What size chainring do I want?

[maxwolfie]

I have a hardtail MTB that is used mostly for trails

At the moment it has a 2 x 10 chainring setup (38 teeth / 24 teeth)

The cassette is 11-36, 10 speed (Shimano HG62)

I’ve just ordered a Toseven DM02 500w 52V that comes with a 42 tooth chainring as default, should I change this? Other options available are 34T/36T/38T/46T/52T

Would prefer to not have to buy a new chain so thinking about dropping down to 38T….?

 

[Epithemeus]

What wheel size? What kind of speed are you hoping to top out at? Climbing lots of hills? Most importantly would a 42 fit on your bike without ruining the chain line? Often the manufacturer will list a largest size they say will fit.

 

[gromike]

I would think you would want a new chain. Gearing has so many variables. Most want a fast top speed. On my BBSO2 I believe I'm 42 font and same cogs on the back as yours. Full charge, flat, and no wind it does 35mph. That motor goes up the short steep hills that I climb. Shifting to the big cog makes everything easy, but that's on a motor that peaks at over 1200w on the stock controller. My bike with the 36v TSDZ2b needs all the low gearing it can get, the top end -- not so much.

If you plan on doing some serious trail riding, that's a whole other bike.

 

[RunForTheHills]

Since you are going from a 2 X 10 to a 1 X 10 setup, you are going to have to break the chain to remove the front derailleur anyway. Why not replace the chain?

Sheldon Brown's Bicycle Gear Calculator

 

[maxwolfie]

My wheels are 27.5”

I really don’t think I’ll ever need to exceed 25mph

I will be climbing lots of hills, some of which are quite technical

Thinking I should just leave it at the default and see how that goes… can always pay for a smaller one if needed?

 

[gromike]

The smaller chainrings are the hardest to fit into those things. If you are doing the technical ebike thing you need to really plan for that. An ebike that does both road and trail reasonably well is where I set my sights. Since you have a hardtail with FS you're not going to be all that technical. It all depends on how you ride your bike. On the road you compete with cars, and that's why I like to be able to pull some speed.

 

[raylo32]

You should do OK on the hills with the 42, IMO and not need lower. Another consideration is if you carry the bike to the trails or ride it there? If you go to a smaller chainring it will slow you down on the pave portions of your rides.

 

[Yanis]

Another thing to take into consideration is the offset of the chainring : I know that the 42T for the DM motors is dished inwards so it corrects the chainline. I am not so sure for the smaller sizes so you should ask them about this. A correct chainline will allow you to go on the largest rear cogs without problems.

 

[Chalo]

Would prefer to not have to buy a new chain so thinking about dropping down to 38T….?

If your chain isn't very fresh, you'll be throwing away some lifespan of the new sprocket by synchronizing its wear with the old chain. Usually, a using a new chain to go along with every new drive component is the most economical choice overall.

 

[maxwolfie]

If your chain isn't very fresh, you'll be throwing away some lifespan of the new sprocket by synchronizing its wear with the old chain. Usually, a using a new chain to go along with every new drive component is the most economical choice overall.

Should I be getting a new cassette too?

 

[Chalo]

Should I be getting a new cassette too?

Ideally yes. But if you check the chain wear with a gauge, you can figure out what the measured wear state of the chain is. If it's not measurably elongated, then you can assume the cassette it has been working with to be in good shape too. Most bike shops will do that for you if you don't want to buy a gauge.

 

[2old]

If I were you, the above suggestion to try the 42 first seems excellent and unless your current system has at least 50% of its wear remaining, replace it. I use a 32 chainring on a BBS02 system which is ridden in hilly off road areas. This reduces top speed to 20 - 22 mph, but that's enough. FYI, the rear is 11-17-28 spaced 8-speed in order to have a straight chainline, and seems durable.