Bike Fitter Answers The Crank Length Debate! | Side By Side Comparison Of Short VS Long

So today we're going to talk about one of the biggest topics that we get asked questions on all the time in the bike fit studio and that is crank length my name is Dan and I'm a bike fitter and the owner of UK bike fit and we're going to talk to you today about the differences in crank length and hopefully give you a bit of insight on what sort of crank length you should be riding so this is our sizing jig that we predominantly use to do pre- purchase sizing fits with people who either haven't had a bike fit before or are looking to buy a new bike um but it's also got adjustable components on it that we can test the effect of so we can have a look at different stem lengths we can have a look at different bar width and bar reaches um but we're going to use it today because we have a set of adjustable cranks on here um so we're going to have a look at what effect a different crank length might have on your body as you ride so we're going to have a look at the hip angle we're going to have a look at the pelvic stability we're going to have a look at what the feet do with a different crank length we're going to test right up to a 85 mm crank length and then right down to a55 mm crank length so you'll be able to see the effects of riding a different crank length and that will give us an idea around what sort of crank length you should be riding so we're going to start by looking at a 185 mm crank length which is obviously to the extreme the stock crank lengths that come with bikes are normally relatively long compared to what most people optimally need um but by testing this um at an exaggerated length we're hopefully going to show why the old way way of thinking that the longer the crank the longer the lever the more torque and the more power sort of theory is probably not as beneficial as it once was so we've set this crank length to 185 mm and we've got Jen our other Physio and bike fitter riding the bike uh Jen normally rides 165 mm cranks so it'll be interesting to see initially how she feels on the bike and we get those first Impressions and then we're going to have a look at uh exactly how Jen's moving and why 185 mm might not be the best crank length for her so straight away we can see that Jen's pedal stroke looks really slow and labored and really hard work to especially to get over the top of the pedal stroke so how does that feel to ride at the moment H it feels like it's really hard work to get going and like at the top and at the bottom of the pedal strike there's just nothing going on it's like like uh just I'm not putting any power down but at the same time it's hard work to like get my leg ground and I also feel like my hips are just rolling around in the saddle Lads more than we normally would yeah so you can see like Jen's hips rocking up and down or tilting up and down as you get over the top the pedal stroke and like we know like gen's pretty flexible like you do a lot of strength stuff course stuff flexibility work so you know your flexibility is definitely better than average and even you are struggling to like like get around that full pedal stroke without any excess movement at the hips so if you did have a bit of tightness or weak core or you were less strong that would become much more exaggerated in someone else as well so we've obviously gone for 185 mm which is a very extreme crank length but most stock bikes will probably come with between 175 and 170 mm cranks which still for most people is a much bigger range or a longer range than they would optimally need for their bike bit um this is especially true when we're talking about women's bikes and bikes for smaller people so Jen like what's your experience of women's bikes and what sort of crank length they come with and how that feels to ride um yeah so like you said most stock bikes come with 170 to 175 and even if you're looking at like the smaller range of bikes um like most small bikes that I would get would come with 170 cranks which as women with generally shorter legs than men um we would need a shorter crank length than that so I've only seen um Liv bikes coming with which are made specifically for women coming with shorter than 170 mil cranks um and there's also the other issue of like women needing narrow handlebars we're just generally made differently to men we're not just small men so we need bikes that fit our geometry so we've now shortened the cranks on the sizing jig to 155 mm so 3 cm shorter than we were previously riding now to keep the saddle height effectively the same I've now got to raise the saddle height that 3 cm difference to make sure that the bottom of the pedal stroke as it's come up with a shorter crank is relative to the top of the pedal stroke at the same distance so we're going to watch J pedy now and see what the differences are between 185 mil and 155 mil again a big jump but just to illustrate the differences in body position straight away you can see that Jen's hips and pelvis is a lot more stable when she's pedaling going over the top of the pedal stroke looks a lot less effort she's very quickly into the downstroke rather than having a pause at the top of the pedal stroke where she's sort of laboring to get into that down phase and Cadence is increased and everything looks a lot smoother in general how's that feeling yeah it just feels a lot more efficient um and like I'm not having to put as much effort in but so like I'm not losing anything and like the whole sort of circle action has got some power going through it yeah sure so that's one of the big downsides of a a really long crank is that as you're going over the top of the pedal stroke it actually takes quite a lot of that Circle to occur before you can actually start putting power into the pedal again that last top of the pedal stroke is very dead and that's what you'd call typically a dead spot at the top and the bottom of the pedal stroke where the foot or the leg isn't able to produce power into the bike it's just getting carried over the top of the pedal stroke whereas now as soon as Jen's foot comes over the top of the pedal stroke her hip and knee are in a really good position to produce power and that's really what we want to minimize those dead spots and maximize efficiency so we've shown the differences in body position with a longer crank and a shorter crank and now we're going to look at why those differences in body position are important and how they generate power and efficiency through your pedal stroke traditionally the the longer crank was thought to have a longer lever which then would generate more torque through the pedal stroke and more torque equaled more power but more recently that's myth has been dispelled and the reason for that is that a pedal stroke is not just in one position where the foot is at the front of the pedal stroke we obviously pedal in a circle and that torque changes as you pedal through the circle and there are other benefits to having a different body position through different parts of that Circle one of those benefits is to keep the muscles in a more mid-range State rather than being fully contracted or fully lengthened and we're going to show that now with a power curve and how power is affected by muscle length so I'm going to show off my artistic capabilities Now by drawing a power muscle length relationship curve so we've got muscle length along the bottom and we're going to have power up the side and let's have a long muscle length and a short muscle length on this axis so basically when the muscle is short in a shortened position it's not very powerful so it's quite weak in that position and then as we get longer into the muscle belly or extend that muscle out we get much more power and then as we start to come more into a long position with the muscle the power starts to drop off and the muscle becomes weak again it can't access as much power so when we have a crank length that is really long in relation to our body type and geometry the muscles spend a lot more time working in this sort of range so they'll work when the muscle's really long and they'll work when the muscles are in a shortened uh position and the muscles we're talking about really are hip flexors glutes hamstrings and quads so the overall range of power when the crank is very long is probably being produced in this sort of box here so we're still accessing those higher Powers but in relation to the total pedal stroke it's a much shorter um part of that pedal Stroke versus when we have a shorter crank length and we cut out a lot of this weak muscle positions that we don't have to use use then the overall pedal stroke is used or using a much bigger percentage of time when the muscles are able to produce more power so although we're cutting out part of the pedal stroke and cutting out some positions that the muscles can't be used in those positions are relatively very weak to what we can access up here so the benefits of having a shorter shorter crank length is not only position where we're getting more stability and uh less movement of the hips and the pelvis when we're coming over the top of the pedal stroke but in terms of power output we're actually only using the more strong part of the muscle or the strong position of the muscle rather than wasting energy trying to work down here when the muscles are either really long or really short after all of that the big question is how do you know whether you're riding a crank length that's too long for you and ultimately there's a lot of feeling involved so next time you're on the bike just have think around what your pelvis is doing what your hips are doing do you feel like your pelvis is rocking from side to side and lifting up off the saddle each time your hip or knee comes over the top of that pedal stroke um and see if you can feel that that instability is going on um the other thing that you might feel with a crank that's too long is a lot of dead spot in your power output so can you feel that over the top of the pedal stroke your foot's actually having to drop quite a long way down the front of the pedal stroke before you can actually produce any meaningful power um and the same at the bottom does the power then stop before you've actually finished that total pedal stroke so that you're you're not really creating power through the whole of the pedal stroke you're actually missing a large portion of your your movement um and a good way to describe that or think about that at home in terms of the muscle length versus the power is think about your bicep so with a straight arm and the biceps at its longest position the muscle itself is actually really weak it's very hard to hold that power or that force in that position and likewise when the biceps very short and contracted it's really hard to hold in our position against some Force but the peak force of the bicep is when it's in this mid-range so in the middle of the the the muscle length the bicep can produce a lot more force and strength and power and it's exactly the same for every other muscle in your body including glutes hamstrings quads all of those big driver muscles of the pedal stroke um so if you're noticing any of those sorts of feelings of of instability or lack of power through the top and the bottom of the pedal stroke it might be worth trying to change to a shorter crank and just see what that sort of feeling is and obviously if you do want to come for a fit or a sizing and use our sizing jig to explore different crank lengths then I would definitely say that's the best option