Mountain Bike Reviews Forum banner

1 - 20 of 88 Posts

·
Registered
Joined
·
636 Posts
Discussion Starter #1 (Edited)
EDIT: Yes, rim stiffness matters. Here's how much:

http://forums.mtbr.com/wheels-tires/how-much-do-rim-hub-stiffness-matter-1042040.html#post13107931

And here's how much spokes matter:

How much do rim and hub stiffness matter? - Page 2- Mtbr.com

------- [original post] -------

People frequently report superior lateral stiffness with carbon rims. They say aluminum wheels deflect from obstacles where carbon doesn't, or that carbon wheels are easier to precisely position.

I don't see how it's possible to make this conclusion.

This is what I understand affects a wheel's lateral stiffness:

Bracing angle (flange width/height, lacing patterns, elbow position)
Number of spokes
Spoke gauge
Hub stiffness
Rim stiffness

On rims:

It's true a carbon rim is substantially stiffer than an alloy one. We see this in bend tests measuring lateral rim deflection for a given load, when the unloaded side of the rim is braced.

But that's not how the rim is used. One side isn't braced; we're lacing it to a hub. Stiffness in this system is the lateral deflection at the rim relative to the hub. That, it seems, should be almost entirely dependent on the spoke configuration.

The only rim stiffness that should matter is the lateral deflection between the nipple holes and the bead locks. On the worst aluminum rim, over this distance of perhaps 2cm, I can't imagine the deflection could be more than a few millimeters without bending the wheel permanently.

I can see the relevance of rim stiffness as a whole on bikes with caliper brakes (where brake rub is a concern), but not mountain bikes where we only care about the contact patch.

On hubs:

I wouldn't even ask about them except for this post:

Bike Forums - View Single Post - Lateral Stiffness in Wheel -- Do I Care?

The author quotes a major wheelmaker that found their hubs (from I assume 2005ish) were inadequately stiff. Speaking strictly of the hub flanges, they don't seem as if they could flex much at all without permanent distortion. Perhaps they were talking about axle skewers? Even a millimeter or two of flange flex wouldn't imply more than a few millimeters of deflection at the rim because the spoke angles are so steep.

None of this jibes with the 'night and day' differences people report. They seem even more spurious when we look at the stiffness of the other parts of the system:

Tire
Axle
Fork
Stem
Bars

Deflection of the tire contact patch is over 1cm with many tires.

Lateral Force Tire Deflection - Mtbr.com

The fork contributes even more. Here's a Cannondale video that shows deflection of a Lefty and an unnamed fork. They're putting what appears to be 30-150 lbs of force on each one. This may be more or less than the actual lateral force on a loaded wheel hitting an obstacle at speed.

https://www.youtube.com/watch?v=_WlRqcAQr2w

The Lefty bends at least 1cm at the hub. The comparison fork moves 3cm or more. These differences dwarf what we see from the wheel by orders of magnitude (at 1:35 in the video, the test wheel doesn't seem to flex much at all relative to the fork), and we haven't even accounted for torsion in the stem or bar flex.

I'm starting to wonder if people would be able to tell carbon from alloy if they were testing blind. What am I missing?
 

·
Ride Fast Take Chances :)
Joined
·
3,331 Posts
Simple answer is it matters a lot. With 29ers having a super strong rim with offset spoke holes gives equal bracing angles and a stronger wheel for the weight. Strong and light are the holy grail of wheels.
You are underestimating the forces that get transmitted threw the wheels on our bikes. Watch how much his rear wheel defelects.
https://www.youtube.com/watch?v=U8YEMP9rdkI
 

·
Registered
Joined
·
636 Posts
Discussion Starter #4
What I see in that clip is the hub twisting with the rear linkage. Relative to the hub, the wheel doesn't appear to yield much at all. This hub twist is magnified in proportion to the wheel diameter, so I'd argue that rim stiffness (of all the things above) is even less relevant on the rear tire of a full-suspension bike that routes the hub between two thin, 40cm lever arms.
 

·
Ride Fast Take Chances :)
Joined
·
3,331 Posts
The wheel and hub don't stay in line with each other. The rim chips quite a bit and the spokes are all over the place. This is an extremely strong wheels. I ride enve, ibis wheels and when I demo bikes with weak wheels its scary. Almost crashed a niner with Stans XC wheels. They wandered all over the place and I hit a 2 foot jump that almost collapsed the wheel right before my eyes.
 

·
Registered
Joined
·
636 Posts
Discussion Starter #6
I disagree, though the argument doesn't depend on that video. I'm arguing that the lateral deflection of the tire contact patch relative to the bottom bracket will be nearly identical for almost any rim in a given wheel size. I argue this because the amount of deflection attributable to the rim appears to be dwarfed by that of the other parts of system.

Wheel Deflection Test Results Table

This is an interesting chart. For a 25lb load, they measure around 1.5mm of deflection for a typical 32-spoke rim. The deflection scales inversely with spoke counts; the wheels with 20 and 24 spokes tend to have about double the deflection. That we see such a strong relationship suggests that the primary driver of lateral stiffness is the spoke configuration, not the rim itself.

To my earlier point, 1.5mm (the collective deflection of spokes and rim, never mind the rim alone) is still small in the system. If you put a 25lb weight on a 12" bar attached to a steel axle in a rear FS linkage, you'll see the bar dip a whole lot more than 1.5mm.
 

·
Ride Fast Take Chances :)
Joined
·
3,331 Posts
That's with a 25lb load. My personal experience riding weak wheels and forks vs stiff wheels and forks shows there to be a significant difference in how far you can push a component before it will fail. For example my enve AM wheels will break me if I land them wrong and are stronger then my fork. SID forks wander all over the place and you can see them physically twist on impacts.
I put 1500 miles on a set of Zipp 404 it got to the point where I was potato chipping the rim every other ride on g-outs and rough sections of road. I switched to Dura Ace C50. Same weight and depth. They never go out of round on the same g-outs and the feel at speed leaned over is much better. You can tell that the structure of the wheel is not stress anywhere near its limit like the Zipps where.
 

·
Registered
Joined
·
636 Posts
Discussion Starter #8
Within the elastic limits of the system, the displacement should be linear, so you're just looking at the relative contributions of each part.

I did a little informal experiment just now: I squeezed the 32H i29 carbon rim I just assembled against one of the stays. I managed 6mm of lateral movement comprised mostly of spoke deflection, then axle twist, and just a hint of flange flex. I tried the same with the 32H i17 alloy wheel that came with my $1K road bike. Same result (actually a bit less, but within the margin of error).

In neither case does the rim feel like it's contributing anything. The exception is if the rim isn't strong enough to let you set adequate spoke tension. If the load exceeds the tension of the bottom three or four spokes (which is what I suspect happened in the video), the wheel loses lateral stability and flops over with little resistance. Until this point, lateral stability is unaffected.

The load it takes to do this with most wheels is immense; from what I can tell, radial stiffness is functionally identical for all decent bike wheels. (Which also implies there's no difference in ride quality. This is why I'm only focusing on lateral stiffness.)
 

·
Ride Fast Take Chances :)
Joined
·
3,331 Posts
Force from the ground applied to the wheel are supported by the spokes parallel to the ground and above. It compresses the rim at the bottom and tries to force it into an oval shape. The structural strength of the rim determines how many spokes are able to support that load. A very stiff wheel will include a larger portion of the top spokes. A weaker rim will also fold laterally under this comperson forming a chip shape.
The forces that wheels need to absorb is immense. When you pull a rim to the seat stays, most of that movement is bearing and spoke deflection, just like you stated.
edit
Finite Element Analysis of Bikes - The Nooganeer
 

·
Wanna ride bikes?
Joined
·
8,551 Posts
I don't see how it's possible to make this conclusion.
You can grind out numbers online all day, find all kinds of "factual" data, and draw any number of inaccurate conclusions. the guys that know the truth have built and/or ridden multiple wheelsets with varying degrees of success. Pound for pound a carbon rim is much stiffer.
 

·
Registered
Joined
·
636 Posts
Discussion Starter #11 (Edited)
Terrific link Alex, that was exactly the kind of feedback I was looking for.

OneSpeed, I agree that carbon rims are much stiffer. However, because they are located by spokes rather than by the rim itself, I don't think the stiffness much matters.

A good example is what happens when you pair a super-stiff rim with a handful of spokes. When lateral force deflects the bottom of the rim, the top moves in the opposite direction because the rim's rigidity overpowers the spokes. My argument, though, is that the distance the bottom of the rim initially deflects is mostly independent of rim stiffness. You can make a stiff MTB wheel from a noodle rim and a lot of spokes, but you can't make one from a dozen standard spokes, no matter how stiff the rim.

I'd love to find FEA of a wheel laterally loaded in a frame. May attempt to create this myself.
 

·
Registered
Joined
·
1,779 Posts
Rim stiffness affects overall wheel stiffness more than anything else. Any wheelbuilder will tell you this. Benefits of spoke bracing angles have been blown out of proportion by trek marketing cronies.
 

·
RAKC Industries
Joined
·
3,265 Posts
Actually bracing angles matter far more than you are trying to imply. Physics 101. Simple structural engineering 101.

If it was all about trek then why are asym rims becoming reality finally. Because it's the truth.

Now this thing saying that carbon vs allow stiffness doesn't matter laterally needs to spend more time riding a mountain bike. Noodle wheels vs strong/stiff wheels on the same bike, same tension, same hubs you notice a different big time just between types on alloy rims.

Comparing some road rim to some noodle xc rim your comparing practically apples to apples there. A strong and stiff alloy rim or more so carbon is very noticeable on a ride.

Yes frame, axle, hub all matter as well but wheel stiffness matters A LOT. I'm a big guy so a noodle wheel is scary. My road bike wheels are double wall because I'll destroy that featherweight stuff.

Sent from my XT1565 using Tapatalk
 

·
Ride Fast Take Chances :)
Joined
·
3,331 Posts
Lets take an example of 2 mountain bike wheels:
Ibis 942 vs Stan flow mk3. Nearly identical builds of hub/spoke strength. Rim is the deciding factor.
Both are 32 spoke
Ibis is 200 grams lighter
Stans has a 250lb weight limit, Ibis has none.
The big deal is that i'm 200lb and ride terrain that will push the flow to its limit. Once you get close to that limit the integrity of the wheel is compromised and you can feel the flex. Riding the same trails on the ibis rim shows no signs of compromise. In fact the feel is drastically different.
Yes lots of factors go into making a strong wheel. Rim strength is near the top of that list. Wheels are systems and having a weak link kills the whole system. It's amazing how strong and light carbon wheels are relative to the forces they encounter.
 

·
Ride Fast Take Chances :)
Joined
·
3,331 Posts
Actually bracing angles matter far more than you are trying to imply. Physics 101. Simple structural engineering 101.

If it was all about trek then why are asym rims becoming reality finally. Because it's the truth.

Now this thing saying that carbon vs allow stiffness doesn't matter laterally needs to spend more time riding a mountain bike. Noodle wheels vs strong/stiff wheels on the same bike, same tension, same hubs you notice a different big time just between types on alloy rims.

Comparing some road rim to some noodle xc rim your comparing practically apples to apples there. A strong and stiff alloy rim or more so carbon is very noticeable on a ride.

Yes frame, axle, hub all matter as well but wheel stiffness matters A LOT. I'm a big guy so a noodle wheel is scary. My road bike wheels are double wall because I'll destroy that featherweight stuff.

Sent from my XT1565 using Tapatalk
Truth.
 

·
Registered
Joined
·
1,779 Posts
Did i say bracing angles dont matter? Hmm.

Asymmetrical rims have been popular for a long time. Where are they new?
 

·
Registered
Joined
·
636 Posts
Discussion Starter #17
I don't know guys, I guess I'm just not seeing a good physics explanation for why rim stiffness matters above a certain minimum. As Roger Musson points out, you can make a strong bike wheel out of a rim in pieces (though that may be only in radial strength).

Personally, at 180lbs or so, I didn't feel any major changes in tracking between my ENVE carbon wheels and the set of featherweight XC alloy wheels I was using before. I'm not the only one to report feeling underwhelmed. Just want to make sure the emperor has clothes.
 

·
Registered
Joined
·
1,779 Posts
I don't know guys, I guess I'm just not seeing a good physics explanation for why rim stiffness matters above a certain minimum. As Roger Musson points out, you can make a strong bike wheel out of a rim in pieces (though that may be only in radial strength).

Personally, at 180lbs or so, I didn't feel any major changes in tracking between my ENVE carbon wheels and the set of featherweight XC alloy wheels I was using before. I'm not the only one to report feeling underwhelmed. Just want to make sure the emperor has clothes.
I think a lot of it explained by the "Enve effect". It could also be called the "Boost effect". Unless you are an engineer. Then you would just do some graphs and charts.
 

·
Ride Fast Take Chances :)
Joined
·
3,331 Posts
I don't know guys, I guess I'm just not seeing a good physics explanation for why rim stiffness matters above a certain minimum. As Roger Musson points out, you can make a strong bike wheel out of a rim in pieces (though that may be only in radial strength).

Personally, at 180lbs or so, I didn't feel any major changes in tracking between my ENVE carbon wheels and the set of featherweight XC alloy wheels I was using before. I'm not the only one to report feeling underwhelmed. Just want to make sure the emperor has clothes.
There should be some difference in feel unless some other component was the weak spot. Like a sid fork or QR instead of a thru axle.
 
1 - 20 of 88 Posts
Top