splined hub and cranks

This question may sound stupid, but why are “splined hub and crank sets” stronger than “square taper sets”?

Also, do most bikes use splined sets?

The axle (or spindle) on splined hubs is a larger diameter than with square tapered hubs. The larger diameter spindle makes the hub stronger.

The splined connection is a more secure way to attach the cranks compared to the square taper. So the crank connection is stronger and easier to get tight.

Most of the better mountain bikes and road bikes now have a form of splined cranks and bottom bracket. Your typical cruiser bike and inexpensive bike will still likely have a square tapered crank.

Why can’t you just make the axle on a square taper hub larger so that it can be as strong as a splined hub?

You could, but then the cranks wouldn’t fit on. A square taper axle has to be the size that it is because that’s the size they settled on when they made the standard. If you make it bigger then the cranks won’t fit on properly.

But even if you made the diameter of a square tapered hub larger, it still wouldn’t be as strong as the same diameter splined hub. The problem is that the square taper cuts away too much material in the wrong places. It leaves the axle with a smaller root diameter than with the splined crank setups. See this post by Steve Howard for an explanation.

The splined hubs are a better engineering design.

Whilst in a BMX shop recently, I saw some cranks that had a square hole (don’t think it tapered down tho) but it was really large and also had pinch bolts. Dunno how strong they are, the owner said they surprised him as he thought they would just round out. I’ve seen some that are almost round but on closer inspection are hex or octagonal- I forget which, I think the former. They certainly didn’t look very strong. Do either of these fall into the category of splined cranks?

I always thought a splined crank was stonger because the splines more evenly distribute the force on the axle and not because of a larger diameter spindle? The higher the spline count the better the distribution, and thus a reduction in bent and broken crank arms and hubs.



No because they don’t have splines. But aside from that, they follow a similar design. the rounder the shape at the connection point, the greater a chance of the crank not staying tight. But I think an octagon, if the machinig is good on both crank and axle, would be strong enough for unicycling.

For Jason, Yes, the even distribution of force is also an advantage of splined cranksets. I think it’s a combination of the narrower root axle and the limited contact faces that cause square tapers to break.

Could you build a bigger square taper axle? Yes. But as was already explained, you’d have to build a pair of cranks to go with it. Both would be heavier because of obvious need of more material to build them with. One of the nice things about splined cranks is that they’re usually hollow. Lighter, yet still pretty dang strong! But more importantly, made for bikes. Made for bikes means mass-produced. Without mass production, the price goes way up.

They’re not splined, but they are oversized. I’d put them in the category of almost splined. The spindle is oversized which gives it more strength than a standard square tapered spindle. The cranks fit on just like most BMX splined cranks with a pinch bolt.

The advantage is that it is easier to machine the square shape or octagon shape than it is to machine the grooves and more complicated shapes for a splined design. Easier machining means less cost.

Primo has a line called Powerbite that has the oversized square spindle with no taper. Odyssey has a line called Black Widow (not Black Widow Euro) that has an oversized octagon shape. You can see a picture of the Primo cranks at Dan’s Comp. You can see a picture of the Odyssey octagon shaped cranks at Odyssey BMX.

There is a lot more to strength than just counting the number of splines. A well designed 8 spline design could be stronger than a poorly designed 20 spline design.

One important factor is the root diameter of the axle or spindle. Take a look at that link I gave to the post by Steve Howard. He has a nice graphic that shows the root diameter for the Profile, KH 8 spline and the standard square taper. The Profile design has very shallow splines so the root diameter is almost as big as the outer diameter. The KH 8 splines so the root diameter is going to be less. All other things being equal, a larger root diameter means more strength.

But all things are not always equal. There is some engineering design considerations too. The shape of the splines and the way they are cut will make a difference. You want to avoid creating stress risers (areas on the spindle where stresses get concentrated) and angular shapes. A spindle will be more likely to start cracking at a stress riser. A design that avoids stress risers will be stronger. For example, see this page by Sheldon Brown about a broken ISIS bottom bracket. The ISIS spline design is a strong design and is proven, but FSA did a poor implementation of the design. Their implementation created an unnecessary stress riser where the crack was able to start and that’s where the part failed. ISIS is a standard spline design that many manufacturers are using.

So there is more to it than just counting the number of splines.

Do any unicycle parts use ISIS? Are there any plans to use it?

None yet. I don’t know if there are any plans to use it.

One problem is that there aren’t any ISIS cranks that come in the variety of sizes that we would want for unicycling. Where are you going to get a 150 mm ISIS crank? We’d need custom cranks made. Getting custom cranks from Truvativ or Race Face or some other company that makes ISIS cranks would likely be expensive.

I also think the machining necessary for the ISIS design is more difficult than for the simpler BMX splined designs. I think the ISIS design has a slight taper that would complicate things.