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Can i replace a 24/34/42 MTB chainset with a 24/38/48 MTB chainset while still using the same 7 speed cassett, chain and shifters.

My old mountain bike from the 90s had 24/38/48 MTB chainset and when i stood up and pedaled on flat roads i could get to 40mpg while now on my 2019 29er with 24/34/42 MTB chainset & 12-32 7 speed cassett i struggle to reach 30pmg while pedalling like crazy

EDITED: So it appears, i have a Formula DC22 rear hub, not sure if this help anyone to tell me if a 11-28 cassette will fit on it. Trek say i need to buy a whole new back wheel if i want to change the cassette. Here are some images of back wheel and cassette. enter image description here

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  • 4
    Miles per gallon on bike?
    – ojs
    Nov 15, 2020 at 20:08
  • @ojs Miles per gallon of beer, duh.
    – MaplePanda
    Nov 16, 2020 at 0:00
  • @Drexal You do know how to put the rear axle back together, right? You usually don’t need to remove it in order to remove the cassette/freewheel.
    – MaplePanda
    Nov 16, 2020 at 0:03
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    @maplepanda the axle snapped months ago. So i had to remove to it replace. The image was from back then while the axle was out.
    – Drexal
    Nov 16, 2020 at 0:47

4 Answers 4

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Another option - change out your cassette to one with an 11-tooth smallest cog.

Per Sheldon Brown's gear calculator, pedaling at 100 rpm on a 42-tooth chainring and a 12-tooth cassette cog results in a speed of 28 mph. Going to a cassette with an 11-tooth small cog, at 100 rpm that's 31 mph. Getting up to 120 rpm results in a 37 mph speed.

And 37 mph on level ground is really fast on a bicycle - as in "speed reached in the final sprints in Tour de France stages" fast. 30 mph on a bicycle is fast, too. If you're not struggling to reach 30 mph on a bicycle on flat ground, you should be riding in the Tour de France. An average-size rider has to sustain a power output of 400 watts or so to go 30 mph - while riding an aerodynamic time-trial bike. Sustained power output from cyclists who have never raced rarely exceeds 150 watts.

40 mph on level ground while riding a mountain bike with 26" wheels? You would have had to have a 48-tooth chainring and and 11-tooth cog and be riding at 120 rpm to do that.

So if you hit 40 mph on that old 26" MTB, you should be able to hit almost that fast with your 42-tooth chainring on your 29er just because of the larger wheels - as long as you have a cassette with an 11-tooth small cog.

But at the end of the day, bigger gears do not make you faster. The speed you can sustain depends mostly on how much power your body can generate. The biggest limiter after that is how aerodynamic you and your bike are.

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  • Thanks Andrew, strava tells me i hit 47.5 km/h = 29.5mpg max speed on the 2019 bike obviously i probably hit that speed for only 10 seconds or so. But i constantly find where there are clear bits of road and i cycle has hard as i can for 10 seconds or 20 seconds max, i'm pedaling like a lunatic so i just want my hardest gear to be harder for those tiny short bursts. So if i buy an 8 speed cassette instead of the original 7 speed making sure the extra cog is a smaller with 11 teeth this should work? Do u know if i will have to buy a new rear hub to do this?
    – Drexal
    Nov 15, 2020 at 13:45
  • @Drexal You cannot just buy an 8-speed cassette if you have 7-speed shifters. You can buy a different 7-speed cassette with different cogs. Nov 15, 2020 at 13:56
  • Thanks @VladimirF ok so do know what ratio cassette i need to search 11 by ? sorry for these dumb questions. I litterally am in the dark trying to learn about gears etc
    – Drexal
    Nov 15, 2020 at 14:15
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    So i'm now looking for a shimano 7 speed cassette 11-28 on goggle
    – Drexal
    Nov 15, 2020 at 14:23
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Your front derailleur is spaced for +10 and +8 gaps between small and middle and then middle and big ring. If you examine your front derailleur you will see that it has a groove spaced to shift for those gap sizes. Going to +14 then +10 will give poor shifting performance, so it would be best to find a matching front derailleur, or to maintain those gap sizes, e.g 48/40/30

A 40mph speed on a flat road is not particularly plausible on a mountain bike. On a time trial bike in an aero position and tight clothing it is possible. The world record time trial time for 10 miles is around 36 mph @ 450W. https://cyclinguphill.com/time-trial-records/ Since power required to defeat wind resistance is proportional to the cube of speed, and proportional to the frontal area of the bike (CdA), it follows that even on a time trial bike you'd need to be putting out around 555W to do 40mph on a flat road. A MTB I would expect to have almost double the CdA of a time trial bike, so you'd need to be putting out 1000W minimum, which is unlikely.....

If you want to go faster you would be best looking at a bicycle with drop bars as a starting point, because fitting road-like rings to an upright bicycle is essentially a masochistic exercise, because most of that work you will need to do to push those rings will be lost to air resistance, and you will end up in top gear at a very low cadence, which won't be good for your knees. It will be many times easier to pedal quickly if you find a bicycle that is comfortable to ride but minimises your body area exposes to the wind, and if you ride in tight clothes (i.e. lycra).

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There are a few issues that you will run into.

The larger difference between the smallest and largest chainrings means the derailleur has to be able to take up more slack in the chain when shifting from largest to smallest rings. This may exceed the Total Capacity specification of the derailleur. The capacity needed is (difference between smallest/largest sprockets) + (difference between smallest/largest chainrings). If you increase the size of small and middle rings by the same number of teeth, the capacity in not affected (but a longer chain is of course required). You also need to check the max chainring size difference that your front derailleur will accommodate.

You may also find you have insufficient clearance between the chainstay and a larger chainring.

Presumably a 7 speed drivetrain has a cartridge bottom bracket. If you need to replace the while crank you may find you need to replace the bottom bracket as well as different crank models require different axle lengths.

Did you 90s bike have smaller wheels than you current one? Remember than if the Oder bike was on 26” wheels and you newer one has 27.5” or 29”, you effectively have a higher gear ratio anyway, so you may not need a 48 tooth chainring. In fact 662/559 = 1.1, 44x1.1=48.4.

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    Thank you for the reply. Yes 26" on 90's bike, i just find this 2019 bike on 29" wheels is to slow on the roads, i need to replace the chain, crankset and cassett as i have neglected the maintenance. But while i do this. I desperately want to make the hardest gears way harder as i sit in the hardest gears 80% of the time. Even if means adding some more rings to cassett but i will prob then run into hub issues. The bike is a cheap 2019 trek marlin 5 which i would rather not repalce. But just desperately want to order and replace these parts. Do think what i'm attempting is possible?
    – Drexal
    Nov 15, 2020 at 12:42
  • I know it's possible. I had a Trek years ago, an 80's bike, where the largest chainring was easily removable and replaceable, by 5 hex-head screws if I remember it right. My bike shop of the time found a larger one which fit, identified that the derailleur would work with it, and frankly, like wow :-) I've asked about the same retrofit much more recently, and have been told it's usually a lot harder (more needs to be replaced), but it can be done. I'm slowly converting to internal hubs both front and rear (rear is done!!!), or I'd be doing that myself. Nov 15, 2020 at 12:57
  • Thanks Jonathan, sadly the chainset/crankset on this bike is a £20 shimano tourney ty301 with none removable rings, so has to be be binned as its worn on the top ring, thats another reason i would prefer to swap out for a bigger better crankset, so i can just change the largest ring if this happened again but it will literally take me days/weeks of research to find a bigger one that will fit, if its even possible.
    – Drexal
    Nov 15, 2020 at 13:06
  • Thanks for this Argenti, wow i didn't realise what a big job this could turn into. It sounds like the easier route would be to just try and swap out the rear cassette adding a smaller cog. Could u see issues if i did this?
    – Drexal
    Nov 15, 2020 at 13:59
  • @Drexal what’s the cmallezt sprocket size now. On a 7 speed getting a smaller sprocket might be tricky, esp if you have a freewheel rather than a cassette. Nov 15, 2020 at 14:02
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Truth be told, the derailleur can handle the larger chainrings up front. However, you would have to bear in mind that each front chainring would be limited to only a few gears at the rear to prevent the RD from over-extending its pulley arm.

Naturally, I understand that answers here should err on the side of safety and practicality, so it would be much more cheaper to replace your cassette with one that has an 11T cog which would allow you to realize the same/faster speeds without changing your front chainrings.

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  • However, the frame might not handle larger chainrings. Nov 15, 2020 at 15:51
  • Hi @Lien028, thanks for reply. Yes from everyones help i'm now going yo go for a 11-28 cassette instead. But how do i know if it will fit on my hub?
    – Drexal
    Nov 15, 2020 at 18:22
  • @Drexal There is no reason why it wouldn't fit if you have a normal 7-speed freehub, you can install a 7-speed cassette. Just make sure you do not have a freewheel instead.There are various issues at 11 and 12 speeds, but that does not concern you. If you have a screw-on freewheel instead, you would have to buy a freewheel. I do not see a cassette lockring in your picture. If it has threads and you did not use a whip to get it out, it is a freewheel. BTW, clean it. Nov 15, 2020 at 23:47
  • @Drexal I know you wrote it was supposed to be a HG-200 cassette and I did not use a freewheel for some time so I do not want to be definitive, but it really looks like a freewheel to me. I have a HG-201 cassette in my hands right now and it looks completely different. Nov 15, 2020 at 23:55
  • @VladimirF thanks again for the help, after spending all day on this. I have now worked out as you say it is actually a freewheel. Its the trek website and info about the bike saying it had a cassette that has wasted most of my day. I have found a chinese made 11-28 freewheel that looks like it will fit. But its not had great reviews. So after all this wasted time, I will probably just replace just replace the freewheel with a 12-32 replacement. Thanks for all the help vlad
    – Drexal
    Nov 16, 2020 at 0:55

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