What's the biggest chainring you could put on a MTB triple?

I have in mind a new MTB build, a build for speed. The rides I do I tend to include a lot of rural road/gravel path/canal towpaths/cycle paths etc. I’d like the speed of a road bike but with the stopping power of a MTB and the grip of MTB tyres. (no not a CX, I hate drop bars and I want fat tyres 2.2ish)
I have a 26″ at the moment and find myself spinning out on my current 42t big ring going down hills. The plan is for a 29″, fitting a triple to save costs and for the full range of gears. I’ve seen on crc there are 48t available. But could I fit a road chainring to a MTB triple? I guess anything too big and the front mech would struggle to switch up. Is 48t the max I could go with a standard XT front mech? If I was to go 48t on the big ring would I have to fit bigger than standard mid and granny as well?
Any thoughts of any of the above or issues you could see with this idea? Thanks.

I think I have a 44t on my old school MTB, pretty sufficient for link road riding.

I have an LX chainset running 48-36-22 with a XT front mech. No problems shifting between the three at all.

Assuming you stick to 104BCD cranks 46t dirt cheap 48t[/url]
Or a pricier 50t[/url]

Fit a 110 BCD road triple though and your choices will increase quite a bit IMO.

XT trekking chainset is what you want

I remember my first proper MTB came with a 28/38/48 chainset, this was 89/90-ish, and essentially most mid- bottom end MTB groups were just adapted touring kit at the time.

Obviously if you are willing to up the granny tooth count, you’ll get more top end, perhaps look at the available XT and LX touring chainsets. Bear in mind the bigger wheels on your 29er will effectively up the gearing range a bit too.

Maybe you need to learn to spin better – or just tuck and coast on the downhills. You’re not actually going to be spinning out your top gear on a canal towpath are you? 42/11 is almost as high a gear as the top gear on a race bike not all that long ago. Fit a standard 44t chainring to a 29er and you have pretty much the same top gear as a road bike with 53/12 which is plenty high enough to road race with. Will save a lot of hassle compared to going for something non-standard which you won’t use much.

80?

Pfft. I ran a 52t on my GT Talera 8)

(Not for long mind)

When I was 14-15 I put a 50T big ring on my MTB to make it easier to keep up on the club rides I went on (on slicks), as I only had the one bike. That gave me something like 24-32-50. It just about worked but I’d not recommend it, the shifting between the middle and big rings was a bit iffy because the gap was so big. Probably would have been ok with something like a 36T middle ring, though.

First time I rode with it on I remember hitting 49-and-a-bit mph down Holme Moss and being really annoyed that I hadn’t quite made it to 50…

I’d also be concerned about smashing a huge chainring against rocks.

Cheers lads, so it looks like yes I would have to fit larger mid and granny as I thought. I’d love to do 50mph on a bike, I love speed, cycling is ace!

Still pedalling? You’d have made 50 if you’d stopped and gone for a full tuck.

I put a 52t on my Cannondale M400 circa 1992. I had Specialized Turbo S tyres on. I spun out very soon going down Garrowby Hill. I was still spun out approaching the AA box at the bottom.

Don’t forget just going from a 26″ to a 29″ wheel will increase your gears and speed by around 10%. So combine that with the XT trekking 46T chainset and your up about 20%. Enough?

Still pedalling? You’d have made 50 if you’d stopped and gone for a full tuck.

I’d have gone flying off the road at the rapidly approaching first hairpin, too… 😀

Make sure you have enough chainstay clearance 😯

I’ve managed low 40 mph on my Langster and that’s 42:16, spin and tuck.

I fitted a budget Shimano 48/38/28 trekking chainset to my commuter/ex MTB but I had to change the middle ring to a 36 because I couldn’t get the standard MTB Deore front mech I already had to work between the top two chainrings.

Changed the rings on my commuting MTB from the standard 22-32-44 to 24-36-48.
Don’t use the 24 at all, 36 sees the chain very occasionally but the 48 is ideal for my daily bimble purposes. Cranks are LX and the mech is an XTR.

If i could fit the 48 in the middle i would ditch the other rings completely.