point, but there seem to be two different ends available for grub screws.  Type 1 is countersunk, so it leaves little circles on the aluminum eccentric.  Type 2 is pointed, and leaves centerpunch-like craters.  I know from past experience, if the eccentric is scarred or tracked from one type of set/grub screw, a switch to the other can help.  I'm not sure what types are available in the diameter and thread pitch Thorn use, but it is easy to make pointy-types from countersunk ones; not so the other way 'round.

As for chain/cog wear, I have several restored bicycles that still use 5-speed freewheels, and I wanted to keep them authentic.  Making a template from the trailing profile of the existing teeth -- and reversing it -- makes a dandy pattern for recreating leading tooth profile using a high-speed die grinder or Dremel Moto-Tool or one of the many variants.  I just inscribe the proper tooth profile using a diamond or carbide scribe on blue layout die, then have at it.  One can usually do this several times before the cog teeth are beyond use; one really just has to remove the little "hooks" that have formed.  Yes, a lot of work when replacements are readily available, but the option of choice when the only alternative is the custom-machining of a whole freewheel's worth of new cogs (and heat treatment of same).  You likely won't need this option for your Thorn for at least 50 more years!     I've found even old freewheels can be rehabilitated with new springs wound from piano wire.  I got that idea from Ian Hibell, who made some repairs in that manner and then told about them in some old publications.

I am still trying to wrap my mind around the idea of rotating several partially-worn chains into the mix using the same chainring and cog.  I'm wondering if it really pencils out in the end wrt extending overall drivetrain life.  So far, it seems about a wash on the tandem's timing chain (running between alloy chainrings).

Oh!  I saw a Rohloff-hubbed bike (non-Thorn) with eccentric today. The owner was using a Rohloff chain guide atop the chainring, though the slack looked fine.  A belt-and-suspenders approach to solving chain derailment?

Fun stuff.

Best,

Dan.

[Relayer]

JimK,
make sure the dimples do have a gap between them, otherwise they just run together in a groove and the eccentric can move easily.

I am sure the eccentric dimples on my stolen Thorn were further apart than 2mm, but I waited until the chain was just about dropping off before adjusting the eccentric.

I still have to adjust the new bike chain, not slack enough yet...
Julian.

Like Julian I didn't adjust my EBB until after my chain came off twice in one day.  I was mindful to be careful not to overtighten the chain when adjusting, although I could feel some friction of the chain over cogs when I rode - this wore off fairly quickly. 

I am therefore fairly sure the eccentric dimples on my EBB will be more than 2mm - however I do wonder if I have perhaps created a problem for 'second time around' with dimples too far apart?  Worst case scenario would be replace eccentric shell.

[JimK]

I do wonder if I have perhaps created a problem for 'second time around' with dimples too far apart? 

The pointy eccentric screws make conical holes in the eccentric shell that are maybe 3mm in diameter. So probably the ideal pattern would be to have a hole every 3mm. If you have holes 6mm apart, then you can just make a new hole right in the middle.

Seems the worst case would be if a person stumbled into making a pattern where the holes were about 5mm apart: just a little too close to allow new holes between existing holes. My guess is that that's still not a severe enough problem to warrant replacing the eccentric shell. One needs to be able to start from a situation where the slack is 60mm, move to the next tighter hole in the shell, and still have a bit of slack in the chain so one isn't stressing the hub bearings.

My guess (informed by a bit of geometric analysis) is that the whole system measures out so that the worst case spacing is small enough to allow any necessary slack adjustment - this sort of system has been around a long time! The tricky spot is when the eccentric is oriented at 12 o'clock - the least rotation of the eccentric makes the biggest change in chain slack. Anyway, the nastiest situation would just force a person to allow the slack to grow a bit beyond 60mm before the notch spacing permitted tightening.

 

[JimK]

An experiment failed today!

Nice weather seems to be settling in so I decided to take off the studded winter tires and put on some new 26x2 Duremes. Might as well add on some other projects: mount some Buddy Flaps, and check the chain tension. Ach, the Buddy Flaps went on a bit crooked! I have never been much of a precision craftsman!

The chain T1-T2 was up around 65mm... sure I could just tighten up the EBB, but my idea had been to rotate chains. The present chain is chain 3 which I already tightened once. So I went rooting around in my parts bin and pulled out chain 1 which was only on the bike a few months and got removed as soon as it got loose. But now chain 1 is totally frozen up! It doesn't look rusty, but almost every link is very stiff, and many are unbudgable. Ouch!

I guess somehow I need to get chains good and oily before storing them away!

Seems I need to inspect chain 2 as well!

[Danneaux]

An experiment failed today!

Oh, no! It may have been Edison who said something to the effect that an unsuccessful experiment is never a failure if you learn something from it. Looked at that way, today was full of success! Still, I know what those days are like and you have my empathy. There's times when I just know I daren't touch anything or I'll break it. Fortunately, most times it isn't a problem.

Jim, hang onto those chains. They can be revitalized by a soak in diesel if they aren't just covered with rust but merely stiff. Even so, they might be good for a bit more use. I've brought a number back from the dead.  A generous spraying with Tri-Flow does wonders, too; just allow it to sit overnight in something like a plastic cottage cheese carton to contain the mess.

I know this is rank heresy, but I've never gone the chain-rotation route, and prefer to simply replace my chains when they closely approach their wear limits. Doing this, I've experienced remarkably long chainring and cog life, and the total cost seems to be about what it would be if I swapped worn chains into the mix. I have a few bikes with old (really old!) freewheels and cogs, where I've revitalized them by using a high-speed die grinder and a template to evenly remove the hooks from the cog teeth and reprofile them. There's usually plenty of metal, and formerly hardened cogs can be re-tempered easily enough through heating and quenching. This worked a treat on Dad's 1938 Hercules. I think arranging my derailleur gearing so my favorite gears run with little chainline deflection helps extend drivetrain life, too. While I'm at it, I shift my derailleurs so they're slack after riding. Over time, it makes a difference on spring tension. That won't apply to Rohloff hubs, of course.

You may find a skewed BuddyFlap to be an advantage, as water will sheet cleanly off the low corner instead of blowing back randomly, so another success there. All is not lost here, either. Just ovalize the two holes (opposite sides) in the BuddyFlaps and they'll line right up vertically. The bolts that come with them have plenty of flange size to cover the adjustment you'll need, no problem. Any chance of some pics? I sure like my front BuddyFlap, and think it adds a bit to Sherpa's overall appearance. I moved the original SKS kick-flap from the front to the rear fender.

How do you like the Duremes?

Best,

Dan.

[JimK]

an unsuccessful experiment is never a failure if you learn something from it.

Yes, I am trying to learn some bike mechanics along with trying to building some physical endurance.

They can be revitalized by a soak in diesel

Thanks for that! I looked at chain 2 and it seems fine. I have sprayed some lube on chain 1 & will look for a suitable container for a more serious soaking.

I've revitalized them by using a high-speed die grinder

Ha! What a blast! It's certainly true that my scheme is intended to reduce wear on the chainring and sprocket. I dream about having lots of nifty tools and the skill to use them, but I doubt I am to to make it quite as far as grinding with a template! Hmm, maybe just a simple hand file, a round file... I might have one of those someplace. Actually my life seems to be headed into ever more nomadic patterns, trimming the kit back almost to what fits on the bike. If we move into a space that's half of what we have here or less, ah, will I be stuck locking the Nomad up outside somehow? In a college town, no less? Life can get difficult!
 

I shift my derailleurs so they're slack after riding.

I remembered to do that with my torque wrench! I put 13 Nm on those EBB bolts - that'll make a nice dent!

Just ovalize the two holes in the BuddyFlaps and it'll line right up vertically.

I had some struggle with the front flap. The SKS fenders have a brace right near the bottom that interferes with the mounting of the flap. The SKS fenders come with a front flap, but I think mine may have been damaged when the bike was shipped from SJS. Anyway it was barely hanging on & I removed it some months ago. I drilled a third hole in the flap and also had to run out to get a longer screw to handle the extra width from the brace. Yeah someday I may try the ovalize trick but right now the silly things are on and I will just see how they handle the water as is and enjoy the advantage of the broken symmetry as you outlined it!

How do you like the Duremes?

I just went out for half a mile to make sure I had things put back together right. They're a lot quieter than those studded tires! Plus for my test ride I had my bags off and my water bottles off so that must've cut the weight practically in half. Wow, the bike is a lot spritelier that way!

Looks like Sunday will be my day to get out for a ride.

[Danneaux]

You did fine, Jim; all will be well tomorrow.  

If we move into a space that's half of what we have here or less, ah, will I be stuck locking the Nomad up outside somehow?

Oh! Don't forget this gives you a license to become creative. You can always store Nomad on-end from a convenient hook.

This would be the perfect opportunity to look into Andre's stem-rotator and a pair of MKS quick-detach pedals. Just twist the stem so the handlebars are inline with the, unclick the pedals, and you could likely store your faithful steed behind an open door.

I've had friends who've stored their bikes in car trunks (you've done it already! Not so convenient, but pretty secure!).

Or, you could do what my friend Miguel did when he lived in the dormatories at University of Oregon -- he stored his handbuilt Toei under the bed! It worked great, and kept it secure from thieves who regularly stole bikes from the racks downstairs. If you want more suggestions about storing More Bikes in Less Space, give a shout and I'll be happy to help. I'll stop here so I won't dilute the thread. I'll start a new one here: http://www.thorncycles.co.uk/forums/index.php?topic=4028.msg18135;topicseen#msg18135, as this is a common problem for many.

Best,

Dan.