Benji opines on what is right and wrong with the world of mountain bikes right now.
Words by Benji, photography as credited
Choosing a mountain bike is like buying a house. I don’t just mean that it’s way more expensive to get a decent one than it used to be. I mean that the selection process is governed by three things. But instead of ‘location, location, location’, mountain bike choosing is dictated by ‘geometry, geometry, geometry’.
Or it should be anyway. People still choose bikes due to what spec is the fanciest at any given price point (which is hugely overestimated in importance). Or which bike just looks the coolest (which is a sound and honest reasoning that I’m totally fine with).
Comply with me
I’m firmly of the opinion that geometry trumps all. Experience has shown that a bike with a ‘poor’ spec but the most suitable geometry is significantly more capable – and fun – than a bike with ‘great value’ spec but ill-suited geometry.
It’s more than a decade since Mondraker came out with the genuinely revolutionary concept of Forward Geometry (nutshell: massively more reach) and recent years have seen a massive cooling off of geometry change. Even among the MTB Illuminati (of which I am the current bursar), there’s a general air of reactionariness seeping back in. ‘Bike geometry is sorted now’ is an oft-heard sentiment.
If anything, I predict The New Thing will be flex. Or chassis compliance to give its Sunday name. Compliance will come to the fore for two reasons. Firstly, it’s just something new to bang on about now that ‘bike geometry is sorted’. And secondly, ebikes. Ebikes are rad. But ebikes ride hella stiff. They’re like riding a piece of scaffold down the trail. Scaffold with a load of D cell batteries in it. A bit of ‘feel is something that analogue bikes can still uniquely offer’.
Tall tales
So to answer the question ‘Where are we going next with mountain bike geometry?’, the answer is: nowhere. ‘Bike geometry is sorted now’, remember? However, to empty my spleen via its system of vents, I will now pontificate about what should happen next with mountain bike geometry.
Speaking as one of the lankier streaks of piss out there on the hills, my general theory is that Medium mountain bikes have been okay-ish in terms of geometry. But I would also point out that in the post-Forward Geometry era, Small is the new Medium, in terms of reach at least. Essentially, my reckoning is that the current geometry of Small size trail bikes serves its diminutive rider very well. Anyone of average height (175cm) or above is being short-changed. Literally almost.
Proportional geometry is the thing that should be what we do next. But it will cost manufacturers more money as they’ll have to produce more frame parts. And the current proportional geometry is nowhere near where it needs to be – adding a mere 10mm of chainstay from Small all the way up to X-Large? Seriously?
There is also a circular elephant in the room here: wheel sizes. You know how children’s bikes are sized by wheel size? We should be doing the same for grown-ups. Small folk on 26in? Medium on 27.5in? Large on mullets? X-Large on full 29in? This is a debate for another time.
Percentagize me
Taking a recent Singletrack World Magazine MTB of the Year winner – the Scott Genius ST – I’m going to put it through my patent pending Benji Percentagizer™ and work out what would happen if we were to properly scale up the geometry into a Medium, Large and X-Large if we were to use the reach figure as the baseline (seeing as reach is the only measurement of distance that’s appreciably changed in the past decade or so).
Scott Genuis ST geometry
| S | M | L | XL | ||
| A | head tube angle (-0.6° setting) | 63.9° | 63.9° | 63.9° | 63.9° |
| B | head tube length | 90mm | 100mm | 120mm | 135mm |
| C | top tube horizontal | 570.1mm | 602.2mm | 631.3mm | 659.3mm |
| D | standover height | 728.5mm | 738.5mm | 753.5mm | 768.5mm |
| E | BB offset | -33mm | -33mm | -33mm | -33mm |
| F | BB height | 342.5mm | 342.5mm | 342.5mm | 342.5mm |
| G | wheel base | 1,195mm | 1,229mm | 1,263mm | 1,294mm |
| i | BB centre to top of seat tube | 380mm | 410mm | 440mm | 470mm |
| j | seat angle | 76.8° | 77.1° | 77.2° | 77.4° |
| K | chainstay | 440mm | 440mm | 440mm | 440mm |
| L | reach | 430mm | 460mm | 485mm | 510mm |
| M | stack | 617mm | 626.1mm | 644.2mm | 657.8mm |
Percentagizer geometry
| S | M | L | XL | ||
| B | head tube length | 90mm | 110mm | 130mm | 150mm |
| E | BB offset | -33mm | -31.35 | -29.78mm | -28.29mm |
| K | chainstay | 440mm | 448mm | 456.25mm | 464.65mm |
Why have I chosen these three metrics? And only these three? Fundamentally, I think geometry in terms of head angle and seat angle should be the same for all sizes of riders. Same goes for ancillary things like stem length and crank length – off-road should just be undertaken on short cranks (160mm or less) for clearance reasons (fit easier ratio gearing if you’re worried about losing leverage). And as for standover and seat tube lengths, well, they should just be as low as possible innit. End of.
Oh, and this particular read-out from the Benji Percentagizer™ is for trail bike geometry ’cos that’s what the Scott Genius ST is. And at the end of the day, in my opinion, it’s trail bikes that most of us are interested in.
How have I arrived at these new geometries numbers? Unfortunately, that is top secret information while I await the patent currently pending for the Benji Percentagizer™. However, if you own a calculator, I dare say you can find my workings.
Hit your numbers
Let us now pore over the new geometry numbers. I’m happy with most of them. There’s only the head tube lengths that I have an issue with. Which is to be expected as the standard head tube lengths on the Scott Genius ST are very short. An X-Large trail bike should have something approaching a 140mm head tube length in my opinion. There’s only so much you can sort out with changing to a different rise of handlebar.
With short head tubes your handlebars are too low. This makes it harder to loft the bike over things. It also makes your hands and shoulders ache after an hour or so.
With short chainstays, the bike just gets too wheelie-happy on ascents. It also makes for jittery fifty-pence-piecing around corners. And there’s just generally not enough grip being levered into the front tyre contact patch, which is not good, anywhere. If you want short stays for jumps, get a 26in wheel jump bike.
Idle thought: Should the Benji Percentagizer™ have an alarm system that goes off and rejects numbers out of certain bounds? ‘AWOOGA!! These chainstays are too short (that’s not my puppy, its wheels are too small)’, type of thing. Yes, probably. It is pure vapourware after all.
I approve this message
Are there any bike brands out there right now that are offering Benji Percentagizer™ Approved geometry? The short answer is: no. The longer answer is: yes in specific sizes.
Obviously, the Scott Genius is still around for the Small size riders. For Medium frame size riders, the Cotic Jeht is a close candidate for a Benji Percentagizer™ Approved sticker. With Large and X-Large pilots, things get a bit trickier. After a probably-not-thorough-enough trawl, I could only really proffer the Orange Stage 7 for X-Large riders. I could not find a Large size bike with anything close enough to Percentagizer™ length chainstays. Maybe you could send in your suggestions on a postcard? Perhaps an email would be better, thinking about it.
Yes, there is the Geometron G1 but that’s arguably stepping out of most folks’ trail bike remit due to its 175mm of travel (personally I’d be fine with a G1 as a trail bike but that’s a story for another time).
If you’re finding all of this rather hard to follow… just pick the bike that looks coolest. You’ll be fine. Bike geometry is sorted now anyway, right?
I’m making my own frames with geometry that a magazine journalist says doesn’t work – so I must be doing something right 😀
One thing that is curiously never mentioned – it is generally easier (and therefore cheaper) to design and make tubular metal frames with with long chainstays and straight (hence steep angled) seat tubes.
And if they raised the headtubes people would complain they lacked stand-over clearance.
There’s no benefit to long headtubes beyond making them long enough that the headtube, steerer, spacers, stem and bar combination is adequately stiff.
Reach is reduced by raising the bars (whether by stem, spacers or bars as long as you keep the grips to steerer relationship constant) but if the frames reach is 20mm longer on a given size, then for a constant bar height the actual reach is still >20mm longer if there’s any extra headtube at all.
Only because slamming the stem and flat bars is fashionable. A 250mm headtube would look equally fugly. We’d all end up riding bikes like Emily Batty.
If you want a ‘perfect’ fitting bike with flat bars, flat stem, no spacers, it’d have to be a custom build for you otherwise geometry needs to be literally be the lowest common denominator so that the most people possible can then pick anything from a flat bar to +80mm rise.
Correct, but the reduction in reach from fitting a higher rise bar is much less than the reduction from adding spacers under the stem. I’d much rather have a longer HT if it means I achieve/maintain my desired stack without faffing around with spacers (and losing reach) or very high rise bars. Doesn’t mean all frame sizes need to have longer HTs but increased proportionality for XL frames would help IMO.
There’s a few Deviates among the folk I ride with and I’m really not a fan of the short HTs their designer favours: 121mm on a XL, and that’s an integrated headset too so not height gained from that. Just looks out of place on a XL frame.
Even on my XL G1 with 140mm HT I have 15mm of spacers under the stem and a 40mm rise bar and am still tempted to try higher rise bar. I’m not even that much of an outlier in height at 187cm. It’s be harder to achieve the same position with a shorter HT such as the above-mentioned Deviate.
”but the reduction in reach from fitting a higher rise bar is much less than the reduction from adding spacers under the stem”
Only if you accept the steering feel changing. You can’t raise the grips on any bike without reducing the reach – the stack and reach are firmly linked by the head angle.
That is dependent on the shape of the bars though and how you rotate them.
In your example you’re effectively lengthening the offset between the bars and the steerer tube (like running spacers + longer stem and flat bar).
For most people they would want to keep that constant, either by picking bars where the rise was added inline with the steerer rather than by bending them forwards, or by then running an even shorter stem,
You would, someone who likes a low front end wouldn’t. And the frame needs to fit both.
And you’re not ‘losing reach’, as we’ve already agreed, if the grips are ~300mm above the fork crown on a Large, and the XL has 20mm longer reach, the bars are still 20mm further forward.
You only get less than the advertised reach increase if you do something nonsensical like compare two sizes with the same number of spacers and different HT lengths. If you compare two sizes with the same steerer tube length (but the larger one has any extra HT at all) then the reach increase will always be MORE than the advertised value.
If you want a taller headtube and more reach than an XL, then the solution isn’t to increase reach and stack but keep calling it XL, it’s buy an XXL surely?
But presumably he’s made a few sales to people who want their bars ~140mm above the fork crown that would otherwise not have happened as you can add rise but can’t take it away (in an aesthetically pleasing, not going to crack your top tube in a crash with the stem way).
I’m in a similar boat as at 6ft but with short legs (31″ jeans) I tend to end up riding either “Large" frames with quite an upright position and swept back bars (Stooge Moto bars FTW!), or for more ‘efficient’ riding either XC or Road I tend towards frames that fit bigger than stated (i.e. labeled 56 for the seatube but have reach figures more like a 58-59) which gives me more space to tuck my elbows in and get low. Which is a roundabout way of saying for me at least the longer the frame is the LOWER I want the front end because it gives me the room to tuck in without having to hunch.
I get that tall people (usually) want longer head tubes, but do you think bike companies don’t put them on because they make bikes look uglier? I’m not saying that this is a good reason to have shorter head tubes, but I have always thought that shorter ones look nicer.
Bike geometry has a way to go as it still isn’t taking into account basic principles. If you got a group of 10 men lined up, with each one 5cm taller than the other I guarantee the difference in torso lengths would be much smaller than the difference in leg lengths. Yet for some reason we still size bikes on total height. Most bike brands would recommend that a man who is 5ft 9” should ride a Medium frame and a man 5ft 11” a Large. The % difference in the reach of a Medium frame compared to the Large frame will be a larger % than the torso % difference of each man.I agree that as a sizing metric a mtb’s reach is the most important. If we are to size bikes properly though we need to stop thinking about total height and base it on a combination of torso, arm length and flexibility.
“If you got a group of 10 men lined up, with each one 5cm taller than the other I guarantee the difference in torso lengths would be much smaller than the difference in leg lengths."
I have no idea what you’re basing that statement on – I’ve never seen anything to suggest that is true. In fact the range of leg lengths on mens’ trousers tends to suggest the opposite!
I was going to say, that’s the exact opposite of my personal experience. Average legs and a much longer torso.
Source?
Geometry change seems to be (rightly) settling down lately, it’s getting pretty good. The biggest issue I still see now is there are brands giving effective seat tube angles that have little bearing on reality at saddle height. The difference between some bikes that should be the same angle is quite shocking.
I wouldn’t necessarily object to a taller headtube, but it’s got to be done in combination with the rest of the geometry. As a taller rider I’ve typically run low-ish bars. For one thing I’ve got long arms, so can reach down and still maintain the same general body position. But often I’ve been running lower bars to compensate for overly slack seat tubes. Which creates its own issues, like closing down the hip angle. A higher front end/stack isn’t any good without a steeper seat tube. Otherwise it feels like I’m falling off the back. Front end/stack has gotten higher in recent years. Seat tubes often haven’t gotten steeper by enough to overcome that shift in body position.
I think the next thing is probably going to be lower BBs matched to shorter cranks. Some people are doing this already but I reckon it’ll become more mainsteam. When you look at the more extreme older bikes, this is what really stands out- I just bought myself a 2016 geometron for a laugh, and what looked absolutely bananas in 2016 now frankly just looks pretty sensible, except that BB drop.
I guess a lot of this depends on what you want the bike for. I haven’t got a very modern progressive geometry bike but whenever I ride one I find the steep seat angles great for short distance trail riding… but as an old bit I wonder how long it would take me to adapt to all day, every day backpacking type riding.
“It’s just something I remember from Uni, basic anatomy. A quick google though brought this up”
The sample size in that paper is pretty small, and the lack of difference between ethnicity is at odds with another study I read. That study found a higher LBR for people of African heritage and a lower LBR for people of Northern European heritage (which makes sense for cold weather evolutionary adaptation).
Still using a Whyte T129, with various upgrades, just the frame now really. My only issue is rear wheel clearance for larger tyres. Geometry is just fine. I prefer to get and ride rather than stress over a degree here or there.
This is probably correct because leg length is affected by nutrition in pre-teen years as well as genetics, torso length will be genetics only. Greater wealth and thf better nutrition has meant that the average height in China has increased in the last 20-30 years and a lot of that will be leg length averages.
We’re more sensitive to half-mast trousers than t-shirt hem position? : )
I think the next thing is probably going to be lower BBs matched to shorter cranks.
Could be – already a driver of different size kid’s bike design beacuse there’s a lean angle test in ISO that gives a minimum, as with MTBs. But a low kid’s bike saddle is good, a low MTB BB pins the bike down and makes getting the front up less easy so not always better? BB drop probably has an ideal for a ride feel aim as well as the need for pedal clearances.