How road/gravel bike sizing compared to mountain bike sizing

Go bigger!

I’m 5’8”/172.5cm

I ride a gravel bike with 385mm reach and a 60mm stem, and it’s a touch too long I feel (although that’s the ETT really not the reach).

I also ride a mtb with 455mm reach and a 40mm stem, and it’s perfect. Another 10mm would be fine too.

I’m 181cm and on a 475mm reach with a 35mm stem, which feels right for me.

For comparison I'm 178cm tall but with short legs and the reach and stack on my gravel bike are 406/590 and I use a 60mm stem. 130/140mm travel mtb is 484/643 and I currently run a 35mm stem (started with 50mm but it feels better on steep stuff with 35mm). I'd happily have even more reach on the mtb if I could get it without having to have a longer seat tube.

Reach can be much more variable (for use/preference for a given rider height) on an MTB than a drop bar bike imo.

I wouldn't base your new MTB size off your current drop bar bike though. Better to try a couple of bikes and see how the feel. Longer reach is good on MTBs but for quick laps at Ashton Court or similar rolling woodland single-track a very long bike might feel a bit much.

This is going to sound luddite, but if you’re, say, a medium on a road bike, chances are you’re a medium on a MTB.  Probably more accurate than mm.

Then test ride a few.

I'd agree with Jameso, especially the bit on trying a couple of bikes (do you have MTB friends?).

At your height, I suspect you'd be on the cusp of medium and large frames - probably wanting a reach of 460 to 470mm as a safe middle ground in terms of geometry.

I'm 178cm tall and my bikes are 501mm and 495mm reaches

It depends what you want the bikes for really, if you're going to be sat down and pedalling (XC stylee) than a shorter reach will be OK, if you're going to be stood up and doing a lot of "body English" then go longer.

Regardless; at your height, I'd be looking at longer reach.

I suspect you’d be on the cusp of medium and large frames

I suspect that 180cm is firmly in the L category for most bikes these days

I typed out a long reply and hit the wrong button and lost it all.

I think you could go a bit bigger reach at your size - especially if picking a bike with a steep seat angle - otherwise you’re going to be cramped sat down. Although stack also comes into that - and whether you’re buying a hardtail or full suss bike.

Depending on budget, in your position I’d be inclined to go for a ‘down country’ style full suspension bike with no more than 120mm travel both ends. It’ll be both comfortable and efficient and quick round AC / LW but would also do a job if you went to Bourton Combe which is also very close and a bit more tech / harder going on a hardtail. 50 acre woods is also hard going on a hardtail unless you’re used to it.

Thinking the likes of a Specialized Epic Evo / Transition Spur / Trek Top Fuel etc would be good for what you need - but they can be quite pricey. This Rocky Mountain seems ok for the money and is a similar kind of thing:

https://www.paulscycles.co.uk/bikes/mountain-bikes/rocky-mountain-element-a30-full-suspension-mountain-bike-2022-orange__9114

If you want to try a hardtail then the bike shop on the golf course at Ashton court hires out Whyte hardtails which are fairly decent. I’d say have a go on one of my bikes for size but the saddles would be too low and I don’t think I have enough slack on the dropper post cables to put them up.

Happy to show you round the local trails though.

If it helps, I'm 187cm and bikes sizes as follows (reach/stack):

Gravel: 454/645 with 70mm stem and shallow, extra-wide flared drops
Hardtail: 510/641 with 45mm stem
Trail FSer: 519/640 with 45mm stem
Silly big FSer: 511/645 with 45mm stem

So basically my gravel bike's quite a bit shorter reach than my MTBs. I like a long bike so mine are all very much at the lengthy end of the spectrum.

Sounds like you are a large

go by the manufacturers recommendation...unless you can demo back to back

you aren't sat down pedalling 50 miles out on an MTB, you're dynamically moving around the bike.. so comparing like for like isn't exactly wise IMHO

Long time roadie and CX rider. Moved to mountain bike about 10 years ago or so.

I’m a bit shorter than you, 5 foot 8. Road and CX bikes were generally a 52, or a 51 for a Cannondale Supersix…love smaller shorter road bikes. Current gravel bike is a small.

I made the mistake of thinking I’d only ride easy woodland trails, so got a XC hardtail with 110mm travel. I sized similar to my road style, at the time it was a 16.5 (small). The geometry is completely different, I’d have to be careful not to knock the bars with my knees when climbing out of the saddle. Over the years I’ve found I prefer a longer bike, my 160mm travel full sus is a medium/large or 18.5 inch frame.

If I was looking at mistakes I’ve made with purchases since moving over to mtb, I’d have missed out on the small travel hardtail. 120/130mm full sus would be my ideal. My 160mm is too much for most local rides and I don’t really get the opportunity to ride in North Wales as much as I used to. And I’d have sized up on my first couple of bikes (relative to road sizing)

397 with an 80mm stem on gravel bike, 515 with a 35mm stem on Enduro bike.

There are too many variables, you can't even compare mountain bike to mountain bike based on one number alone.

This is going to sound luddite, but if you’re, say, a medium on a road bike, chances are you’re a medium on a MTB. Probably more accurate than mm.

Then test ride a few.

^^This^^ isn't bad advice TBH, check the manufacturer's charts and sizing advice and go from there.

You are coming to this "cold" and comparing MTB reach numbers to road/gravel bikes is sort of redundant, the controls setup and intended use are so different a ~400mm Drop bar/100mm stem Vs a ~750mm riser/40mm stem put your hands/arms/torso in such different positions (for good reason) that the layout of the frames isn't really comparable.

Presumably answers to the above might change depending on whether it’s a trail or XC bike? I haven’t made my mind up on that front yet which is probably a whole other discussion!

Again it may well be worth your while doing some test rides to see what sort of bike suits you're intended uses best.

You spend so much time standing up on the pedals, size is less critical than your road bike. Unlike road, I too would say go bigger and use a shorter stem. You're a medium/large depending on maker. Try a couple. Standing up on the pedals, not just sitting and spinning. See what feels comfortable, but I'd avoid going too small for an MTB.

Thanks all for your really helpful responses. In that case my road sizing is going of out the window - you read all these reviews about long, low, slack geometry being amazing but based on road/gravel riding, overly long and low sounds really uncomfortable so the numbers all look uncomfortable! I really hate being too stretched out on the road.

I have rented a Whyte 801 hardtail from Pedal Progression at Ashton Court before. I can't find a geometry chart for that version but it was a large and felt fine (although I spent most of the time getting used to the 780mm wide bars which felt enormous compared to anything I'd ridden before!)

I also rode at the Dare Valley Gravity Family Bike Park and rented a Marin San Quentin 1 hardtail there, also in large, which had 464mm reach with a 45mm stem and 599.7mm stack. Again, that felt fine (not that I did much pedalling as there was an uplift).

@joebristol nightmare on losing your post! Thanks very much for the offer, which I would love to take you up on sometime. A downcountry full suspension bike sounds great, but I think I'm sold on the relative simplicity and ease of cleaning/maintenance of a hardtail (hopefully that won't be a mistake as @w00dster had on his XC hardtail - I was sort of moving in that direction, so that's a great cautionary tale).

upintheair

(although I spent most of the time getting used to the 780mm wide bars which felt enormous compared to anything I’d ridden before!)

780 is on the larger end of bar size, and as a general rule, the wider the bar, the shorter the stem you want - just bearin mind that you can cut down MTB bars, which isn't really an option for curly bars.

@honourablegeorge I saw a Reddit post the other day where someone had cut down their curly bars in the middle and held the two halves on mechanically with a normal stem faceplate - can’t see any problems there!!!

Yeah - that's the kind of madness I was thinking of when i said "isn't really an option", there's always one

I’d try and ride an xc hardtail before committing to one. I’ve built myself broadly a trail hardtail - but at the robust end of things with 2.6” tyres. Keeps it comfier than something with 2.2-2.3” tyres.

You’ll find yourself standing up quite a lot on a hardtail vs either road or a full suss bike. It’s not the downhill rocky stuff that’s the issue - more the pedalling along sort of stuff that’s just a bit uncomfortable sat down with no rear suspension.

I think in your use case I’d just go for a middle of the road trail hardtail - which one that is depends on budget.

Bottom end - Voodoo Bizango perhaps. Bit higher perhaps a Bird Zero 29 / Vitus Sentier / Nukeproof Scout sort of thing, higher again (proper bling) something like the Yeti Arc.

I am similar dimensions to you - 179cm tall with an 83cm inside leg - and I ride a large mtb with 470mm of reach and a 50mm stem. I would say that it's pretty much spot on size wise.

I would also mention that I reckon a hardtail mtb is too similar to a gravel bike. If I were buying in your position it would definitely be a full sus. As Joe says above, it will be so much more comfy, even just for pottering on bridleways.

@joebristol I know what you mean re the uncomfortable pedalling along stuff! I'm reasonably happy going down things like the right hand side of Bourton Combe (the nice fast bit at the top, slabby bit near the bottom, then the right hand descent to the road) on my gravel bike, but even more so with a set of 650b 2.25" tyres on. The things I really don't like are lumpy rocks where I start bouncing all over the place. I have a real problem with the bridleways going north off Cadbury Camp Lane and emerging onto Clevedon Lane for example.

The hardtail bike I keep coming back to is the Merida Big Trail. It has various great reviews. Maybe as @Jamz says I should be thinking about turning my road bike into my commuter and going for a full suspension, especially as my gravel bike is on 2.25" 650b tyres for the summer. The Vitus Mythique looks like a bit of a steal at the moment.

<p style="text-align: center;">I think that Merida has fairly decent reviews from memory. The track you’re talking about in Bourton is broadly just a fireroad climb for me. There’s another rocky fireroad down the middle which is much rougher and rockier + loads of unofficial stuff that has a couple of spicier features</p>
That new model Vitus Mythique looks alright actually. I think it was reviewed fairly recently - maybe on bike radar?

Haha classic gravel bike vs mountain biker – your fire road climb is my fun descent!

The Merida got a great review in STW  https://singletrackmag.com/wp-content/uploads/2020/08/merida-big-trail-600-first-ride-review-all-the-hardtail-youll-ever-need-for-just-1500/  and 9/10 in MBR  https://www.mbr.co.uk/reviews/hardtail/merida-big-trail-600-2022-review

All the reviews of the Merida talk about tiki NBB advantage of the short seat tube and sizing up to make it super long, which is partly where my original question came from. The large has a reach of ‘only’ 455 but the XL is 475.

The Vitus got 10/10 in MBR  https://www.mbr.co.uk/reviews/full-suspension-bikes/vitus-mythique-29-vrx-review

So it sounds like I can’t go too far wrong with either! I would like to do the Trans Cambrian Way and Cycling UK’s soon to be announced bikepacking route in North Wales which is pushing me towards the hardtail.

I’d lean towards the full suspension bike tbh - any kind of long ride the full suss is much more comfy. As well as a bit of a skills compensator. 120/130mm is on the shorter travel end of trail and it’s surprising how efficient most full suss bikes are now.

Unless it’s a smooth fireroad / road I find my 160mm travel / 35lb 29er full suss bike a better climber than my 140mm forked / 30lb hardtail.

On the Merida 475mm reach on a full suss bike would probably be ideal for you. On a hardtail that sounds fairly long as that’ll go up with fork sag - although I’m not sure how much by. Will make it better for faster / rockier stuff as it’ll be more stable than a shorter bike - but might be slightly harder to weight the front wheel for turns / won’t be quite as nippy on twisty Singletrack.

That said, the Whyte you rode was probably fairly lengthy on reach as they usually are - if it’s still the same as the 2019 model it would have been reach 461 / stack 628 / seat tube 481 (really long) / seat angle 74 / ett 641

Just had a look at the Merida review - that’s the xl with the 475mm reach that was reviewed. Perhaps the size down might do the job - has a 455mm reach static.

Thanks very much for all of the input. I’m definitely going to go for the Merida Big Trail because I’ll also use it for my short commute. Now I can’t work out whether to go XL or L!

- Many reviews say size up (so XL) for longer reach, more progressive geometry and therefore more stable.

- Merida’s website has a calculator which based on my height (180cm), inside leg (85cm) and arm length (60cm) says size L (tending towards XL rather than M).

- Seat tube is v short so stand over height is not an issue.

- Heart says go for the XL - mtb sizing is v different from road/gravel bike sizing and stability sounds good!

- Head says that the 20mm difference in reach probably won’t be that much in real terms, I will be doing some seated pedalling on this bike and I am worried about being too stretched out (I really do hate being too stretched out).

- The stem is 50mm on both so there is little/no scope on going longer on the L if it is too short, but some scope in going shorter on the XL if that’s too long (shorter stem, but more sweep on the bars).

I am probably overthinking things!

The stem is 50mm on both so there is little/no scope on going longer on the L if it is too short, but some scope in going shorter on the XL if that’s too long (shorter stem, but more sweep on the bars).

Eh?

Other way round surely? It would be easier to fit a 70 or 80mm stem to the L to get similar reach (to the bars) to the XL but to shorten the XL you'd need a ~35mm which is fine but it all depends on how you want the bike to handle.

Just ignore the numbers and buy what fits.

You know that no two manufacturers size the same.

Use geometry comparing software to overlay frame images to compare shapes with what you like.

Right - large ordered! Thanks again!