I’ve ordered a 40T chainring off eBay for £13.
One of the few I could find that was compatible with the 12 speed Eagle drivetrain, and had the 3mm offset I require. There were some others with 42T, but they were all +£70.
This cheap one should give me an idea of how a better geared MTB will perform for me.
@Shuji_Sakai can you elaborate on this comment a bit please? My understanding is that FTP makes no difference whatsoever to the resistance sent to the trainer, unless you are doing a workout in ERG mode and therefore the power target is higher - and even then, it just the power target sent to the trainer.
Got the 40T chainring today.
Just fitted it to my crank, and then went to put it onto the bike.
Unfortunately it’s way too big!
Offset is correct, and I don’t think any offset adjustment would make this fit.
Glad I only paid £13!
Looking at the size difference between my original 32T and the 40T, I think I’d be lucky if a 36T would fit without catching the frame.
I did try to add a photo, but it says I can’t embed pictures in the post? Do I not have enough posts or something?
Not sure how many posts you need sorry but you wont be far off I should think. Send pic me on zwiftpost at gmail dot com and I’ll post it up. I’m curious about a couple of things - what is the offset of the new chainring (assume 6mm?), what is the frame (and size), and what spacers are on both sides of cranks now? I’ll see if I can do a bit of digging. Spacers are fine to use but you have to do the calcs and make sure you are leaving enough thread etc.
Pictures sent - thank you.
The bike is a 2017 Stumpjumper Expert - 6Fattie, in size Large.
The new chainring is 3mm offset, as per the original. The original chainring was the one that came with the SRAM Eagle groupset. The new chainring is a cheap eBay 40T that said it was compatible with Eagle X01.
As far as I could see, there were no spacers. Bearings, bearing cover, and then the crank arm was straight on. If I was to add a spacer big enough to get the chainring not to touch the chainstay and have a few mm to turn, I’d need at least 5mm. The gap in the photo is with the chainring pressed against the chainstay. I assume I’d have to space both sides the same - in which case I’d need a longer spindle (or whatever the bit that goes through the middle is called?!).
Appreciate any help!
The pictures sent to zwiftpost (at) gmail (dot) com bounced back. I think this is my 10th post, so I can maybe post them myself now?!
sorry, its zwiftposts (with an “s”)
6Fattie will likely explain it - chainstays are wider to cater for Plus tires whereas non Plus bikes keep them narrower hence they can easily accommodate large chainrings.
If you went with 0mm offset to move the chainline closer to the outside of frame, that will have helped but from what you describe, you would still need a 2/3mm spacer. Based on your estimate, thats not a lot of margin to allow for frame flex even it did fit! (whether that would be an issue on your bike I dont know).
Technically, you can add quite a bit spacer wise but limited by having enough thread. 3mm should be easy, but I wouldnt be thinking anything above 5mm would be good. Regradless of tech side, whether you would be comfortable/notice the offset is another thing. Going down the path of longer crank shaft (spindle) starts getting $$.
[edit] - i incorrectly suggested 6mm whereas should have stated 0mm as most optimal you could get
Here’s your pic’s Peter…
Yeah thats not good and 2-3mm spacer and 0mm offset would still be marginal. Only other thing you could do is find Bottom Bracket with reduced width and use that on the non-drive side to help push the crank out further on the drive side (I had to do recently on a bike and it now has Chris King and Praxis BB components) - that way you could use 4-5mm spacer.
I’m comfortable playing with this sort of thing as I have loads of spares in my bike cave BUT I would suggest you seek a knowledgeable LBS to help you do anything further to avoid cost blowouts, or plain screwups. And with any new offset, your knees may simply not like it!
Thanks for uploading the pics, and your thoughts - appreciate it.
I don’t really want to significantly change the bike to get it to work - especially when everyone says I’ll end up buying a road bike anyway!
A £13 chainring that took 20mins to swap was cheap / easy enough to try, but I think a new bottom bracket etc is going to be an unnecessary expense - especially if I have to pay a bike shop to do it.
I may buy a 36T chainring for another £13, and see if that gives me enough gearing just for cruising about for the time being whilst I consider my options.
I’ve spent £1000 on the Kickr, and £100 on the cassette, and so far have used it for about 45mins over the 3 weeks I’ve owned it. Not a good spend so far! I was hoping to be on it for at least a few hours a week. 2 weeks of those were the kids holidays, and we were away for 3 nights of each week - so that’s my excuse for now.
A wee update…
I ordered a 36T chainring, and fitted that without any clearance issues.
Time has been a factor recently, so I’ve only managed to try it once. I rode a route round Yorkshire, which was about 11km and ~260m elevation from memory. I set my difficulty up to 3/4… and the bike was perfect for that.
I don’t think I was ever on my 50 cog, however I was on the one below as there were some 20% inclines.
If I stick with routes like that with decent hills, then the bike should be ok for what I’m wanting. Ideally I’m looking to do maybe 2 evenings a week, 15-20km rides, with plenty of hills. Most of my weekend riding is off-road with 10-20% climbs, so I think this sort of riding would be beneficial to me.
In summer, I probably get out 2-3 times a week… so I’m basically wanting to fill in the missing sessions on the Kickr that I can’t get in winter due to it being dark at 4pm.
It may be that as time goes on I’ll want to be more involved and try and beat times etc… however for now, it’s just a way of keeping my legs moving!
Good to hear. Use Zwifthub.com for a super quick way to view which routes will/wont work for you.
You’ll be good for all routes in Workout mode so you can still get to see the easy stuff if you want to as well.
Thanks - I’ll check that out.
I’ve just done Greater London Loop - which was about 20k and 255m altitude. Legs were going a bit fast on the big downhill… but not so much that they were out of control. The rest of the ride was fine.
I’d been running the free trial until then (still under 50km!), so had to stop mid way up the Box Hill and subscribe 
Time to dig the huge fan out of the loft, and be ready for my next ride - as I really enjoyed that. Had Zwift on my golf simulator screen… projected at about 5m x 2.5m. Almost felt like I was there!!
