bruken

I’m planning on buying a brand new crankset for my wife’s bike. She has been getting back into riding recently and we are both discovering that some parts of the bike are not well suited for her geometry. She is 158cm tall and has an inseam of 70cm. The bike came with 170mm cranks, which by all metrics seem to be way too long. However, I’ve been kind of let down by my LBS who sold the bike. Neither do they think the crank length is a bad match for her nor do they stock cranks that are shorter than 165mm. So I’m a bit on my own now.
I found that Rotor makes a set of cranks called ALDHU 3D+ that go all the way down to 150mm. Unfortunately, I’ll have to order these parts online without being able to try them out or consult with a mechanic first. In any case, the bike is a Trek Lexa SLX with a BB86 and a Shimano 105 groupset and these are the parts I’m looking at.

• Rotor ALDHU 3D+ crank arms (probably going for 150mm)
• Rotor ALDHU Axle 30mm (QF: 147mm, Chainline: 43,5mm)
• Rotor 3D+ 110x5 BCD Spider (mount for chainrings)
• Rotor Chainring Screws Road
• Rotor Pressfit 4130 Steel Bottom Bracket (I guess this is like a BB86 -> BB30 adapter?)

I have 3 questions before I feel confident in ordering(it’s gonna hurt my wallet).

1. Are these all the parts I need?
2. Can I use the previous Shimano 105 chainrings on the Rotor cranks or do I need to buy Rotor specific chainrings?
3. Will the Shimano 105 front derailleur be able to properly shift these Rotor cranks?


Any input is appreciated!

tagaproject6

Is she having issues with the current cranks, i.e., knee pains or any pain at all? If not, what benefit do you believe that shorter cranks will provide? 158cm is not that short to warrant extremely short cranks.

mac_flyer9

Shimano makes 160mm cranks in the 105 groupset, as another option. Its what I use (164cm and 71cm inseam)

Russ Roth

Don't be too disappointed in the shop, most shops put their trust in the brands to provide the appropriate length cranks and brands have happily winnowed things down to 3 lengths based upon some random formula. The idea that crank length should have more variables isn't fully mainstream, just look at crank lengths on kid's bikes.
If your shimano crank is 5x110, chainrings should stipulate that they're 110 or 130. Typically 53/39 will be 130bcd while 50/34 or 36 will be 110. Does look like the appropriate BB and crank setup, only thing I have run into is that the tabs below the chainring bolts are too long to fit the crank, in the past I've solved this problem filing the chainring just enough to fit the crank.

aaronmcd

I disagree. I raced for a few years on whatever my bike had, I think 172.5. I'm almost 5'-10" or 177-178 cm, but I always felt like my legs were too short and I couldn't get low due to lowering my seat so much to reach the bottom of the stroke. I finally just went for it and got 160mm rotor cranks with my new Power2Max (a few years back) and it was the best decision ever. I was able to raise the saddle half inch and therefore get an inch lower in the front, and not feel like I was pedaling humongous circles. Everything about it felt tons better, and I'm notoriously NOT a picky person. I really wish shorter cranks were more popular and I could get them stock. I got a new bike with 170s and it's ok but it's a gravel bike so I'm naturally more upright.

However, I do agree that if it isn't causing issues or discomfort and she isn't racing or performance oriented it's not such a big deal. I had an old road bike that my boss gave my that was too big for me by at least 2cm, and had super long 175mm cranks. It wasn't nimble but I wasn't racing on it and it was good enough for 20-40 mile social rides.

popeye

Be careful what you wish for. Those shorter cranks are going to make climbing harder.

aaronmcd

Not if gearing adjusted to compensate, or if gearing is already low enough for anticipated climbing. But yeah each 5mm crank difference requires ~1 extra tooth in the big cog to compensate.

Dertam

I’m 166cm on 165mm crank feels great compared to 170 hard to find a 160mm

aaronmcd

Yeah super annoying how hard they are to find. Also super annoying how all the old threads I dug through a couple years back didn't make it clear as day how amazing shorter cranks are for those of us with shorter legs. Short cranks - not only for track and time trials.

Carbonfiberboy 

I think you are right about wanting 150mm cranks. The math works. I found stoker cranks that short! Actually, re-drilled 172.5 cranks. She loves them. Yes, if our tandem runs out of gears on very steep hills, we're slower because the cadence drops, power = force*speed, and there's only so much force available. We don't have a whole lot of climbs of over 8% here, so that's a trade-off that works for us, with our current gearing, here. You could probably do calculations based on information you have with the long cranks to see how that will work.

As above, you could get a machine shop to redrill cranks of 172.5 and longer, cutting off and cleaning up to look like new.

phtomita

I am 165cm and 71 inseam.
I also got the 160 mm 105 R7000 for myself and don't fell much difference from the 165 mm I used to ride.
Last year when riding a lot I had some knee pain. This year I had a stretch with lots of miles but my knees didn't complain so far.

One thing I noticed is that on a 40 miles loop I am slow on 4 rides I did when compared with riding with 165 mm cranks,
Planning on swapping back to 165 mm and see if my theory is right or not.
There is always weather, wind, your energy etc to consider, but taking average will give me some more data .
Currently I see some 5 to 10 mins difference when compared with last year when I used to ride with 165 mm.

Edit - btw, I tried some 147 mm cranks but was too odd and was getting some cramps frequently, so i give up.

Marylander

A lot of people find just the opposite.

Marylander

I'm looking into getting a set of cranks from here for my daughter who is 150cm tall:
Shortened FSA Vero road or compact double or road triple

popeye

Show your math

Iride01 

The whole idea of proportional sizing for cranks is bunk in my opinion. The math only shows how long you can reasonably go, not how long you must go. I rode a bike with 175 cranks recently and during a turn while pedaling, my crank hit the ground and thankfully only caused tightening of the pucker factor.

I've ridden longer cranks and they always wear me out. I'm happier with my 34.5" legs on 165 mm cranks. I've never struck the ground while pedaling a turn in 165 cranks. At least not that I can remember.

smashndash

You have a BB86 frame. 30mm spindles are a suboptimal fit. Is 160mm 105 not short enough? Is the rider in question trying to get really low and race? If not, the ease and reliability of the 105 crankset might outweigh the performance gains of the rotor.

https://www.hambini.com/bb86-bottom-...-dub-crankset/

I believe you need the 110x4 spider in order to use Shimano rings. Not sure if it’s actually compatible. Obviously the 5 bolt will not work.

https://rotorbike.com/catalog/defaul...der-110x4.html

I’d recommend trying this bottom bracket:
https://www.bbinfinite.com/products/...20970840555579

It’s expensive, but it’ll perform better than what rotor has to offer, most likely.

If the rider wants to get low and hammer, then yes, I think 150 is close to optimal. It’s biomechanically better to err slightly on the shorter side than the longer side.

smashndash

I would say that going for the longest crank possible is like going for the biggest chainrings possible or the lowest stack and the longest reach possible. A really competitive and strong rider who wants every advantage should do so, but it makes less sense when the only goal is to noodle around. Perhaps very short cranks could compromise power in some situations (though there is almost no evidence to suggest this), but if they’re more comfortable, they might be preferable.

In this case, however, it seems like the shortest crank available and the most optimal for hard riding are similar, so there isn’t much to argue - unless you’re a bike shop “professional” fitter.

popeye

Power is torque x rpm. Gear inches being equal if your climbing in your lowest gear a shorter crank is going to suck if you are riding at the limit of your ability. Its just math.

Iride01 

So how much more torque do I get from 10 mm of length that I can't easily make up for by shifting to a lower gear? For me the benefits of shorter cranks are I can keep a higher cadence without my knees pounding into my chest. I feel like I use less energy overall with shorter cranks. And there is that pedal strike thing too I don' like. I pedal through many turns even when banked.

If I liked to stand up when climbing or had a low cadence, I"d probably be happy on long cranks. But I don't do either, so long isn't for me.

bruken

Thank you all for these thoughtful responses. You guys are quick!
I've gathered from the research I've done in the past week that crank length is one of those controversial topics in bike fitting and I don't mean to be stirring the pot too much. I just want the best for the wife.

She is complaining about some knee pain, but it was obvious long before that. Her hips are rocking more than they should despite making all the appropriate saddle adjustments. The angle between her torso and thighs is also rather sharp in the 12 o'clock position. Moreover, she has difficulty maintaining a cadence over 80 explaining that it just feels so taxing and that she starts bouncing.

I must have missed that! I only saw the 165mm when I was looking around. Although we were already glancing at the 150 range since many of the formulas are pointing in that direction. And from what I gather it's the safer bet to go for shorter rather than longer.


I sort of get that the bike shops dance to the flute of the manufacturers. But if I should make a prediction, just like with 50/34 having become mainstream due to a change in attitude towards cadence among the pro teams, in a few years we will see shorter cranks shipping on entry level bikes.

It's a 50/34 on the current bike and it's a 5-point mount, haven't measured but probably 110mm then. Thanks for the filing tip!

My area is notoriously flat though. But please elaborate!

Looking at some formulas, some based on body height and others on inseam land her between 149cm and 153cm. Sure, those formulas aren't the be all and end all. But it seems like it's a safer to go for shorter rather than longer.

I found mike's site and had a look around but didn't find what I needed. Also the drilling part seems a bit sketchy, but maybe I'm just a novice here. But it's a long wait time and high shipping cost to him now it seems.

Thanks for the info. I've seen some Hambini videos. He seems to do his due diligence with these things. I'll look into to that first link as soon as I get the time.

We actually have 110x5 on that bike, so I guess it works then?

I'll add that bbinfinite to my list of consideration. As a follow up though. How easy is it to tinker with your bottom bracket? Or at least the things I would have to be doing. Do I need special tools? Might I have to grovel at the feet of my LBS in the case of having bitten off more than I can chew?


PS. I removed the URLs since I am prevented from posting because of having less than 10 total posts.

smashndash

Drilling is indeed sketchy unless you have cheap and heavy solid metal cranks.

110 x 5 shimano? So I assume it’s an older group? What groupset is it?

Also I’ve found that most of the people who say you shouldn’t go with shorter cranks are very very tall and are running quite short cranks themselves. If you’re 6’ 2” (not uncommon for fast cyclists) and run 175mm cranks, that’s like 146mm for a 170cm cyclist. Of course there’s more to it than just height, but proportionally, these tall people should be running 180-200mm cranks. And yet I have never seen one.

I have yet to see any evidence to suggest climbing is harder with shorter cranks as long as you get easier gears - which is extremely easy with today’s gravel boom.

Installing pressfit stuff is doable at home. BBInfinite has videos to show how. It does take tools, not necessarily specialized ones but specialized ones can make things a lot easier.

popeye

Divide the length of the shorter crank by the longer and that is the percentage. Read my post again I said gear inches equal, what do you do when you are already in granny? If you are power challenged rather than aerobic then you have a case for spinning. Some of us are mashers and are more efficient on standard cranks. I have had a few strikes and one airborne launch for quite a distance but I would have been faster with less lean lean and more traction. No free lunch. I ride short cranks track but that is to carry the higher cadence needed for fixed gear. Compact cranks also may favor a shorter crank depending on you. I ride 53x39 11-25 and kick on fast rollers. The bottom line is the shorter the crank the harder you will have to push all things being equal.

Iride01 

That's pretty much been my thinking, but too many seem to tout long cranks for long legs as the only solution.

For a long time I've said that if a person is a masher, then they'll do well with longer cranks. Those that can't mash due to knee pains, might like learning to spin a higher cadence and using lower gearing. I spin because my legs aren't the strongest. If I mashed I'd probably have knee pain because mashing usually will have me standing up and putting more power into my knee. Spinning I can use a ridiculously easy gear and for my area, I have plenty of them.

I currently use a 52/36 with 11-30 eleven speed for the rolling terrain here. I'm usually in the big ring for all but the few really steep grades. I like to climb at 75 to 85 rpm if relaxed climb. Higher rpm if pushing for a best. I have quite a few lower gears left for steeper climbs.

Russ Roth

Its like anything, there can be trade offs to everything, for a shorter person or shorter legged person a shorter crank might provide a more consistent power output through the whole downward stroke of the pedal. I see this a lot when watching kids pedal bikes with 152mm cranks, the seat might be close to optimal height but the knee still comes to or above horizontal, which is sub-optimal for putting power into the pedal stroke not to mention bad for the knees, same can hold true for adults with too long a crank; if the pedal has to hit the 2 o'clock or 2:30 position before the leg is able to start applying max strength then it might not be less effective then a leg that can't apply as much strength due to a shorter lever but can do so closer to the 1 o'clock position and doesn't receive the strain of having to move through a bad position to begin with.
As others have mentioned the ability to get lower and still maintain pedal without having your knees hitting you is another one; switching from 175 to 170mm allowed me to get lower and faster overall.

bruken

I just went to measure it and it's definitely 110mm. The bike is a 2015 model and most of the components are Shimano 105. Specifically reading off the cranks it says "FC-R563/R565/RS500", but that doesn't mean much to me.


I also read the Hambini article on BB86 and 30mm crank axle and I see the point of refraining from that practice. Fortunately, Rotor also has a 24mm axle for their cranksets. Does this mean that that axle will fit perfectly in my current bottom bracket despite being from different manufacturers(Rotor and Shimano)?

So I would go for crank arms, axle(24mm) and spider mount(110x5)? Only these three things? I'm just fishing for confirmation here to calm my nerves once I decide to order