My brother almost made a mega mistake with his road cranks

from Rookie’s keyboard,

Hello, friends

I can’t sleep tonight for whatever reason (too many energy drinks…too many worries on my mind…who knows).

The good news besides being awake at 1 a.m. when everyone is sleeping is that I got a great idea for a post that my brother accidentally gave me one year ago when he did something that I’d put in the cycling equivalent of the movie Idiocracy – he tried to put some beaten old-school road cranks on his hardtail (before switching to BMX, he was a hard-headed MTB rider).

So, why is that a bad idea? Are all cranks…just cranks?

Where do I even start? 

First, the obvious – road cranks are rarely strong enough for legit, ultra-gangsta MTB riding. This is mostly true for the lighter carbon models, of course. But the aluminum road cranks often don’t have the needed strength either, albeit they are much safer than the carbon sets. 

I have old French aluminum cranks on my road bike, and while I like their looks a lot, I’d never trust them enough for aggressive riding on a hardcore or an enduro bike, for example. 

But let’s say that you don’t plan on putting your MTB to the limit. In that case, the strength of road cranks will be sufficient. 

However, rigidity and toughness are not the only obstacles. The main problem is architectural so to speak. 

Bike Frame Anatomy

You see, MTBs are designed for super wide tires. And guess what? Wide tires require wider chainstays (the lower tubes connecting to the bottom bracket). Otherwise, the tire would rub against the frame. 

Retro road cranks

Road bikes, on the other hand, operate with slimmer tires. For instance, a 2.3″ MTB tire is fairly normal. Meanwhile, anything over 28mm on a road bike is considered ultra-wide. Consequently, the chainstays of road frames are much closer together. 

Road and MTB cranks reflect those dimensions. MTB cranks have an “outward sweep” known as Q-factor precisely to create clearance for the chainstays. Whilst many road cranks are STRAIGHT. Look at them if you don’t believe me 

So, in some cases, road cranks would hit the chainstays when put on an MTB frame (it depends on the frame model, ofc.)


Bottom Bracket Differences

Even if the frame is narrow enough for the cranks, the issues don’t end.

The bottom brackets (a.k.a. the part connecting the cranks to the frame and allowing them to spin smoothly) of MTBs and road machines differ in width too.

The bottom bracket (BB) goes into a part of the frame known as the bottom bracket shell (BBS).

MTB frames have a 73mm or a 68mm wide BBS. The standard for road bikes is 68mm.

So, when the MTB BBS is 73mm, the road cranks will be pushed further out and cause a crooked chain line resulting in sub-optimal power transfer and gear shifting.

A logical solution would be to install a 68mm bottom bracket, but this method doesn’t help much because the bottom bracket shell remains wider (73mm).

A 68mm bottom bracket on a 73mm frame is a big problem in and of itself because:

1. It pushes the drive side crank arm out by 2.5mm and widens the chain line.

2. It pulls the non-drive side crank in by 2.5mm and reduces the chainstay clearance even further.

Even if somehow you fix those problems, the result will be a BB stretched to 73mm – a number that road cranks aren’t built for anyway.

The ideal scenario is to source an MTB frame with a 68mm bottom bracket shell. Retro MTBs are your friend in this case.

The Front Derailleur Will Complain Too

If you want fluent index shifting between the chainrings, you will have to couple the road chainrings with a road front derailleur.

Road front derailleurs have a longer, curved cage making the shift onto the big ring easier and safer.

An MTB front derailleur can’t work as well because it’s designed for chainrings with fewer teeth and has a smaller cage. 

Front Shifting Will Become Problematic Too

Front MTB and road bike index shifters are not interchangeable as they pull and release a different amount of cable.

The front MTB shifter will have to be replaced with a road version. (And if one wants to stay with standard MTB handlebars, a flat bar road shifter will be necessary.

If you’re doing a complete MTB to road bike conversion including drop bars, you will need a break-shifter (e.g., STI).

If You Want a Higher Top Speed On an MTB, Get a Trekking Chainset

If your actual goal is to get larger chainrings on an MTB, you don’t need to do stupid things like installing road cranks. Instead, you can just get one of those trekking chainsets. Trekking cranks can be mounted on every average MTB out there.

For instance, there are 48/36/26 chainrings(the number of teeth on them) that will give you plenty of top speed and would be easier to mount. 

+ Bonus(Speed Calculation)

Are you ready for some math?

I will give you an even bigger incentive to go for a trekking chainset by comparing its top speed to that of a theoretical road model. 

To calculate the speed of a bicycle, you will need the following data:

1. The RPM of the rider (the number of full revolutions of the chainring/cranks per 1 minute)

2. The gear ratio (the ratio between the number of teeth on the chainring and the cog in use)

3. The circumference of the wheels

4. The traveled distance or the time needed to cover it.

Example A: Trekking Bike With a 48/36/26 Setup

Bike stats: 

Tires: 27.5″ 

Large chainring: 48T

Small cog: 11T

The highest gear ratio is 48:11=4.36:1 and results in 4.36 rotations of the rear cog per 1 revolution of cranks/chainring.

If the rider is pedaling at 90 RPM, the cranks will rotate 90 times in 60 seconds.

In that case, the rear cog/wheel will make 90×4.36=392 rotations. By multiplying the number of rotations by the circumference of the wheel, we will have the traveled distance.

If the bike has 27.5″ rims with 2.2″ tires, the wheels’ circumference will be about 2185mm or 218.5cm.

Thus, the traveled distance is 218.5 x 392.4 = 853.794 meters.

To calculate the speed, we have to divide the distance by the time (60s in this case).

The maximum possible speed of the trekking bike that could be achieved via pedaling at 90 RPM is 853.794m/60s=14m/s = 51km/h = 32mph

Example B: Some Bike With Road Cranks

Now, let’s repeat the process for a theoretical bike with road cranks. I will use MTB tires for the calculation, though. (Tiires with a larger circumference will produce a higher top speed).

Stats: 27.5″ tires

Large chainring 52T

Small cog: 11T

Traveled distance = RPM x [Gear Ratio x Tire Circumference] = 90 x [52:11 x 218.5] = 90 x [4.72 x 218.5] = 90 x 1031.32 = 92818.8cm or 928.188m.

Speed = Traveled Distance/Elapsed time = 928.188m/60s = 15.4698 m/s = 55.7 km/h = 34.6mi/h

So, in the second case, a 52T road chainring gives us a 4.46 km/h increase over a 48T trekking chainring.

As you can see the difference is too small to justify the conversion. 

And since you’re probably wondering, I’ll tell you. Yes, I did stop my brother from using the road cranks. 

Well, there you have it. Another informative post. 

Time for me to go to bed. It’s 3 a.m. where I live. Good night, fellas.

Until next time

– rookie






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