Why chain she skips?
 

Hi, I've got a Mongoose Somethingorother (approx. 21spd hybrid with motorcycle style grip shift shifters) that I'm trying to fix up to give to a friend. I've adjust the rear derailleur but still, the chain skips on occasion, as a regular occurence, and what I'd like to know is if the problem will be fixed if I buy a new chain. Perhaps it might help if I said I left the bike out two winters, and its all rusted to hell now. Just don't want to buy a new chain if a lot of other expensive parts need to be replaced to eliminate this skipping problem; because then it'd be more cost-effective to buy a used bike on the cheap, that's in better condition (ie. a new freewheel would be a dealbreaker). Reason I'm fixing it up instead of buying a used bike, is that my Mongoose hybrid is the type of bike we were looking for.

(BTW, are Mongoose exclusive to Walmart? Don't know where the bike originally came from, but it seems the quality of the components and all are better than a typical Walmart bike).

afak there are non walmart mongoose bikes and they can be quite nice, but most come from wally these days.

The chain might skip because of frozen links or being too loose. If it has been sitting, it is likely that it never saw avid use, so the chain may not be stretched. To check if the chain is stretched, simply attempt to pull a link away from one of the chainrings (front gears). If it comes all the way off the tooth, it is badly stretched, though if it moves more than a few millimeters it is suspect. They make a tool/guage for a more precise reading, but this method works well enough. To check if the links are frozen or sticky, you can watch the motion of the chain while backpedaling the bike. A more accurate check would be to individually attempt to wiggle each link though its normal range of motion and see if any catch.

I guess the more relevant concern is the condition of the freewheel. The rust is often superficial and can be brushed off with the aid of a solvent. The greater concern is that the teeth are not too worn. If the teeth are completely flat on the top, they are in good shape, but if they look like waves cresting or jagged points, then the freewheel probably needs to be replaced. Maybe someone will come by and post a picture for you.

A few drops of oil might change everything... often it will cure a mildly rusty chain immediately; sometimes it will make a skipping chain skip worse.

After the chain is oiled you can do a quick assessment by (1) turning the cranks backwards to see if it rides smoothly through the rear derailleur and over the cogs, (2) measure 12 pairs of links with an inch measuring tape - the distance from the centre of the first pin to the centre of the 25th pin should be between 12" and 12-1/16"... any longer and it is worn out and replacing it with a new chain will necessitate new cogs at least.

Thanks for the helpful replies. I have done the "pinch-pull" test to check how loose the chain is on the front large and middle cog, and it is definitely loose. It comes off more than a few mm I'd say, if not completely. In fact, just spinning the wheel with the bike upside down, you can see how the chain rides just above the largest front cog (with a gap between the chain and cogs). As a result of this looseness, it sometimes slips off the largest front cog, making it impossible to pedal the bike. Although the derailleur appears to work fine when shifting, the bike sometimes slips gears, particularly in the middle cogs. The other major problem with the chain is that when applying pressure to the pedals, it usually slips gears. Measuring the chain shows just over 24 links from 12" to 12 1-16". Not 25. The links seem flexible though, from wiggling the links.

I had oiled the chain with Pedro's bike oil weeks ago. I thought a major cleaning of the rust on the chain might be enough, but given the above, maybe not. Now the question remains whether I have to change the cogs as well in order to solve the skipping problem. (n.b. I did not do 100km "tours" with this bike. It was just for riding in the city). (The bike is so rusted, I broke two of the brake bolts that hold the pads, trying to adjust the brakes!).

THIS IS THE CONDITION OF THE FREEWHEEL AND FRONT COGS:

http://www.flickr.com/photos/21180446@N05/3714114373/
http://www.flickr.com/photos/21180446@N05/3714114361/
http://www.flickr.com/photos/21180446@N05/3714114203/
http://www.flickr.com/photos/21180446@N05/3714114357/
http://www.flickr.com/photos/21180446@N05/3714114367/

So, uh, what's the question? How to fix it? I'd try a cheapo replacement chain. With that amount of stretch you should probably replace the cassette too, but that wouldn't be as cheap. The front chain rings are likely to be okay, but you never know.

If the cassette is bad too, replacing just the chain will produce skips at the rear. But it should generally be ridable.

it seems like your chain has worn out your cassette and chainring. check out http://www.sheldonbrown.com/chains.html.
look specifically at the third to last photo at the bottom. usually you would replace the cassette, chainring, and chain but since its a walmart bike, it'll cost more to fix it than to buy a new walmart bike.

as dmf said, a cheap replacement chain will make the it rideable, but i think it'll still skip a little.

or if you have the money, you can get a used bike on craigslist.

HYPERMANIAC: I don't really see the signs of sprocket wear in the 3rd photo, but if you do, I'll have to assume it's there. If its worn out the front chainring as well, that would be bad news. But I'm still not sure I've done enough riding on this bike to wear out the cogs that much. The Mongoose is probably not a Walmart bike, but a bike shop Mongoose. The overall quality of the bike is pretty good, with good components (I like the motorcycle style GripShift shifters), not too heavy CroMo frame and rides and shifts well, save for the chain skipping problem. I have looked but can't really find the same quality and type on Craigslist for the cost of a new chain, cassette, bearings and brake parts I would need. But if I have to add front chainring.... then it might not be worth fixing up (the labour I could do myself, except for the cassette, cos I don't have the special tools to remove it). The most important thing is to have the chain never go over top of the front chainring and disengage from it, because that makes the bike unsafe. I actually thought I could maybe fix the chain problem by adjusting the rear derailleur somehow so it removes the slack in the chain, but that may be wishful thinking.

DMF: So, uh, what's the question? How to fix it?

No, the question is what needs to be replaced. Whether the chain needs replacing (or could be cleaned into working), and whether other parts need replacing. I posted 5 photos of the cassette (freewheel?) and front chainring, to help determine this.

Answered above.

the teeth on brand new unworn chainrings or cassettes will look like a trapezoid. when its worn down, it will look like a shark's fin

_
/ \ <== That is a new, and unworn, and a worn one looks like a sharks fin.

The sharks fin is whats making the chain skip, the chain can't hold on the cassette because its worn down. A brand new chain may fix it temporarily and is the cheapest way to go, but the problem is your cassette and chainring. It is rideable, but the skipping won't stop till your replace the parts.

I'd advise you to get a new bike as its cheaper than to fix the bike. Look up brands on craigslist like trek, raleigh, gt, cannodale, fuji, schwin, and there are many more but i won't list them all. I'd stay away from mongoose ones, mostly the ones on craigslist are from walmart and are lower quality.

I took the Mongoose to a dealer today who measured the chainring and confirmed "it is dead" (his cute way of saying its really stretched out). He then immediately went on to say the freewheel needs replacing, but never mentioned the front chainring. When I specifically asked if the front chainring needs replacing he said no, it looks alright (at first), then spun it a few times and said "oh, you're right, it needs replacing". So he could be ****tin' me for all I know... In any case, when I total up his prices, it's just over $100 (CAD) to replace and install the chainring, freewheel and chain. Since I need new brake bolts after I broke the old rusty ones, that price doesn't include fixing the brakes ($10-30). What really kills me is the bike rides like a dream, but costs like a nightmare just to fix a chain skipping problem.

In any case, I have been looking at the local used market and can not get the kind of bikes you mentioned (Trek, GT, definitely NOT Cannondale, Fuji, Schwinn) at the price it would cost to fix up the Mongoose. I can get Raleigh, but whether an old or new Raleigh city bike, the difference between that kind of ride and my old Mongoose hybrid is really night and day. The Mongoose just performs better and is a more comfortable ride, than what it looks like I can get for the price of repairing it. Got a lot of hard thinking to do....

Learn to work on your bikes yourself. Buy tools as you need them and you won't even notice the cost. Will have you a tonne over the years.

I learned to work on bikes myself over 20 years ago, when I completely disassembled every single part on my Norco racer, to repaint the frame. That's not the problem. I simply don't work on bikes often enough to require a freewheel or cassette tool; particularly when they are often made to fit a certain type of freewheel or cassette. For the most part, biking is transportation for me; not a hobby.

Well.. $5 in tools and 15-minutes is still a lot less trouble than $100+ and hours spent shuffling the bike back & forth to and from bike shops.

The $100+ is the cost of parts and labour. The labour is $5 per part. ($15 ttl). So as much as I wish it were so, it would not cost me $5 to fix up my Mongoose, if I did the work myself (nor would the freewheel tools be $5 here, nor would it take me 15m to remove and replace chain, freewheel and front chainring assembly).

As good as the Mongoose would be if the chain problem were fixed, it's looking like it would be cheaper to just buy a used whatever, especially if i have to replace the chainring as well. e.g. I can get a 10 spd Raleigh Record from the mid 70's from my local thrift shop for $60 + tx (racer style bike but with North Road style handlebars) - fenders and rack included. The cost of adding fenders & rack and fixing the Mongoose hybrid is more than double the Raleigh.

Another idea I had, is to buy a cheap hybrid used and then remove the freewheel, chain and chainring and put that on the Mongoose. That would be cheaper than buying just those parts new. I have no idea whether that's actually possible, though...