shutuppostman

Hey guys, still kinda new here so not sure if this is the right forum so apologies in advance if so!

So title says it all. Im putting 26" or 650c on a 48 cm road bike and was wondering if, since the bike is gonna be lower, i'm gonna have to switch out the crank arms as well. Any thoughts?

Also, am i gonna have to get my gears re-tensioned for the new set up? I feel like i would but im not sure. Please let me know!

Thanks!

Retro Grouch 

I've never done it that way but I have installed 700c wheels on a bike frame that was designed for 26" wheels. To tell the truth, I never noticed a noticeable difference in ride height so I'm thinking you won't have very much either.

If it was my bike I'd bolt 'em up and see what happens. If you still have the original equipment wheels, I'd keep them until you find out if the 26 inchers are going to work.

hokiefyd 

I would say that it's not likely. How long are your current crank arms? I'll probably say on the inside of the arm somewhere. I would say that most bikes with 26" wheels are using 170mm crank arms, and many are also using 175mm crank arms. As long as your current crank arms aren't something oddly long (like 180mm or more, you'll likely be fine). Just know that you'll have less ground clearance.

randomgear

a far bigger question is whether the brake pads will reach the smaller rim, unless you are running disc brakes.
No reason to reduce crank arm length, that said, there will be an increased chance of pedal strike when pedaling through corners.

SkyDog75

That's a significant change in wheel size, so yes, there's a chance that your bottom bracket height will be a concern. Without factoring tire width in, converting from 700c to 650c will lower the bike over 2.5 cm, or about an inch. Converting from 700c to 26" will reduce your bottom bracket height more than 3 cm.

(For comparison's sake since it's a common conversion, swapping from 700c to 650b would lower your bottom bracket height 1.9 cm. But the goal of a 700c/650b conversion is to use wide tires, which will gain back some of that height.)

After swapping to a smaller wheel size, you'll almost certainly still be able to pedal while riding in a straight line -- it's cornering that's the problem. As you turn, you lean. With reduced ground clearance, striking the inside pedal on the pavement (and potentially wiping out from that pedal strike) is more likely.

I converted my daughter's bike, a Fuji SJ-10, to 650c wheels, which reduced her bottom bracket height about 2 cm. In part because of the reduced bottom bracket height (and in part because she was 11), I chose to use crank arms that were 5 mm shorter. I also put narrow MKS Sylvan track pedals on the bike to reduce the chance of pedal strike while cornering a little bit more.

I'm not sure what you mean by "re-tensioned".

Changing the wheel size will change the effective gear ratio and thus your speed relative to pedaling effort, but almost certainly not enough to prompt you to change your gear ratios (such as by using a cassette with different cog sizes).

On the other hand, if you ever change from one rear wheel to another, regardless of size, you should adjust your rear derailleur. Even if they're the exact same model. Because of manufacturing tolerances, your cogs may not sit in the exact same spot relative to the frame and derailleur. Any difference can have a negative impact on shifting. A big enough difference can cause the chain to overshoot the big cog, dropping the chain between the cog and spokes, potentially causing expensive damage.

There's another mechanical consideration, as randomgear mentioned above... What kind of brakes does the bike have? If you have disc brakes, they'll work regardless of what wheel size you change to. Cantilever and v-brakes use posts that are brazed to the frame in a specific position for a particular wheel size, so those type of brakes may make a wheel size swap impractical. Caliper brakes are a little more forgiving since you can buy brakes with longer reach. Finding brakes that'll reach 2 or 3 cm further may be problematic, though. And the longer the brake arm, generally the worse the performance.

JanMM

What he said. ^^^^^
If the bike was originally built up with 700 wheels, then the gearing - every combination - will be a little bit lower than it would be with 650c wheels.

CliffordK

Interesting build idea. You're wanting to make the bike smaller? Why not try to find a small 650c frame? How tall are you?

Keep in mind that 650b is the common MTB size. 650c is the common track/tri/junior road size, and mostly only has narrow tires.

I've got a few 650c wheels here that I can take some measurements, and perhaps make some comparison photos if you wish.

As mentioned, you may notice the pedals on corners, otherwise the cranks should be ok. What length of cranks are you using? If you are short, then 165mm cranks might be good, or even shorter.

If the goal is aero, you can go mixed, 700c/650c. There are quite a few forks available. It would change the geometry some, but perhaps not all for the bad, as a steeper seat isn't bad with a more aero config.

SkyDog75

There's not a direct correlation between wheel size and crank arm length. Frame geometry, specifically bottom bracket drop, plays heavily into the equation. The attached image borrowed from Dave Moulton's blog, illustrates bottom bracket drop and height.

Bike frames designed for smaller wheels need to have less bottom bracket drop to result in the same bottom bracket height. Bikes designed for bigger wheels will have more drop, meaning the bottom bracket (and thus your crankset) may be abnormally low if you swap to significantly smaller wheels.

That part we agree on.

downtube42

Agree with other posters; another consideration is tire selection. If you plan to use wider/higher volume tires on the smaller wheels, that will offset some of the difference. And you can generally run wider tires when you go to smaller wheels.

CliffordK

There are very few wide 650c tires available.
Most of the wide tires are 650b.

SkyDog75

+1

The Panaracer Pasela is available in 650x25c and they recently started offering the Pasela PT in 650x28c. The Terry Tellus (made by Panaracer) is available in 650x28c. Those were the only options wider than 23 the last time I looked.

There are some other archaic sizes that use the same 571 mm bead seat diameter as 650c, so tires in those sizes should work so long as they're not too wide for your 650c rim. Some tires are still available for old Schwinn cruisers that use the 26" x 1 3/4" (S7) size, for example. The problem, though, is that modern 650c rims are generally marketed toward the roadie and triathlon crowd and tend to be narrow. A 1 3/4" tire is probably going to be pretty squirmy on a rim whose internal width is only 14 mm like the Sun M14A's on my daughter's bike.

shutuppostman

Thanks for all the responses guys! Super helpful!

So last question: will a 165mm crank arm set fit my current cranks or do i need to switch to smaller cranks to match the arms? Thanks!

CliffordK

Your question doesn't really make sense.

Most modern cranks have a built-in spider, with a few exceptions. Spider sizes vary somewhat, but 5-arm 110BCD and 130BCD are common for older styles, and 4-arm cranksets are more common for modern cranksets. However, if you change cranks, it may be easier to get a whole crankset including rings of the size you desire.

Likewise, you should choose a bottom bracket to match your crankset. Square taper would be common for 165mm cranks, but perhaps there would be other types available too. Anyway, it just depends on what you have already, and what you purchase.

If you're building a small woman's bike, or kid's bike, then consider a triple crankset, although that could require a different derailleur and shifters.

shutuppostman

Ah ok. Well changing out the crankset/bottom bracket isnt an option at the moment but the cranks i do have are pretty standard modern sugino cranks. If "spider" refers to the end of the drive side arm opposite to the pedal, yes, it has 5 things that connect to the cranks. Taking into account everything youve said, im not sure how to rephrase my question. I may be using incorrect terminology and you may have answered my question already but to ask again with what i just said in mind: these shorter arms should have the same length of the 5 extensions that connect to the crank, correct?

CliffordK

Ok, a few terms to help get everyone on the same page.

• Crank (right and left).
• Spider 4 or 5 arms, generally built into the right crank.
• Chainrings. These are the FRONT sprockets that have the teeth that engage the chain.
• Freewheel, rear sprocket set that screws onto the hub (vintage)..
• Cassette, more modern rear sprocket set that slips onto a separate freehub.
• BCD, Bolt Circle Diameter (for the chainrings to connect to the spider).
  110 BCD is common (called Compact), with the smallest chainring generally having 33, 34, or 36 teeth, and the largest chainring often having about 48 or 50 teeth.
  130 BCD is also common (standard), with the smallest chainring having 39 teeth, and the largest chainring usually having about 52 or 53 teeth.
  Of course, different ring sizes can be used, as long as they are larger than the minimums above.
  Other sizes are also common. Campagnolo uses a 110/112BCD crankset (no 33T), as well as 135 BCD crankset. Vintage Campagnolo uses 144 BCD with the smallest chainring being 41T or 42T. A few Campagnolo clones also used 144 BCD.
  Mountain Bikes have introduced a few more sizes, and there are several standards for inner triple chainrings.
• Bottom Bracket: Usually a shaft and bearings that the cranks connect to, although some modern bottom brackets may have the shafts as part of the crankset, and "outboard" bearings.
  Square Taper is a common vintage style of bottom bracket.
  Shimano and other brands brought in a couple of alternatives including octalink and ISIS with several teeth engaging the crankset.
  Bottom Bracket threading is important. Italian and some European bikes frequently use "Italian" threads. Otherwise, most modern bikes use "English" threads. Vintage bikes might use French, or other bottom bracket standards.
  Press Fit bottom brackets are also common with some high-end racing bikes.

So you can re-use the chainrings, if the BCD of your cranksets match.

maddog34

from what i'm seeing... you will be in WAY over your head on the swaps needed.... and the biggest hassle will be BRAKES.

that said, the 26" wheels are NOT 26", the overall height, with a 2" tire IS 26"... the 650 b or c will not drop the bike much... putting a 1 3/8" tire on a 26" WILL lower the bike about 1.5", when switching from a 700c road bike wheel with a 25 mm tire on it...

CRANK arm lengths CAN be bought all the way down to 155mm or so... Origin8 sells them, but you will probably need to special order them... i've put them on BMX bikes for little guys before, and also tried them on my road bike, since i enjoy shorter arms to increase cadence by lowering rotational foot speed... take off torque is reduced quite a bit though, so be ready to REALLY push when launching or climbing!

as i said above... BRAKE REACH will be your main issue when doing the swap you've discussed... try the longest reach BMX sidepull brakes you can find.... a lot of walmart bikes come with them.... but they are not very good in quality, eh? get the aluminum ones... the steel ones are just about useless! Put KOOL Stop brake pads on them to make them work better...


i once converted a set of 175mm crank arms to 147mm for someone... not an easy job...... a special fixture, pedal thread taps, and several measuring tools were needed, along with my machinist skills....


NOW... making the assumption that the reason you want to do the swap is to lower the bike so that it will fit your short height, or someone else's... try a DIRT JUMPER style MTB frame! then make it more road-worthy! a small frame dirt jumper of quality can be found used for about 100 to 300 dollars, which will be less than you will probablt spend on hard parts... then but some Panaracer paselas in the 1 3/8 or 1 1/4 size! Bars and controls can be swapped from your road bike... the cranks will be higher already then, and the stock brakes will be the right reach, too! set the seat to your needs, and ride!

I'm building up a Specialized brand Rock Hopper aluminum dirt jumper right now... it will have 1 3/8" paselas, and disc brakes... building for a young lady that is currently riding WAY beyond the capabilities of her walmart girl's MTB... watched her ride by today STANDING on the TOP TUBE of the frame... YIKES!!! she's 9 yrs. old... and she does knack-knacks while jumping! yes, she needs her wheels trued on a regular basis! The rockhopper is a SMALL size Dirt jumper frame... they also come in medium and large sizes... look for a rather short head tube on the frame... and you can put a solid, non-suspension fork on them, too... better for road use!


"Dirt Jumpers" have a top tube that slopes almost directly back to the rear axle, btw.... this allows extra room for squatting when landing, since the seat can be WAY LOW when all the way down.... your legs act like rear suspension. The fork for her bike is only an 80mm travel one... an old Marzocchi Bomber fork i had lying around the parts room... WORLDS better than the pogo stick on her walmart bike!

CliffordK

I agree, what CAN be done doesn't always mean it SHOULD be done.

There are quite a few 650c bikes available.

Felt F65 Road BIke | eBay
Trek Krx Road bike 26"-Red/Black | eBay

If you are near a big city in the USA, some should show up on Craigslist.

One issue is that many of the smaller road bikes (24" & 650c) are built for kids, and have many compromises, but they can always be upgraded. That Felt above looks nice if the seller is willing to ship.

I picked up a Fuji Ace 650 for my nephew for around $200 a while ago, definitely cheaply made, but it would make a good base platform to work from, and slowly upgrade.

Building the custom 700c to 650c conversion, some dimensions will be improved, but for the most part, it will just be a shorter 700c bike. And for the expense of the wheelset, crankset, brakes, and a few other odds and ends, you might as well buy the 650c bike, and sell what you have.

SkyDog75