[tilemachos]

hello to all you mountain bike gurus. I have a question. I've read some articles about wider rear spacing(for example 148, 157 ) and they all claim that the wider it is the more stiff the frame. On the 148 standard the stiffness comes only from the angle of the spokes to the rim but again someone wrote that wider rear hub results to wider pivot points to the frame thus making a stiffer frame overall.
Now, lets say that someone gets a full suspension fat bike with 197 rear hub, does this mean that its frame is a lot lot stiffer from anything out there? if so , than why doesn't all buy fat full suspension and replace the wheels with conventional ones , lets say 27.5x2.6 , and only having the hub different in order to complete the whole assembly. in general , due to lot wider hub spacing of a full suspension fat bike , does it make a stronger stiffer frame?
for all the above lets say that frame weight isn't an issue and that serious manufacturers start producing fine forks.

 

[Jayem]

Quite the opposite, the frame will be less stiff, as it has to be made wider. The manufacturer may compensate by adding more material and "beefing it up", as in the case of the Pivot 429 Trail, where it's added some frame weight over the last non-super boost version. Same thing with a fork, widening it, keeping everything else the same, would be less stiff for torsion and lateral stiffness.

The wheels may be stiffer due to the wider flanges (but not all boost/super boost hubs even do this), but a majority of the wheel stiffness comes from the rim, so you'll still see some significant variations, wide aluminum vs. skinny aluminum, carbon fiber, etc. The super-boost manufacturers will also go and do stuff like make them with 24 or 28 spokes, often because at the OEM level it's a bit cheaper to do so and it saves weight to make them look more competitive, and that will further reduce the "edge" that you might get from the wider hubs. It's not that the wider hubs don't help to make stiffer wheels, but it's not an end-all and there are still several variables. As I said, the actual rim is a huge variable. Skinny aluminum rims feel like noodles to me comparatively.

 

[tilemachos]

i swear i saw someone write that wide spacing makes wider rockers and pivot points that make a stiff frame. And in general using wide pipes you increase stiffness. In fact that was the selling point of aluminium frames. ''We got the same stiffness with steel just by using wider aluminium pipes'' i remember reading. But as you say , the stiffness game comes primarily from the wheels and not the frame. For a moment i thought that if we used a fat bike frame than we would have a largerly stiff frame. I am really baffled by this. Only by curiosity not that i do rigorous rides and need a stiff frame. it just hit me one day. i always hear about everything getting wider and adding stiffness than i said , ''what the heck , it sounds like a full suspension fat bike will solve every stiffness problem''.

 

[Jayem]

i swear i saw someone write that wide spacing makes wider rockers and pivot points that make a stiff frame.

This is basic physics, a wider/longer structure will allow the forces to act like a bigger lever and flex it easier. Think of a 5" handlebar vs. a 50" handlebar, which do you think will flex more? This is why 29er wheels are inherently more flexy than smaller wheel sizes (and part of the reason for moving to boost). This is why the fox SC 32 chassis is narrower than the regular 32, to try and make up for weight reductions in other areas and maintain enough stiffness.

 

[Fleas]

This is basic physics, a wider/longer structure will allow the forces to act like a bigger lever and flex it easier. Think of a 5" handlebar vs. a 50" handlebar, which do you think will flex more? This is why 29er wheels are inherently more flexy than smaller wheel sizes (and part of the reason for moving to boost). This is why the fox SC 32 chassis is narrower than the regular 32, to try and make up for weight reductions in other areas and maintain enough stiffness.

Solid explanation.

-F

 

[tilemachos]

ah i see. didn't know tha fat bikes are so long to make such a difference.

 

[Harold]

ah i see. didn't know tha fat bikes are so long to make such a difference.

I was told by Industry Nine employees some years ago at a show that the hub shells of fatbike hubs flex noticeably more than narrower ones (but they all flex). This is at least in part responsible for a higher incidence of hub bearing failures on fatbike hubs. This makes sense - the only dimension that differs on fatbike hubs compared to their non-fat counterparts is the width. The diameters are the same. Material thickness is the same.

One thing that you might be thinking of that is somewhat confounding is that by making something like a pivot point wider, the bearings can be spaced a bit wider to support the axle of the pivot, and that does lead to increased strength. So while the longer pivot will be more flexy if all else is equal, wider bearing support will increase overall stiffness of the assembly. Larger diameter of that pivot axle will increase stiffness of the pivot axle itself.

We saw this when hub manufacturers moved from a freewheel to freehub design. The axles of freewheel hubs bend and break pretty easily because there's a decent length of unsupported axle between the dropout and the most outboard hub bearing. Freehub designs gave the axles more stiffness because it allowed for the bearings to support that axle better.

I also saw some of these things when comparing the pivots of my first FS (from 2003) to my 2nd (from 2014). On my first FS, the pivot axles were just plain old steel bolts. Nothing fancy. Those axles were also much narrower than on my 2nd FS (meaning, the bearings were spaced more closely on the older bike). The older bike had noticeable lateral flex in the rear triangle. The newer one had changes that increased the stiffness through wider pivot bearing spacing, larger diameter pivot axles, and better material placement in the rear triangle itself, even though the general concept of longer axles being more flexy is true, those effects were mitigated by other stiffness-increasing changes.

 

[plummet]

This is basic physics, a wider/longer structure will allow the forces to act like a bigger lever and flex it easier. Think of a 5" handlebar vs. a 50" handlebar, which do you think will flex more? This is why 29er wheels are inherently more flexy than smaller wheel sizes (and part of the reason for moving to boost). This is why the fox SC 32 chassis is narrower than the regular 32, to try and make up for weight reductions in other areas and maintain enough stiffness.

This might sound like a solid explanation. But it is wrong when you consider hub spacing.
Why? because the hub forms part of the rear structural triangle. The wider and closer you get to optimal triangulation the stiffer the rear triangle will become laterally.

You use 29 wheel in your example to explain that it has become weeker.... Think about what you have done with a 29 wheel compared to 27.5 or 26. You have reduced the triangulation angles by making the daimeter of the rim bigger but not increase the width of the hub to compenate.

Optimise the triangulation and you optimise the stiffness of the rear triangle.

Go get some tooth picks and glue several rear triangles up with various spacing options and tell me which one is has the most strength?

 

[Jayem]

This might sound like a solid explanation. But it is wrong when you consider hub spacing.
Why? because the hub forms part of the rear structural triangle. The wider and closer you get to optimal triangulation the stiffer the rear triangle will become laterally.

You use 29 wheel in your example to explain that it has become weeker.... Think about what you have done with a 29 wheel compared to 27.5 or 26. You have reduced the triangulation angles by making the daimeter of the rim bigger but not increase the width of the hub to compenate.

Optimise the triangulation and you optimise the stiffness of the rear triangle.

Go get some tooth picks and glue several rear triangles up with various spacing options and tell me which one is has the most strength?

We were talking about frames, not hub spacing. The point about the 29er rim was because a significant amount of wheel stiffness comes from the rim.

 

[plummet]

We were talking about frames, not hub spacing. The point about the 29er rim was because a significant amount of wheel stiffness comes from the rim.

Yes i know. Thus the wider the rear triangle is the more stiffness it has because it has an improved triangulation. Thus going from 135, 142 to 148 gives you greater stiffness all other aspect being equal.

 

[tilemachos]

then a fat bike with 197 rear spacing and much wider bearing points are marginally better at stiffness, because the rockers are the same length as a conventional 27.5 but way wider. Thus excluding that a full suspension fat bike with a 27.5 wheel is much stiffer than a regular full suspension and design one.
for all the above , this comes to my mind only using a conventional wheel to the fat bike not the monster wheel it regularly uses.

 

[Lone Rager]

I'm with plummet on this one.

In general though, many design features may or may not produce a desired effect depending on the details of the implementation and other factors. Does a wider hub spacing and splayed out chain stays and seat stays produce a stiffer frame? Possibly but not necessarily.

 

[HerrKaLeu]

No matter if wider spacing makes the frame stiffer.... it makes the hub weaker. Many more fatbikes with freehub problems due to the longer more flexing axle.

 

[E. Bryant]

This is a complex question, and thus there are no simple answers. There are a lot of give-and-takes as have been pointed out, and so it's nearly impossible to provide a generic response.

Ultimately, you're connected to the ground via squishy tires, and thus I generally don't regard stiffness as being a major factor in selecting structural components.

The single best reason for not going with 197mm spacing on everything is that eventually the Q-factor (width of the cranks) becomes annoying. Most riders will notice that before they pick up on any difference in stiffness.

 

[Jayem]

The single best reason for not going with 197mm spacing on everything is that eventually the Q-factor (width of the cranks) becomes annoying. Most riders will notice that before they pick up on any difference in stiffness.

Don't agree with that, there is testing that showed a minimal loss of power for wider Q factors, but that is more about conditioning and what you are used to, rather than being an actual issue. If it was, no one could ride their fat-bikes around here in the winter.

The discussion never really changed. Moving the bearings outboard further is reducing the lever arm for the axle, but increasing it on the frame, a surface that may be significantly stiffer, but as you say, it gets to be "depends" due to the axle size, bearing size, frame stiffness, and many other factors. Making a pencil-sized tube with small diameter axles may save weight, but may cost you stiffness. Making giant tubes and structures to support your 157 super boost frame may make for a very stiff frame with the structures moved outboard, but at the cost of weight.

 

[kapusta]

I think you need to specify which kind of "stiffness" and "flex" we are talking about.

One kind of stiffness refers to how well the frame resists forces trying to twist the wheel from the outer edge (tires). This affects how the bike tracks on rough and uneven ground, or when you land something badly. I THINK this is mostly what is being discussed here.

The other kind of flex comes from hard pedaling. The chain being outboard of center is trying to bend the frame. On some very flexy rear ends (like my 2000 Superlight and 2008 MKIII) I could get the rear tire to rub the chainstays under hard pedaling even though it looked like the rear tire had plenty of clearance.

As far as I can tell, having a wider rear spacing will help with the first type of flex. Think about it: if you had a wheel with a really long axle sticking out either end, and I asked you to hold it by the axle and try to resist my twisting the wheel, you would grab it as far out as you could.

As far as the second type. I think that is more complicated for the following reason: along with the wider spacing often comes a chain-line farther outboard from the center-line of the bike. This is the case with boost and fat bikes. The farther out the chain line is, the more leverage the chain has in bending the frame.

So does any of this really matter much? I really don't know. I am guessing on the the margins, sure. But There are a lot of other factors that go into how stiff a frame is in either sense. I am sure there is benefit to having the wider hub flange spacing, but nobody ever seemed to give that much though as they got narrower due to disc brakes. Again, I am sure it made a difference, but I for one did not notice more flex going from non-disc to disc hubs.

I think the reality is that most companies have figured out how to make adequately stiff frames.

Jayem mentioned the Pivot 429 frame getting heavier in the Super-Boost version. If that was indeed due to the wider rear spacing I would be curious to know if that was to address the first or second sort of flex I mentioned.

Also, I do think Q-factor is an issue. I am fine with the Q-factor on my fat bike, but I prefer the way my regular (non-boost) cranks feel.