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Nearly every mountain bike update this year has boasted geometry that is longer, lower, and slacker than before. But what does this even mean? In this episode of the Singletracks podcast, we define long, low, and slack as they pertain to mountain bike geometry. We discuss what’s driving this shift, the tradeoffs involved, and what this means for current mountain bike owners.
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I would like to comment about something that you failed to mention about longer top tubes. As top tubes have gotten longer, stems have gotten shorter. In addition, seat tube angle has also gotten steeper which pushes the rider more forward on the bike which helps keep the front wheel on the ground while climbing. With all these geometry changes, I find the best way to compare fit on different bikes is to add the top tube length to the stem length which I will call “size length”. My ten year old bike has a 580mm top tube and a 100mm stem totaling 680mm size length. My new LLS bike has a 630mm top tube and a 50mm stem totaling 680mm size length. In my opinion, all other measurements such as reach, stand over height, or seat tube length do not help me understand the fit much. Bike manufacturers should use “size length” (top tube length + stem length) as the best way to determine fit.
That’s a really good point and a big oversight on our part. Size length is a good idea and maybe the industry is moving that way slowly. Forever, companies keyed in on top tube length and now it’s reach. I still think reach is way more helpful than TT length though. Also, stems are really easy to swap out based on personal preference so maybe that’s why you don’t see bikes listed like you suggest.
Love the show. Not sure what was up with the audio at the beginning. I almost skipped this one but it wait clipping a couple mins in.
*it (the audio) quit clipping
Spinning off of what PlusBikeNerd said, clearly the steeper seat tube has been a big part of the LLS trend. I like to say LLSS — long, low, slack (front) and steep (seat tube angle).
Maybe it should be LLSSS – long, low slack, steep, and short(stem length)!
If you look at these long bikes, it seems to me the top tube measurements have mostly stopped getting longer. The seat tube is getting steeper which is one of those “balancing” effects that Greg was mentioning because if you were primarily lengthening the top tube soon you would be bent over too far when seated. What I am seeing (along with the steeper head tube angle raking out the front end) is that the bottom tube is lengthening horizontally by adding a horizontal section either in front of the BB or just behind the head tube or both. This effectively lengthens the “reach” measurement — the horizontal distance between your feet and hands — which is critical when you are standing in the attack position on descents (it keeps you more centered on the bike). Then if you offset this lengthening with a steepened seat tube angle, then your TT measurement doesn’t get crazy long for when you are seated, keeping on on top of the bike for climbing). I think horizontally (i.e. “long”) one of the main problems our older bikes had was that they were too short in the “reach” measurement, so when we stood up in the attack, we were in danger of going over the bars. Now there is a lot more bike in front of us in the attack position.
slacker head angle gives added resistance against steering movements when going straight out on even ground, but also means added self-steering when the bike or ground is tilted to the side. you have to compensate a greater force when the front wheel passes not perpendicular over a for example a rock.
ergo compensate with overly wide handlebars.
the effect is obvoius when you bike along frozen tracks at winter. too slack bikes throw themselves sideways on and off the often elevated trail of knobbly ice, for example after cars or from people walking. i guess this is one reason you dont see many slack fatbikes.
Agree about the stupid low bottom bracket height trend. Nearly all bikes with 5 inch or more travel have BB’s almost 1/2 inch too low for safe pedaling technical trail. Low BB’s are only good for commuting on pavement and buff fire roads. Or DH parks with minimal pedaling.
Sagged BB height should be no less than 12 inches for safe technical trail riding.
We use dropper posts and bend knees when standing to lower our weight center, not low BB’s.
Intense has consistently kept their BB height appropriate for the bike’s travel. I don’t know any other brand these days that has safe BB heights for riding technical trails.
1+ to that. and after having used normal height bb on small wheeled bromptons and moultons for many years with their excellent both stable and agile handling in traffic, i am totally convinced that the concept that bb center should be below wheel center is a myth.