New Chainring 34T to 38T

[pmac]

My current Focus Jam 2 has a 34T chainring. After 1800 km my rear sprocket needs to be replaced. This is because I tend to use the top few gears. I want to get a larger chainring so I use a wider range or rear gears. I was thinking a 38T. Would that be a bad idea? I don't know what mount type is used so I'm lost when looking at the range of chainrings. Here is one that says it is specifically for a Bosch CX Gen 4. Would that do? Any other suggestions?

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[Jackware]

I'd check that the larger diameter chainring doesn't hit the chain stay

38t =approx 19 inch so divided by pi = approx 6 inch diameter

 

[raymal]

My current Focus Jam 2 has a 34T chainring. After 1800 km my rear sprocket needs to be replaced. This is because I tend to use the top few gears. I want to get a larger chainring so I use a wider range or rear gears. I was thinking a 38T. Would that be a bad idea? I don't know what mount type is used so I'm lost when looking at the range of chainrings. Here is one that says it is specifically for a Bosch CX Gen 4. Would that do? Any other suggestions?

ethirteen eSpec Aluminum Bosch CX Gen4 Direct Mount Chainring Black

Buy ethirteen eSpec Aluminum Bosch CX Gen4 Direct Mount Chainring Black Now at Pushys. Lowest Price, Secure Shopping and Fast Delivery from Australia's Best Online Bike Store

www.pushys.com.au

What Jackware said - I went from 34t - 36t and it's a close fit at the chainstay

 

[pmac]

What Jackware said - I went from 34t - 36t and it's a close fit at the chainstay

I pulled the trigger. Lets see if it fits.

 

[Ricky_1973]

I pulled the trigger. Lets see if it fits.

Did it fit?

 

[pmac]

I've not tried it yet. It's still sitting on my desk. I measured it. It's 6.37" diameter.

 

[lezuzlak]

is it ok for the bosch software to get bigger chainrings? no error codes?

 

[Hamina]

is it ok for the bosch software to get bigger chainrings? no error codes?

I don't see any logical reason why it would give an error coder. It stops assisting based on rear wheel speed.

 

[badnwelsh]

I've not tried it yet. It's still sitting on my desk. I measured it. It's 6.37" diameter.

Is it fitted yet? Eagerly awaiting an update as looking to do the same on my jam2

 

[pmac]

Its still not fitted. I need some stupid spider tool.

 

[Ash233]

@pmac, any updates? I have the exact same bike and want to go to 38T also. Keen to hear if there are any issues. Cheers

 

[Jonathan56]

@pmac, any updates? I have the exact same bike and want to go to 38T also. Keen to hear if there are any issues. Cheers

Any updates from anyone who has tried fitting a 38t and had success with the install would be appreciated.

 

[Jonathan56]

Its still not fitted. I need some stupid spider tool.

Did it fit okay? Thinking of doing the same thing, on a Scott eride genius 900

 

[pmac]

Did it fit okay? Thinking of doing the same thing, on a Scott eride genius 900

I doesn't fit. It looks like it does but it just touches the chain stay.

 

[millemille]

The bosch software uses the gear ratios as part of the calculations for the motor torque output in the dynamic modes (Tour+ and Emtb). If you change the ratios away from the factory ones it will confuse the software.

 

[Planemo]

The bosch software uses the gear ratios as part of the calculations for the motor torque output in the dynamic modes (Tour+ and Emtb). If you change the ratios away from the factory ones it will confuse the software.

How the hell does the Bosch system know what gear you are in in order for it to make those calculations?!

 

[millemille]

How the hell does the Bosch system know what gear you are in in order for it to make those calculations?!

If only it had some way of knowing the rpm of the motor and the rpm of the rear wheel and could divide one by the other....oh, wait....

 

[Planemo]

I appreciate it knows rpm and wheel speed, but does Bosch really go as far as calculating ratios? Is there any data on this? I have never heard of say a change of cassette (or chainring) being of any concern to the firmware, thats all. If it is a 'thing', can dealers alter it? I have only heard of wheel size being changed for the obvious reason.

 

[boBE]

I changed both chainring and cassette (on a Specialized) and did not notice any change in motor output. Maybe Bosch does this, ymmv...

If you really want to use the larger chainring you could maybe space it out 2 mm. This would alter the chainline but only by about one space or less.

 

[millemille]

I appreciate it knows rpm and wheel speed, but does Bosch really go as far as calculating ratios? Is there any data on this? I have never heard of say a change of cassette (or chainring) being of any concern to the firmware, thats all. If it is a 'thing', can dealers alter it? I have only heard of wheel size being changed for the obvious reason.

I spent some time recently with the technician at my LBS (Stanley Fearn's in Matlock, big thumbs up from me) going through the Bosch dealer software with him and learning from him (I've been having some problems with battery life and wanted to get to the bottom of it, which we did) and one of the things he showed me - partly because they'd just changed the front ring on my bike and I had wondered at the time if a different size was an option - is that at a dealer level the gear ratios stored in each bike's system are fixed and can only be changed by someone with manufacturer level software. This is because the Bosch system uses the gear the bike is in as one of the factors to calculate how much torque to deliver.

 

[Planemo]

one of the things he showed me

Interesting. So what did you actually see on the software they had? Something like a greyed-out section showing the current chainring and cassette ratios?

 

[millemille]

Interesting. So what did you actually see on the software they had? Something like a greyed-out section showing the current chainring and cassette ratios?

I'm in there again later this week, I'll see if I can grab a photo of the laptop screen...

 

[Peaky Rider]

I'm in there again later this week, I'll see if I can grab a photo of the laptop screen...

Ahh! Good old Stans. Three miles down the road from me and I bought my Trek Rail 7 from them last week. Surprising just how many Rails they had in stock.

 

[millemille]

Ahh! Good old Stans. Three miles down the road from me and I bought my Trek Rail 7 from them last week. Surprising just how many Rails they had in stock.

Took a relatively novice riding buddy in there on Saturday morning and he walked out £6k worse off, but the owner of a Whyte e160rs. We'd been up at Black Rocks half an hour earlier and he'd been complaining about his POS Carrera Emtb not being up to the job so I nigh on frog marched him down the hill to Fearns.

Apparently I should have a government health warning "knowing this man can lead to serious damage to your wallet..."

 

[Peaky Rider]

Took a relatively novice riding buddy in there on Saturday morning and he walked out £6k worse off, but the owner of a Whyte e160rs. We'd been up at Black Rocks half an hour earlier and he'd been complaining about his POS Carrera Emtb not being up to the job so I nigh on frog marched him down the hill to Fearns.

Apparently I should have a government health warning "knowing this man can lead to serious damage to your wallet..."

Yep, he has a row of Trek and White e-mtbs at the back of his 'showroom', despite there apparently being a shortage of such.

 

[millemille]

Yep, he has a row of Trek and White e-mtbs at the back of his 'showroom', despite there apparently being a shortage of such.

There is definitely a shortage, pretty much all of their stock is out on the shop floor and they can't get any more until well into next year. 18 months ago they had plenty of stock out the back and if they didn't have it out the back it was a matter of weeks to get it in....