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Bigger=Stronger ? Calling BS on Super Boost

Aug 28, 2018 at 6:01
by Antoine Besqueut  
The Knolly guy is going to call me an Armchair Engineer but I'm going to go for it. I might be barred for life at Knolly bikes for critical thinking.
After this article about super boost 157 I grew tired of the argument of "it's wider so it's stronger" which is the main selling point of Boost 148 and SB157.

Making something bigger to make it stronger is level 0 of engineering. Prehistoric Humans already knew the trick when choosing the right bludgeon for mammoth hunting. Even Sram though of it when releasing DUB.

Putting my life in danger, riding non boost wheels

The premises are true : larger hub equals flatter spoke angle. Let's do some quick maths.
My hypothesis is : you want to approach the maximum tension allowed by the rim on the drive side (F2) to reach maximum strength. The resulting axial force (T2) must be balanced on the non-drive side (T1) resulting in a tension F1.

drawing of a hub
my crappy schematisation of a hub

With the above drawing with a2 going longer, you get a bigger axial force T2 for the same tension. That is the desired effect since T2 will impact directly your wheels' lateral flexibility.

Here is a spreadsheet, detailing the maths for 27.5" and 29" wheels. No rocket science here, just geometry. Feel free to correct if I made mistakes.

maths crap

And the results :
axial strength gain over 142 hub
Both 157 hubs improve lateral strength, Boost 148 doesn't improve much
tension difference
Tension imbalance gets worse with SB157

SB157 isn't any stronger than 157DH but results in more uneven wheel. Boost 148 barely improved anything.
Even tension is more important to me since it impacts directly on durability. I'm not the first one to talk about this :
American Classic 2017
Bill shook didn't use all the free space but balanced the spacing instead

Building strong wheels was never a problem, but they tried to solve it nontheless. And with it cames load of crap : plus tyres, crappy chainline, heel rub, chainring limitations. The goal was to have both short chainstays and 2.8 tyres, too bad I want none of them. Really short chainstays are only good if you're a front charging rider, give sharp handling but require precise body balance.

What would be other ways to go ?
One is offset rims, WTB is the brand that come to my mind about this. Another one is non symmetrical stays, which has been tried already.

Super Boost 157 reminds me of Spinal Tap :
bigquotesWell, it's one louder, isn't it? It's not ten. You see, most blokes, you know, will be playing at ten. You're on ten here, all the way up, all the way up, all the way up, you're on ten on your guitar. Where can you go from there? Where?Nigel Tufnel

At least I'm ok with one thing in the Knolly pie chart :

Author Info:
Whipperman avatar

Member since Nov 1, 2014
1 articles

  • 2 0
 Good blog. I agree with pretty much everything here. Smile It's cool to see someone else who likes to think about these things.

Going back to the original discussion where you directed me to this blog, I'd be interested to hear your views on the bit where you state "My hypothesis is : you want to approach the maximum tension allowed by the rim on the drive side (F2) to reach maximum strength." You seem like a smart guy, and you have the maths. I'm sadly ill equipped in that dept
  • 2 2
 "Even tension is more important to me since it impacts directly on durability" You didn't make a case for this. It's also untrue.
  • 1 0
 I disagree. Even spoke tension is the holy grail. It is possible to build a perfectly straight wheel with very uneven tensions. However it won't stay straight for long, a few weeks of little more than riding to the shops and back is usually enough for them to get a wobble on. An evenly tensioned wheel will likely stay straight through many months of abuse, bad line choices and sideways landings.
  • 1 0
 @gabriel-mission9: You also have not made a case for this. Re-stating an opinion is not a case.

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