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  1. #1
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    New question here. Crank arm length and chainring size proportional?

    I'm 5'8" with 29.5" inseam. Currently running 175mm cranks which I think is too long for me. I'd like to experiment with 165mm cranks. I'm currently using a 32t ring and 11-40 cassette. To supplement the 165mm crank length accordingly, should I use a 30t ring as well or keep the 32t? Advantage/disadvantage with changing ring size whilst going with shorter cranks?
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    Shorter cranks will give you less power to the cranks, so you'll either want to use a smaller chainring, or suck it up. It'll only push you up a single gear in back, so unless you're running out of gears, I'd live with the 32T chainring.
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    Good question!

    I'm similar height to you, and still running a 175mm. Basically because that's what I have, and its a fairly expensive exercise to try other sizes (unless you can borrow some from somewhere?)

    I did have a set of 172.5 cranks, to match my road bike, and they did feel good. But they were double, so I sold em.

    So, with a shorter crank length, you'll need to raise your saddle by the difference in length (10mm if you got to 165mm) to achieve the same leg extension at the bottom of your pedal stroke. And on the upstroke of your pedal action, your knee won't need to bend as much when your foot goes through the dead spot. This could be a good thing for us with short legs, but you may want to raise your bars too.

    If you work on the fact that the power that you can put out being fairly constant, the shorter crank length will give you less torque, so to achieve the same power, you'll be spinning more in a lower gear. This may or may not work for you. I have roadie friends using very short cranks (150mm) based on some info here: https://www.powercranks.com/cld.html
    and they've said that it only took a short time to adapt.

    Things might be a bit different on a MTB, since there are times when I feel you need the extra torque that the longer crank gives, ie up a steep pinch climb, and I do ride a singlespeed where this is more apparent, but maybe this is in my head, and contradictory to the info on that website...

    Give it a try, let us know how. I'd be keen to try a 170 crank if I could borrow one.
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    Seems like 10mm is a pretty small adjustment, less than 1/2 of an inch. I understand the theory but is a difference this small really noticeable to the rider? Just curious.

  5. #5
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    When knees are in UR chest....the less torque there is....Found this riding 125mm on road bike, and 135mm on the CX and MTB. Peeps are always asking me, how I can climb and talk at the same time.....no knees in the lungs=better breathing....Wished I had know this thirty yrs ago.....Also, had to change the way I ride some steeps, would spin-out from more power....needed more weight on the rear wheel.

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    Good info. Do you notice any reduced torque on pinch climbs? Say when you may hit the climb but you don't have time to change gear, and you just have to grind your way to the top (like riding a singlespeed!)
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  7. #7
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    xcbarny....actually feel like I have more power with less circumference to spin, but I ride a Rohloff on the CX and MTB.....

  8. #8
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    Good to know. What cranks are you running?
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  9. #9
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    Quote Originally Posted by watts888 View Post
    Shorter cranks will give you less power to the cranks, so you'll either want to use a smaller chainring, or suck it up. It'll only push you up a single gear in back, so unless you're running out of gears, I'd live with the 32T chainring.
    Isn't that arguable? Isn't power dependent on torque and torque dependent on leverage? If my crank arm is too long causing too much knee bend or bad hip angle or inadequate glute/quad/ham muscle contraction, doesnt that mean I'm not getting the most amount of leverage? Which in turn means I'm not applying the most torque possible which in turn means I'm not outputting the maximum amount of power? What if a shorter crank arm means I can apply more leverage because of better angles and whatnot.

    I'm not running out of gears in the back, in fact I'm rarely in my lowest two.

    My theory is that a shorter crank will optimize my pedal stroke since I can spin easier. Constant spinning means I can keep the bike moving especially over techy stuff or steep climbs instead of stalling whilst trying to mash. Again, just a theory.

    Thoughts?
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  10. #10
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    I think you're on the right track. Obviously longer crank arms give you more leverage, but due to biomechanical limitations, you may be able to apply more force to the pedal, when the crank isn't to far away.

    Power is a function of torque and cadence, so if you reduce the torque, you can still achieve the same power, by increasing your RPM (power = torque x angular velocity)

    I think a smaller chainring will help you increase your cadence slightly, but I guess just give it a try, and see how it feels.
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  11. #11
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    I have custom made Power Cranks on the Road bike, running White Industries ENO on the CX and Mtb, which I had a machinist cut down to 135mm. There R people who will cut down UR existing cranks (BikeSmith Design) or may have to go with Square taper BB.... Turn3racing....The cranks are not closer(U raise the saddle to compensate) your spinning a smaller circumference, thus keeping knees in a more powerful position, and out of UR lungs....
    Attached Thumbnails Attached Thumbnails Crank arm length and chainring size proportional?-dscn3367.jpg  

    Crank arm length and chainring size proportional?-dscn3369.jpg  


  12. #12
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    This article contradicts the need to run a smaller gear:

    “MY OWN CONTINUED RESEARCH AND EXPERIENCE SUPPORTED MARTIN AND HIS ASSOCIATES’ FINDINGS. BUT THERE WAS A RECURRING THEME I WAS NOTING IN MY WORK WITH ATHLETES: IN ORDER FOR SHORTER CRANKS TO ‘FEEL RIGHT’ TO MY RIDERS, THEY HAD TO START TURNING BIGGER GEARS.”

    Crank Length-Coming Full Circle | Bike Crank Lengths Shorten
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  13. #13
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    I would caution this application of short cranks vs long. The world of time trialling and multi discipline events are not the same as MTB. In TT events (I used to do these) I always used 165mm and found an even cadence was easier to establish, however, I was riding courses that were flat, smooth, and fast surfaced, with steady pace riding. In MTB, low speed, irregular surfaces, hills and objects to surmount, riding inconsistent cadences are the order of the day, and torque does count here. In looking at the variations, analyse your environment before you make decisions. Unless you're badly miss-matched to your cranks (like 5' pushing 180's) 5mm = 10mm in overall leg movement in a full revolution, it's not much. However, I do notice the difference between 2 bikes I have, 1 at 180 and the other at 172.5mm. I feel comfortable off-road on 180's, but can pedal better on the road with the 172.5's as I am in a steady state of riding. I have a 34" inseam. I don't think at the OP's sizing that 175mm is particularly exceptional, but it depends on whether you can keep steady cadences or are confronted with obstacles putting the cadence all over the place.

    Your call.

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  14. #14
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    Timely post. There was some good information on the subject posted in the Women's Forum recently that I stumbled on. It convinced me, after seeking the proper measurement of inseam, to go with 170mm instead of 175 that by default, I have always used. To begin with, the trending of low bottom brackets had me pedal striking on our rocky trails like crazy on my new bike. Also, I have always suffered a bit of pain across my left knee cap after about 9 miles of climbing output. It's my longer leg and MRI proved no abnormalities or injury. After reading up on the info from that recent thread, I decided to make a change and just put a new Race Face Turbine Cinch 170 crankset on my Stumpy Expert Carbon Evo. There's a bit of adjustment, I'm 3 or 4 rides into it. After making all the necessary seat and ergo adjustments, my knee issue is feeling like it's gone! This is HUGE for me. I'm positioned better on the bike for climbing. Finally, pedals strikes on the uphill climb as well as downhill are pretty much gone. I needed "just that much".

    I'll go dig up that thread and link it. The beginning measurement to determine the correct starting point was your properly measured inseam in millimeters times .216, i.e., 800mm x .216= 172.8

    http://forums.mtbr.com/womens-lounge...th-995982.html

  15. #15
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    I find I'm more comfortable on 170 mm cranks than 175s. If you think you might be, I'd encourage you to try it. Or whatever size.

    There's a lot of contradictory material out there. Pez Cycling did a round up a while ago and concluded that you should use whatever you're most comfortable with.

    I wouldn't change my chain ring right off the bat. Maybe you won't ride any higher cadence with shorter crank arms, but you'll be comfortable developing a little more force. Who knows? There's a ton of individual variation here, even if someone is able to identify some common trends.
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  17. #17
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    This may be an interesting starting point for if your cranks are the wrong length:

    Indicators your cranks are too long.

    You are the first to get out of the saddle when climbing in a group.
    You feel your upper body 'bobbing' vertically when spinning fast and lightly loaded.
    Your transmission occasionally 'clangs' as you hit a dead spot at the top of the stroke.
    You prefer a low cadence i.e. 75rpm or less.
    Your knees / hips / lower back hurts.
    Your knees come uncomfortably close to your chest when on the drops or tri bars.
    Indicators your cranks are too short.

    You feel strong on hills but seem to struggle on the flats.

    BikeDynamics - Bike Fitting Specialists - Crank Arm Lengths

    My knees are generally fine, but I regularly get lower back pain. I'm beginning to think that going to 165mm would be advantageous for me, but dam I've got 5 bikes. This could get expensive!
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  18. #18
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    Shopping for a new ride right now and thinking about getting the proper crank length for my 30" inseam. Every time I ride few times hard in the same week my butt muscle hurt and I get all sort of back pain. Just getting over a 2 weeks sciatic nerve pain fiasco...

    If I crash some numbers I should be 165mm, big difference from 175 on my Stumpy. I'll try to aim for 170mm if I can...

    Good read! Thanks

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  19. #19
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    Quote Originally Posted by Eric Malcolm View Post
    I feel comfortable off-road on 180's, but can pedal better on the road with the 172.5's as I am in a steady state of riding. I have a 34" inseam. I don't think at the OP's sizing that 175mm is particularly exceptional, but it depends on whether you can keep steady cadences or are confronted with obstacles putting the cadence all over the place.

    Your call.

    Eric
    According to several calculators, you should be running 185mm

    I can't keep a steady cadence. On the slow techy stuff I find myself mashing to get over obstacles which I don't always clear and if I do then I'm gassed and the rest of my ride suffers.

    This is one of the trails I ride which illustrate the tech I'm talking about. Another one. All those 6"-12" step ups add up over time especially since that trail averages about 3 hours and it has this elevation grade.
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  20. #20
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    According to several calculators I should be running ~190's, I tried 180's for awhile and totally hated them. 170-175 for me.

  21. #21
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    Are you using your 32/40 gear much?

    I don't have any bikes set up 1x. But I know if I don't have about an 18" granny, I really miss it. With an 11-40 cassette and still riding a 29er, I'd want a 24t chain ring. (Could I even buy that?)

    People can adapt to a pretty wide variety of crank arm lengths, and the difference between 165 mm and 175 mm is really quite small. So gear selection seems more likely to be the problem to me. If I was going 1x, I'd want a 42t granny cog. That would bring me to a 26t chain ring, which I know I can get as a direct mount ring from Race Face, and maybe some others.

    The pricing's a bit much for me, but I have some sympathy with what SRAM has done with a 10t cog and new driver to go with a 1x system.
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  22. #22
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    Quote Originally Posted by phoeneous View Post
    Isn't that arguable?
    Of course it's arguable. Everything on the internet is arguable. My point is soley from a physics standpoint. Longer crankarm = more torque. It's math and can not be dis-proven. Bringing biometrics into it, all bets are off. Shorter crankarms can help provide better torque for someone where the longer crankarms don't work well.
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  23. #23
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    And after all that I decided on 170mm. Didn't want to take a chance with having too short of cranks and living with it for several seasons. Besides lbs had a good deal on rf turbine cinch in 170mm. Thanks all for the input.
    What the hell is an aluminum falcon?

  24. #24
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    Quote Originally Posted by phoeneous View Post
    And after all that I decided on 170mm. Didn't want to take a chance with having too short of cranks and living with it for several seasons. Besides lbs had a good deal on rf turbine cinch in 170mm. Thanks all for the input.
    Post a follow up! I put the same set up on but I stuck with my 2x because I love it. So far it's good. If it stays that way, I'm going with SixC carbons but I didn't want to throw the $$ at it till I had some hard evidence to go on.

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    Quote Originally Posted by Oh My Sack! View Post
    Timely post. There was some good information on the subject posted in the Women's Forum recently that I stumbled on. It convinced me, after seeking the proper measurement of inseam, to go with 170mm instead of 175 that by default, I have always used. To begin with, the trending of low bottom brackets had me pedal striking on our rocky trails like crazy on my new bike. Also, I have always suffered a bit of pain across my left knee cap after about 9 miles of climbing output. It's my longer leg and MRI proved no abnormalities or injury. After reading up on the info from that recent thread, I decided to make a change and just put a new Race Face Turbine Cinch 170 crankset on my Stumpy Expert Carbon Evo. There's a bit of adjustment, I'm 3 or 4 rides into it. After making all the necessary seat and ergo adjustments, my knee issue is feeling like it's gone! This is HUGE for me. I'm positioned better on the bike for climbing. Finally, pedals strikes on the uphill climb as well as downhill are pretty much gone. I needed "just that much".

    I'll go dig up that thread and link it. The beginning measurement to determine the correct starting point was your properly measured inseam in millimeters times .216, i.e., 800mm x .216= 172.8

    http://forums.mtbr.com/womens-lounge...th-995982.html
    I'm 5'10" and have a 33.5 inseam (from the ground), so under the assumption that somewhere around half of your height is in your legs, you should be 5'6-7", right?

    That should put you on a small frame, medium frames generally have 175mm cranks, do smalls not downsize cranks, or is it a case of sizing up and ending up with the wrong sized components?

  26. #26
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  27. #27
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    You were absolutely right about having more leverage, and will not need a smaller chainring with your 170mm cranks. It's only going around one time per revolution, regardless of crank length, so if you genuinely needed 170s, they are not going to make things harder, only easier.
    I have a 30" inseam, and would like to tell you what a difference going from 175 to 170mm cranks made, what with not feeling like having to power over a hump at the top of each pedal stroke and all, but I can't, because I almost immediately switched to an oval chainring, too. They're a great combo, IMO. Made everything better overnight. Big difference, and I was making it over obstacles that I'd never been able to on my very next ride.

  28. #28
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    Quote Originally Posted by OwenM View Post
    You were absolutely right about having more leverage, and will not need a smaller chainring with your 170mm cranks. It's only going around one time per revolution, regardless of crank length, so if you genuinely needed 170s, they are not going to make things harder, only easier.
    I have a 30" inseam, and would like to tell you what a difference going from 175 to 170mm cranks made, what with not feeling like having to power over a hump at the top of each pedal stroke and all, but I can't, because I almost immediately switched to an oval chainring, too. They're a great combo, IMO. Made everything better overnight. Big difference, and I was making it over obstacles that I'd never been able to on my very next ride.
    I've been thinking about an oval as well despite not really knowing much about them. Can you describe in detail how it made everything better? No more dead spots in your pedal stroke?
    What the hell is an aluminum falcon?

  29. #29
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    How it works is simple. Makes it easier(small side, 30T on 32T oval) where you have the least leverage, but "harder" on the downstroke where you have the most(34T on 32T oval). Does not feel harder at all, only easier to maintain cadence. It really does feel "more round" than a round chainring as a result, and as many others have also reported, I'm able to use a higher gear for the same terrain with no perceived increase in effort-which means it's faster. It's particularly noticeable in higher gears, and when your pedal stroke gets slowed down on climbs.

    As your knee comes up, and is at a mechanical disadvantage, the oval gets smaller.
    Crank arm length and chainring size proportional?-20151120_121421_01.jpg

    As your foot comes down, and you can deliver more power, the oval gets bigger.
    Crank arm length and chainring size proportional?-20151120_121406_01.jpg
    ^^^That's an Absolute Black direct mount 32T, btw

    Editing to add: I may see a bigger difference than many people because I'm working toward single speed, and staying in the same gear all the time. I struggle a LOT with even moderate uphills if they're very long, and often end up standing and mashing in slow motion. That will really make you appreciate the oval!

  30. #30
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    Mostly run 175's because they are most commonly available.

    I run one set of 180mm cranks on one of my single speeds and they work quite well. Tried them on another single speed bike and didn't like them so I'd say some depends on geometry...

    I am going to install 180's on my hard tail (with slightly larger rings) and see what happens.
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  31. #31
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    Quote Originally Posted by phoeneous View Post
    I'm 5'8" with 29.5" inseam. Currently running 175mm cranks which I think is too long for me. I'd like to experiment with 165mm cranks. I'm currently using a 32t ring and 11-40 cassette. To supplement the 165mm crank length accordingly, should I use a 30t ring as well or keep the 32t? Advantage/disadvantage with changing ring size whilst going with shorter cranks?
    Almost identical size I have been running 170 cranks for ages. They are simply more comfortable and I gain some ground clearance. I would not change the chain ring, I doubt crank length makes any difference 30 vs 32 but even if it does and you keep your 32 the result would be that "on average" you will spend more times on bigger cogs ... you got 11 of those!

    Having said that aybe in the smallest gear you might feel a difference in some technical situation .... maybe ...

  32. #32
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    Quote Originally Posted by watts888 View Post
    Of course it's arguable. Everything on the internet is arguable. My point is soley from a physics standpoint. Longer crankarm = more torque. It's math and can not be dis-proven.
    But this is what you claimed:

    Shorter cranks will give you less power to the cranks.
    That is not only arguable, it is wrong. Torque and power are not the same. Longer cranks have more torque at the spindle but lower cadence means the power doesn't increase. Power is more or less directly related to foot speed, not cadence or crank length.

    You should study up on your "physics standpoint".

    The problem with varying crank length is that it affects fit and ground clearance. You need to consider crank length and frame fit together. I had a bike with a low BB and a long chain stay. Using a 160mm crank worked great on that bike. I've used up to 185mm on other frames for the same reasons. Power is never the issue but you should make sure your gearing is right.

    The OP is thinking right to keep the chainring and crank in proper proportion. There are two gears on a bike, the first is the crank/chain ring ratio and the second is the cog/wheel ratio.

  33. #33
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    Definitely try different setups to see what you like. Already been said, but the longer "throw" 180-85s on the HT or rigid can have more of a powerband feel when you mash on it. And the 170-75s on FS can help with a little less burn from the back leg on extended/steep chunky downs
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    I was able to borrow a 170mm crank for my ride yesterday. Fitted it on, and raised my saddle a few mm - not sure if I raised it exactly 5mm.

    I didn't touch my saddle fore / aft position, but it possibly felt like I was sitting further back on the saddle.

    The ride I did was solo, on mostly undulating singletrack, some climbs, no more that a few mins long, 2 hrs ride total.

    This is typically the sort of ride where I would get a sore lower back. Doesn't happen on the road, and it doesn't happen on a group ride where I'd have more rest stops.

    I think it's seated pinch climbs that put strain on my lower back, as I'm putting the power down, and keep my front low to keep traction on the front wheel.

    I'm currently running an XC hardtail, with a Doval oval chain ring. My back actually gets worse on the duelly, since I sit down more on climbs.

    First thing I noticed was how much easier it was to spin the shorter cranks. Initially, I felt like I needed a lower gear on the climbs. As I got more used to them, I found that I was spinning up the gear quickly, and then changing to a harder gear, so it actually felt like I was riding faster. Conversely, on pinch climbs, I would struggle a bit more, so learnt to change down a gear sooner.

    From riding a singlespeed, I'm a bit of a masher. Often I don't change gear much. With the shorter cranks, I was changing gear a lot more often - both up and down.

    Back pain was greatly reduced. I still felt the odd twinge hear and there, where I thought it might have come on, but I was still good by the end of the ride.

    Overall, it was a good first try, I'll give it a few more rides to see if its worth investing in a set. I did set a number of PBs on the climbs, but this could also be down to some recent weight loss.

    I can see shorter cranks working well for me on my road bike also, but conversely, I don't think they'd work on the singlespeed, but then maybe if my legs got a bit stronger - the better spinning could help me.
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    Quote Originally Posted by OwenM View Post
    How it works is simple. Makes it easier(small side, 30T on 32T oval) where you have the least leverage, but "harder" on the downstroke where you have the most(34T on 32T oval). Does not feel harder at all, only easier to maintain cadence. It really does feel "more round" than a round chainring as a result, and as many others have also reported, I'm able to use a higher gear for the same terrain with no perceived increase in effort-which means it's faster. It's particularly noticeable in higher gears, and when your pedal stroke gets slowed down on climbs.

    As your knee comes up, and is at a mechanical disadvantage, the oval gets smaller.

    Thanks, that makes complete sense!
    What the hell is an aluminum falcon?

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    Last Post: 01-19-2011, 04:21 PM

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