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I have a cheap ($100) 26" Walmart mountain bike (Hyper Summit) that has a triple chainring (24,34,42). It appears there is no easy way to remove the granny gear since it has what looks like some type of rivets. However, the BCD (Bolt Circle Diameter) appears to be 58mm which is a standard size. I would like to go with a 20 granny chainring cuz I like to ride thru tall grass and need a lower gear. My rear gears are a 7 speed freewheel with a 28 as the largest cog so currently, 24/28 is my lowest gear which is about 22 gear inches (26" tires). I will eventually be changing to a 34 largest rear cog (Shimano MegaRange 7 speed freewheel) but I also want the change the front granny chainring for a double gear reduction which I know I will like. So any ideas how to get the granny gear off? It connects in 5 places (starfish pattern). My goal for my lowest gear is 20/34 which will give me about 15 gear inches.

I was thinking if I can somehow get those rivets out without damaging anything critical, I can probably use some washers and chainring bolts to handle any difference in hole sizes. It seems like it can work with just some light machining. Once the chainring is off I can have a bike mechanic work on it. There is always a brute force method too. For example, cutting the teeth off of the 24 chainring and attaching the 20 chainring to it. However I prefer trying to bust out the rivets first as the BCD of 58mm matches up with available 20T chainrings.

UPDATE: I am moving forward on this project so will update when I get some actual results. It might take a month or so to get the parts and schedule a bike shop appointment. I think at minimum I should be able to get the different freewheel gearset installed but not sure about the 4 tooth smaller granny gear. If I can get both it should be awesome cuz that will then be a low gear with 15 gear inches which is very low.

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    rivets are common on lesser drivetrains. That's one of the limitations you take on board when you go down the cheap route - stuff just isn't designed to be upgraded. As you move up product ranges, rivets change to nuts and bolts. But as things stand, your options are to replace all three chainrings (if you can find something suitable), which might also involve work at the back, or to live with it.
    – PeteH
    Jan 20, 2016 at 13:17
  • PeteH - I don't want to give up that easily. What would be fun to try would be if I can contact the manufacturer of the bike and see if I can get a 2nd chainring set to play with. That way I wont disable my bike while I am tinkering. If I take my time, I might be able to bust out those rivets and get the smaller granny chainring on there. If the FD can't handle the 22 tooth difference between the small and large chainrings, perhaps I can just not use the largest of them and go with a wider range rear freewheel to compensate (11-34 instead of 14-28 for example). 34/11 is very close to 42/14
    – David
    Jan 20, 2016 at 13:55
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    Be aware that rear derailleurs also have a working range, so if you up the T on the cassette, you might need to rethink that too. But good luck with your endeavours
    – PeteH
    Jan 20, 2016 at 14:10

2 Answers 2

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Depends on your budget, your requirements, and your mechanical aptitude. Plus how far down the slippery slope of knock-on upgrades you want to do on a BSO bike.

Since the crank is riveted together, its unlikely to accept any bolt-on replacement chainrings. Though its possible the makers used rivets instead of bolts but used normal bolt chainrings. You'll want to pop the chainrings off and have a close look. It may be possible to buy a MTB triple chainring like what you want, but it won't be walmart-grade or price.

Second problem is Front Deraileur capacity. Most front mechanisms have a maximum tooth difference that they can swing, and for your existing FD is already coping with a difference of 18 teeth. A difference of 22 teeth might be too much for it. Hard to know.

Finances comes into this as well - remember a walmart bike is at the bottom of the quality scale. I see little point in throwing good money at a crappy bike, when you can save up and get something better. Don't discount used bikes either - a 5-10 year old used bike could cost as much as a new chainring triple crankset, but have a lot better quality components.

You say you want to ride through long grass - can you expand on why this needs such a crawler gear ratio? I can ride (very slowly) up a 25% grade on a 26/24, and a 20/34 would be quite a low gearing.

You'll have better luck with the rear cassette upgrade. I upgraded my 7 speed MTB from a maximum 28tooth to a 34 tooth megarange and it worked perfectly. All you need is a long rear deraileur, and many bikes already have them because they fit the "look" even if the bike doesn't need them. This will also decrease the tension on your chain - as the front chainring gets smaller chain tension increases.

TBH don't discount the value of an engine upgrade - ride your bike further for endurance or harder for increasing power. This helps immensely, as does weight reduction of you and your load.

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  • When I ride over grass (especially thick grass), the bike has lots of rolling resistance. In my current 24/28 lowest gear ratio, it moves the bike 5.8 feet per crank rotation. If I change to a Shimano MegaRange MF-TZ31 7 speed freewheel (a fairly simple upgrade I should be able to do myself and the freewheel + tool to remove/install it total should be about $25), it will give me 4.8 feet per crank rotation. If with the MegaRange I am also to change my granny gear from 24 to 20, that will give me 4 feet per crank rotation (and about 15 gear inches). It is not such a radical gear I need it.
    – David
    Jan 20, 2016 at 12:57
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    All bikes have a lot of resistance when riding over thick grass...
    – Batman
    Jan 20, 2016 at 14:37
  • - Batman - some more than others. The ones with those very wide tires should be better at distributing the load and making it easier. It is amazing how when transitioning from pavement to wet thick grass, how much the rolling resistance changes. I can go from my top gear which is 42/14 to my underdrive gear (24/28) and the low gear is still not low enough. I need 20/34 to be happy so I am going for it. This is a realistic scenario for me cuz I like to ride near retention ponds edges which is grass. The MTB I got didn't come with low enough gearing which is somewhat surprising to me.
    – David
    Jan 20, 2016 at 15:30
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    I guess BSO is a nicer way of saying POS.
    – David
    Jan 20, 2016 at 17:01
  • @david Almost - BSO implies cheap construction and features to appeal to people who don't ride a lot. However its still a rideable bike. POS suggests its dangerous or otherwise unrideable. My suggestion would be ride it as-is, and save up for a better bike as you work out what your requirements are. Consider a suitable used bike too.
    – Criggie ♦
    Jan 20, 2016 at 19:08
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You aren't going to be able to get those rivets out and replace the smallest chainring; it's not going to happen. Chainrings and the holes for the bolts are designed for tight tolerances and you'll never get it back together in a functional manner.

Do I think you should try? Absolutely.

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  • Remember to post progress pictures!
    – Criggie ♦
    Jan 23, 2016 at 9:51
  • I am moving forward on this. Yes I will post some pictures if someone tells me how. I will show both the before and after pics. I can ask some buddies at the local motorcycle shop for some advice and perhaps they know a machinist that can make it happen for not much money. It is very important that I lower my lowest gear substantially and a nice side effect is that I widen my gear range from 3.5 to 4.2 just with the granny gear change and if I also widen the cog range, I can get a gear spread of over 5:1.
    – David
    Jan 25, 2016 at 11:20

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