jim p

My large ring is trying to down shift about every 10 pedal strokes. The chain will start dropping to me middle ring but will shift back up to the large ring in about 1/4 pedal stroke. If I let the deraileur rub along the chain then the chain will not try to down shift. The small and middle ring don't try to auto shift.

Is this a sign of a worn chain ring? I checked the run out on the ring and the ring appears to be running straight. I am not a strong rider and if the frame is flexing I better start looking for a cracked frame.

If you have any ideas about what I can do to keep the chain on the large ring without having to let the deraileur drag, please let me know.

dddd

Assuming that the chainline is within the normal range, i.e. that the bb spindle length is correct, it sounds like possibly just a bent tooth on the big ring.
That's all it takes to derail the chain. I deliberately bend teeth on the middle ring when downshifting needs to be more assertive.
Even if the tip of a tooth is bent over, that can significantly assist the downshifting process to a fault, especially as the big ring causes maximal cross-chaining when using the bigger cogs in back.
The fact that it's BioPace should have no effect, since while the gap between the teeth and the front derailer cage varies, there is normally no contact between the chain and the cage except while shifting.

Look for a suspect tooth on the big ring, and bend or bevel the right-hand side of any distorted tooth, or you could slightly bevel all of them to assist chain retention.
Out on the road, I've used a rock against the rotating chainring to bevel all the teeth at once, but with this side of the teeth the crankarm would get in the way unless the ring is removed for some tooth massaging.
Any missing or broken-off teeth will also allow the chain to cross over when the chain is crossed.
Start with a clean chainring and use magnification and good light as needed to discern any bad teeth.

Bill Kapaun

You might fill in more of the details like the fact you switched from a double to a triple like you did in the other forum you posted this in.

https://www.bikeforums.net/showthread...ce-chain-rings

ultraman6970

Find out and patent it, a lot of guys might like it.

jim p

I cleaned up the large ring and took a close look at it and it does not appear to have any problems. I did notice that the teeth are lower by design at some points around the ring. Both the small and medium ring also have these lower teeth at the same locations.

I checked the wear on the chain and it is about 3/32" longer over a 12" section. The chain also had a very little tendancy to want to stay on the large chain ring when I back pedaled the crank by hand. The bike is an 8 speed and the chain line on the large ring seems to line up with 6 th gear or maybe 7th gear. The bike was trying to shift when it was in 5 th gear. I didn't test the shifting in 6th 7th, or 8th gear. I am thinking that the top 3 gears work perfectly.

It looks like I need to change the chain due to elongation. Is there much of a chance that this will fix the problem?

onespeedbiker

Assuming all you did was bolt on a Biopace triple to replace a double crank of some kind (Shimano?). The most obvious answer would be not using the correct spindle length for a triple. Square taper cranks are all mated to a specific length spindle; in some cases a triple of one model will have a shorter spindle than a double of another..you said the chainline has the big chainring lining up between 6th and 7th gear. With a triple 8sp the chainline should line up the middle chainring between 4th and 5th (with the big chain ring probably between 5 and 6). With your being between 6-7, it seems to indicate your crankset is too far outboard by 3-5 mm (probably due to spindle that is too long). This could easy explain the chain being pulled of the big chain ring when in the lower gears (1-5). Also the big chainring may be beyond the range of the front derailleur, also resulting if the chian being pulled off the big chainring.

However I would also suggest you replace you chain as it has lengthened to the point I replace my chains.

jim p

I am going to replace the chain. I use cheap chains and they only last 500 miles or so. I don't understand how some people get 3000 miles out of a chain.

The bikes shifts ok as far as I can tell I have a downtube friction shifter for the front and it does have a triple front deraileur.

Even if my chain line is off by 3 mm, it would seem that I could cross chain by 12 mm or more and get away with it. But this is not the case for my setup. Also it seems that once the bike is shifted that the type deraileur would be a non issue as far as the chain staying on the chain ring. For proper operation the deraileur should not be touching the chain after the shift has been made.

Thanks for the suggestions. Getting bikes to work well can be interesting.

cny-bikeman

1. I've never seen a chain that will last only 500 miles under any kind of normal use. However, If you cross chain a lot and push high gears often you probably could manage to to so.

2. These two statments imply that you do have a triple, but you have not directly answered the question of whether you converted to a triple (possibly without changing the BB spindle), which would indeed shift the chainwheels out. Also, what you can "get away with" is not dependent on how things seem to you - cross chaining with a bad chainline can indeed cause problems. You did not specify in the original post in which gear(s) you are in when the problem happens. If you want good help you need to provide good information

3. That much is accurate

jim p

I didn't replace the bb. It is pretty short and if I made it shorter to get a better chain line with the large ring then the small ring would be rubbing on the frame.

ThermionicScott 

3/32" over 12" is pretty darned bad. Do you ever lubricate your chains?

dddd

It takes me 5,000 miles to wear a chain that far.
Cross-chaining couldn't cause such rapid chain wear, but at some point can derail a chain.
If this is "industrial" chain, it won't be appropriate for bicycles, but a $12 KMC "Z" derailer chain always lasts a couple of thousand miles or more on a road bike.
3/32" over 12" is worn out to the point that rapid sprocket wear will follow.
What conditions is the bike ridden in? Swamps?
I have to agree that the bb is appropriate if the little ring now has only 1 or 2mm of clearance with the frame.

cny-bikeman

We still need to know what front/rear combo you are using when the problem occurs.

jim p

The auto shifting is happening in the 48 tooth front gear and the 4th largest cog on the rear. I think that the 4th cog is a 20 tooth.

3/32" wear over 12" is 0.7% wear. I thought that you could go as high as 1% wear. What percentage wear do you guys shoot for as the max wear on a chain?

As far a chain lube, I drip some motor oil on the chain about every 100 miles. Then I just let the chain sling the oil all over the rear wheel and wipe things down about every 2 weeks. Since I only do hill repeats it takes me about 6 months to put 500 miles on the chain. I did make an exception this weekend and rode 63 miles on a course with only about 3000 feet of climbing.

jim p

What happened? Did you guys get tired of pulling my chain?

rccardr 

Could be. But I'm still intrigued. If the inner ring is close enough to the chainstay that you think a shorter spindle would make it hit the stay, you're fine with the BB. Assuming your triple FD has a straight cage front to back and top to bottom, it's not the FD- especially since you have a friction DT shifter up front. And I've never seen a case where even extreme cross chaining will actually cause a big ring shift. So, three things remain:
* How close to the big ring does the FD cage sit? Should be about the thickness of a nickel as the cage passes over the teeth on the LARGE LOBE of the BioPace ring (there will be more space when the small lobe is under the cage
* Is the large ring installed properly on the crank arm? There's a small shaft attached to the outside of thebig ring and it should be placed UNDER and toward the crank arm. The middle and small rings have a 'V' stamped into them, the V should be installed so it's in the same orientation as the shaft on the big ring, but facing inward
* Is the large ring so worn out that the short/shar toothed teeth simply can't keep it on the ring?

BTW, pics might help.

onespeedbiker

Well yes and no. While the majority of time the chain will not be rubbing on the RD fence, if installed properly the FD should be low enough so if the Chain tries to derail off the big ring, the cage will stop it; this is why rccardr wanted to know;

Also the reason your was because you pretty much shot down eveyones' idea that was trying to help..

jim p

Thanks for the additional comments. I will try to get some pictures up so that you guys may see something that needs to be fixed. I have been working on the cv axles of my truck for the last couple of days and it has been raining in the evening so I have not been riding or doing much further testing on the bike. I will get my wife to help me post up some pictures maybe later today or tomorrow.

This problem is not something that I can't live with but it is a problem that I would like to have some answers as to what is causing this down shifting.

I would also like to apologize for coming across as being a know it all or smart A. I did think that some were pulling my chain and I was waiting for someone to ask me what color shirt that I was wearing. I should not have been thinking this way.

The little pin on the large ring is lined up with the crank arm. I am thinking that this is to keep the chain from dropping between the large ring and the crank arm as well as an indicator showing how to orient the chain ring. The cage of the deraileur is about 1/8" above the top of the tallest teeth on the large chain ring.

I know that this must be puzzling to you guys because you can't look at the bike and I am looking at the bike and I can't tell what needs to be done.

Thanks again.

dddd

Modern drivetrains specify 0.5% wear for chain replacement.

Previous recommendations were for 1%, with replacement of all chain and sprockets.

Something like hill repeats is very stressfull, to the point that a timely replacement of chains is advised.

I can't believe you wear them out that fast. Which brand and model of chain?

DannoXYZ 

Motor oil is not a good lube for chains as it was designed for pressurized system that floats the rotating parts away from each other. Better oil to use has more extreme-pressure & temperature additives such as lubes used for hypoid gears. Check out the Chain-L oil, it's amongst the best. I get 8-10k miles out of a chain before 1/16" wear. Every 250-miles, I'll spray WD-40 through the chain and wipe off all the old crud. Then lube with oil that has the better additives package.

Your auto-shifting could be due to the extreme wear. This causes the next tooth to engage the chain-rollers too soon and the chain may then ride higher on the tooth rather than slip into the valley in between.

Another idea is the oscillating pressure-changes from the pedaling-stroke. This causes waves of lateral-motion to move up and down the length of the chain. A left-moving wave coupled with the bike rocking to the right can throw the chain off. Try unbolting the BioPace and rotating 144-degrees forward (2 bolt-holes). I've found through lots of testing that this evens out the power-delivery. This rotation places the larger lever/radius of the chainring in use when there's the highest leg-force. And puts the smaller radius when there's little force. The result is you have more even torque-transmission to the rear-wheel. It's the opposite of the original BioPace idea that exaggerates the up-down lobsided pedal-stroke.

jim p

DannoXYZ, I think that you are correct on all points. I went outside a few minutes ago and with the bike supported I tried to get it to act up. I was only pedaling the bike by hand so the chain was able to stay on the big ring but I did notice that the chain was riding up on the teeth of the big ring.

I am thinking that a new chain will fix the problem. I am also going to look for some chain L oil. Where might I find this oil? Would something like STP oil treatment work as chain lube?

I also like hearing about rotating the chain rings forward by 2 bolt holes. I have been riding circular rings and I don't like the pulsing feeling of the biopace rings. I feel like I am going up hill in a series of lunges. It is just not a smooth motion or application of power. I also find that I bounce more when I try to spin the biopace rings as opposed to spinning the round rings.

It looks like it is going to rain in a few minutes so instead of riding I may just go ahead and replace the chain.

jim p

I just replaced the chain and it is no longer riding up on the large ring. Checked the radar and the rain looks to be a couple of hours away so I am going out for a quick test ride.

jim p

Test ride report. You guys were right about the chain wear being the problem.

I put the bike in the big chain ring and the big rear cog. Well it shifted back down to the middle ring. So I put the bike in the big ring and the 2nd largest rear cog and the chain stayed on the big ring. So I consider the problem solved because all I wanted to be able to do was to run the big ring and the 4th largest cog on the rear and this works perfectly.

Now for the fine tuning. I am riding along and looking down at my large chain ring. I think that I see the large ring moving from side to side when I look at the cage of the deraileur. Now I had called myself checking to see if this ring was running true. As some one mentioned there is an indexing pin on the large ring which points to the crank arm. Well the crank arm is in the way when you are trying to measure the run out on the large ring so being lazy I didn't check directly behind the crank arm. It looks like the chain ring is bent away from the crank arm by maybe 1/16". My guess is that the chain fell off the outside of the large ring and jammed between the crank arm and the large ring bending the large ring away from the crank arm.

So I would like some advise on how to straighten the ring. I want to be lazy and just grab it and the crank with some vice grips and squeeze it back into alignment. But before I do this I would like your opinions on if this will work without breaking the spider arms off the crank.

Thanks to all of you guys for your patience with me and for helping me sort out what was going on.

dddd

Usually you have to first determine if the crankarm spider ars are bent, or if it's the ring itself.

I have reliably straightened hundreds of crankarm spiders using a 3lb hammer and broom-handle dowel as a drift punch, applied to the base area of the spider tabs where the chainring appears to meet the crankarm.
It's a job for someone with experience bending metal, and I have never broken a tab off doing this.

OTOH, if you try to bend a chainring while it's mounted to the spider, there's a LOT of leverage and I HAVE broken a crank spider tab doing that.

Trueing cranks and chainrings is a skill that isn't so much fun to learn, probably why so few mech's are confident enough to have a serious go at it. It's also more profitable to sell a new part, even if the replacement chainring is a poor-shifting aftermarket part.
It has been worthwhile for me though, since it saves a lot of parts, and a true-running chainring really improves the ability to adjust the front derailer to perfection.

I wouldn't rule out some judicious force applied to a mounted chainring, but be forewarned about the amount of leverage you're playing with. I think I was using an adjustable wrench when I broke that tab, but it is also possible that it was already broken before I tried bending it. FWIW, it was a cheap, cast Suntour GPX crank.

Bending a chainring back, with it removed and on the bench, is almost impossible.
The ring part may be straight, but if the tabs are then even slightly out of parallel, the ring goes out of plane as soon as the chainring bolts are tightened. It doesn't help that the rings are cold-rolled and relatively springy.

Might be a good time to look for some newer round chainrings?

DannoXYZ 

You can find it here: https://www.chain-l.com

You can use an adjustable wrench like this:

https://4.bp.blogspot.com/_Y9sYeduiCG...jimlangley.jpg

I would just insert the wrench 1/2" down over the ring. Start first where the ring has arms going down towards the spiders, get those spots aligned. Then do the span in between.

jim p

I got the ring back almost perfect so maybe it will even ride big to big now. I may do this for a test but I wont be riding this way on a regular basis.

I took a look at the chain L oil. I have to get some and see how it works. All the concoctions that I have used don't even come close to giving me the chain life as one application of the Chain L oil. Some users are saying that they are getting 2000 miles between lubes and 4000 miles before the chain reaches its wear limit. This must be some good stuff.