Front 3 sprocket crankset change

sjpt · Jul 12, 2023

We need more info.
How many teeth on the current 3 sprocket crankset?
Is the bike electric? If so is the drive front, mid or rear?

saneagle · Jul 12, 2023

Wayners said:
Front 250w hub and im peddling a bit fast at 19 to 20mph although no motor assistance at that. I was thinking if I can gear it up a bit, the ride will be a bit nicer with my leg speed. Think I'm missing the 7th gear at the back

Generally, you can put any crank on most bikes. I often swap for road bike ones because you only need two rings when you have a motor. They're lighter too. I think I have 48T/34T on mine.

Cadence · Jul 13, 2023

A photo would help.
Most 3x 6 set-ups on older MTB's have a 48-38-28T riveted front chainset. Some had a 42-34-24T and if yours is this size the larger chainset should be straight swap if it is the same make. Shimano ones fit a 122mm bottom bracket spindle to give a 50mm chainline (centre of seat tube to middle chainring). They are freely available. If the derailleur is a friction type (rather than indexed) you might be able to re-use it. In either case you won't be using the small "granny" ring any more with electric assist.

I have an old Giant MTB with a 3x7 set-up and had the same problem as you. I have (2 days ago) fitted a DNP Epoch 11-28T 7 speed rear screw-on freewheel to replace the existing 14-28T.
I've only managed a short test run so far, but the smaller 11t sprocket has solved the "spin out" problem I had above about 18mph.
One should fit on your existing rear wheel hub, but you would need to measure the distance between the rear frame drop-outs, whch should be 135mm or more.
You will also need a 7 speed shifter and possibly a new derailleur,. It's a good idea to replace the chain and cable as well.
DNP 7-Speed 7S Screw-on MTB Bike Freewheel Nickel Plate 11-28T Bicycle | eBay

Jodel · Jul 13, 2023

There are some things to think about if you are changing the crankset. You might find it worth looking at a few articles like these for background info:

A complete guide to front derailleurs: types, mounts and compatibility explained

Banish your bewilderment with our comprehensive guide to the front derailleur

www.bikeradar.com

Cranksets explained: everything you need to know about the heart of the drivetrain

A comprehensive guide to crankset sizes and standards

www.bikeradar.com

When rebuilding bikes over the years, I've run into a few issues and it's worth taking some factors into consideration. If your present setup is riveted together, you'll need to get a new crankset and ideally a new chain and freewheel. Not trying to sound gloomy, but it's better to be forewarned. Off the top of my head:

If you have an older MTB, the chances are that you'll have a square taper bottom bracket, so lots of cranksets will be available. The 'Q' factor on your replacement triple ring crank may be different and you might need a new bottom bracket. If the BB axle is too short, your inner ring might foul the frame chainstay, if too long, your front derailleur might not have enough 'throw' to reach the outer ring. A BB axle that is too short might also cause the pedal crank arm to hit the chainstay.

On an older MTB your front derailleur should allow for a larger outer ring - some more modern FD's will only allow outer rings up to about 40-42 teeth. The other thing to consider is the capacity of the front derailleur. For example, a lot of Shimano FD's have a total range of 20 teeth with a minimum difference of 12 teeth between the middle and large rings. You can normally persuade most set ups to work, but sometimes they need a bit of fettling.

Some of the above comments are 'worst case' and the chances are that you'll be fine just changing the crankset, but a bit of research in advance might save you some grief / cash!

Bonzo Banana · Jul 13, 2023

Cadence said:
A photo would help.
Most 3x 6 set-ups on older MTB's have a 48-38-28T riveted front chainset. Some had a 42-34-24T and if yours is this size the larger chainset should be straight swap if it is the same make. Shimano ones fit a 122mm bottom bracket spindle to give a 50mm chainline (centre of seat tube to middle chainring). They are freely available. If the derailleur is a friction type (rather than indexed) you might be able to re-use it. In either case you won't be using the small "granny" ring any more with electric assist.

I have an old Giant MTB with a 3x7 set-up and had the same problem as you. I have (2 days ago) fitted a DNP Epoch 11-28T 7 speed rear screw-on freewheel to replace the existing 14-28T.
I've only managed a short test run so far, but the smaller 11t sprocket has solved the "spin out" problem I had above about 18mph.
One should fit on your existing rear wheel hub, but you would need to measure the distance between the rear frame drop-outs, whch should be 135mm or more.
You will also need a 7 speed shifter and possibly a new derailleur,. It's a good idea to replace the chain and cable as well.
DNP 7-Speed 7S Screw-on MTB Bike Freewheel Nickel Plate 11-28T Bicycle | eBay

I personally think a new crankset is a better long term solution. Those DNP freewheels aren't that high quality and if he is the 11t all the time the freewheel teeth will wear out quickly and he will have to replace the whole unit. A new higher teeth crankset either 2x or 3x means he can still use higher quality Shimano freewheels and with 14T as the highest gear it will last much, much longer. The new crankset may mean the smaller cogs of the freewheel are all getting use so more even wear. Something like this would be my suggestion;

Double Chainwheel set Crankset chainset 170mm square taper | eBay

Double Chainwheel set. 170mm Crankarm length. JIS square taper Bottom bracket interface. Alloy Crank Arms. You will receive both sides (left hand arm included). Horbury Wakefield. Removable chainrings.

www.ebay.co.uk

If this isn't low geared enough a 14-34T megarange freewheel can be used to give more of a emergency granny gear for when the battery has run out of charge. F42T xR34T is fairly low gearing for all but the steepest hills.

Nealh · Jul 13, 2023

A coupke things to be wary of ;
1. The front derailleuer may not handle the larger chain wheel.
2. The chain may need extra links added.

Bonzo Banana · Jul 13, 2023

Nealh said:
A coupke things to be wary of ;
1. The front derailleuer may not handle the larger chain wheel.
2. The chain may need extra links added.

Probably need to replace the chain which is pretty normal when you replace the crankset. The crankset I have linked to should be easy to use for a front derailleur designed for a much wider range 3x crankset. Mine only has a 10 tooth difference between the two rings. I don't think a basic riveted 3x crankset ebike would have a front derailleur with a set frame position. I'm fairly sure it will be height adjustable on the bottom of the seat tube to move it up slightly for a 52T chainring. A 3x front shifter can be handy for a 2x crankset as you can use the limiters to create 2 slight position variations to stop knocking on the front derailleur in some gears. Difficult to explain in writing. I personally prefer a friction shifter for the front derailleur as it allows for small tweaks more easily to stop the chain knocking against the front derailleur guide. Definitely nicer for touring.

saneagle · Jul 13, 2023

Wayners said:
That's what I need. Do you buy one made or swap the sprockets only? @saneagle
Don't think my 3 crack sprockets come apart. Looks like they are riveted together

I normally swap out what's there for the cheapest hollowness crankset, which is about £70 because it saves a lot of weight. The set includes the BB. If you have a square taper type BB, you can just replace the cranks, which is a lot cheaper.

How many teeth on your large chainwheel at the moment?

saneagle · Jul 13, 2023

Wayners said:
Yes square type crack fitting.
42 teeth

If you look on Ebay, there are many cranks with one, two and three rings. Search for "48T chainset". The prices range from 99p to £hundreds. Choose any with the square hole in as long as the both pedal arms are included.

There are different pedal thread sizes. Most are the same. In the worst case, you might have to buy new pedals. Whichever one you get, you'll need to unscrew your pedals from your old crankset. You'll also need a crank puller tool. You'll also need to completely reset your front derailleur to change gear correctly, and you'll need a new chain of the correct length, which will be 6 links longer.

48T will make a 14% difference to your pedal speed. You can get a 50T one if you think that's not enough, or even 52T.

Cadence · Jul 13, 2023

Wayners said:
I have a new Shimano derailleur, it's a 7 speed running on a 6 speed freewheel.
I didn't think I could swap the 6 speed freewheel for a 7 speed?
I have the smaller 3 sprockets set on the front unfortunately, so thought it would be better to swap for bigger set. I only really need the one bigger sprocket as other 2 smaller ones are useless to me.

You should be able to run a 7 speed freewheel if your rear frame drop outs are 135mm, but unless the smaller sprocket is smaller than what you already have (I'm assuming 14T) it won't offer any advantage other than you have a closer range. This is why i've fitted a DNP 11-28T freewheel. There are mixed comments about its quality on the 'net, so I'll see how it goes.
Now you've said that your largest chainwheel is 42T your best move is to replace the chainset with another one. Is yours Shimano? If so the easiest swap would be a 48-38-28T Shimano triple which are cheap and easily obtainable, and the chainline will be the same. They are often described as 7, 8 or 9 speed - any of these will work. Ideally you should replace the front derailleur to match, but your existing one can probably be made to work. You just move it up the seat tube until it just clears the big chainring. If you have friction shifting on the front rather than indexed (which I suspect a 3x6 may have) it will be a lot easier to get working.
You could try a bigger double chainset (52/42 or similar) but bear in mind that you might need a different length bottom bracket to maintain the existing chain line and the bigger the chainring, the more likely that you will need a different derailleur. The bigger chainring might not clear the chainstay.
In all cases you will need a new chain and probably a new cable.

Front 3 sprocket crankset change

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