chaadster

That gets to what I was asking Psimet about, plus the very basic question of breakpoints, meaning, what is defined as shallow, what as deep, and is there a depth at which they start acting/feeling the same? You know, like, is a 45mm carbon effectively as vertically stiff as a 60mm relative to 17mm aluminum? What about compared to 30mm alu rims?

It all gets complicated, that's for sure, but it still strikes me as a little odd to think about wheels as a comfort item. I think I probably pay closer attention to how my gear works and feels than most, and relative comfort between wheels has never been a point of consideration. Different feel, yes, but nothing I'd call comfort. It's interesting to consider what I might be missing out on, though.

Now these, however, are a different matter:

bikepro

+1. I have a wheel set with 18 spokes/radial in the front and 20/cross 2 in the rear. In trying to find a less harsh ride, I finally switched to Vittoria Corsa Evo CX, with latex tubes. 23 mm front and 25 mm rear. This was a huge improvement, even over the same size Continential 4000s with butyl tubes. The Vittorias likely won't last as long, but the ride makes it worth it.

Psimet2001 

Haven't had much of a chance to read your response when people aren't talking to me so maybe I missed something in it, but in general -

Rims deflect. Spokes are in tension - not compression. There is nothing that holds a rim from deflecting inward when it hits an obstacle except for it's own mass and area moment of inertia.

Think of an I-Beam. We can get a deeper/stiffer i-beam for the same mass of a smaller square beam. The i-beam won't deflect as much - and the system has exactly the same mass. This is completely due to how the mass is arranged around the center of mass - Mass/Area moment of inertia.

An extruded aluminum rim is essentially an "i-beam" that is round.

Hold the spoke count the same and go from a shallow heavy rim to a deeper rim of the same weight and the shallow one will feel "softer" - or quite simply will deflect more. Increasing the spoke count, in essence, increases the support of the basic rim structure.

Shallow rims (we like to refer to them as 'box section rims') are light in many cases but also fairly soft. They are super easy to build on but build up at a low tension and need many spokes to support them. Deeper more modern alloy rims in more complicated extruded shapes have changed the equation by adding a ton of stiffness into the system. It's how and why we are able to have low spoke count radial laced "cool looking" wheels.

On those prebuilt wheels with low spoke counts - they purpose those wheels for OE builds coming out of factories where the end customer is not known. That's why they have to "beef them up" for the likelihood that a larger rider will be on them. Well the appropriate way to do that is with the correct spoke count but that makes the wheel look less sexy, lowers it's aerodynamic efficiency, etc. So they compensate by putting extra mass into the rim itself - making it a stronger "i-beam" section. Less likely to bend and much stiffer. The result is that for a lighter rider you end up with a harsher feeling ride as the rim is way less likely to deflect under the stresses they subject the wheel to. With a heavier rider you get a rigid ride but often break spokes when you jump out of the saddle and apply pressure, or have the wheel feel "floppy" when out of the saddle.

Carbon is carbon. Sure is smooths out the feel of the ride a bit but it's just about like putting a nice tire on an alloy setup.

Don't mix ideas about why we use carbon fiber in wheels. Carbon fiber is a HORRIBLE wheel material when you step back and look at it. It prefers to break/crack instead of bend when it hits obstacles. The resins used tend to experience problems when under high heat - such as when braking. Carbon is a thermal insulator instead of a thermal conductor like aluminum.

So why do we use carbon? Ever try and make an aerodynamic foil shape out of aluminum? It gets REALLY heavy REALLY quickly. Carbon drops that weight out of the equation and makes deeper aero sections possible at a weight that's acceptable for the market.

There are some who use it simply for the weight savings resulting in shallow section carbon rims that are really light. But now you just have a REALLY poor performing wheel material and not much of it. I've never been a fan of shallow section carbon rims. Get a well made shallow section alloy rim that is proven to perform.

The other nice part about carbon is the stiffness you can achieve. A nice stiff section rim - many OEM's will under spoke it because they now have the strength of the material to support the same variety of rider loads over the smaller spoke count - making the wheel more aerodynamic and lighter in the process. In reality though if you take that section and put a higher spoke count on it you can create some amazingly cool wheels. the lateral stiffness of the system increases, the carbon gives it that buttery road feel, yet it's still light and extremely responsive. You stop thinking about them and they react intuitively.

So....yeah. Of course tires and tubes make a difference. A big difference. ...but so do wheels. So do frames, handlebars, cranks, stems, etc. If you're lucky enough to have ridden a lot of different designs and materials you start to appreciate how well you can combine them all to get the qualities you want overall in the system.

Psimet2001 

tubular rim construction drops a substantial amount of weight off of the edge of the rim and provides a wider angle of attack for the tire itself.

Good clinchers are great. Deep section carbon tubulars are amazing. Lighter and less expensive in most cases. The only reason all wheels are tubular is simply due to the gluing. It's a big obstacle for most of the riding public - we know as people have voted with their wallets over time.

SirHustlerEsq

I weigh 180lb, run a 19mm bead-to-bead rim width, GP4ks at 70psi front and 80psi rear. Its very, very smooth. I run 60psi in my 28mm Gatorskings on the CX bike when riding it on the road.

rpenmanparker 

Rob, concerning the above assertion: I want to test a contrary idea on you to get your response. Of course the spokes are in tension and the spokes nearest the point of stress (say contact with the road) can't keep the rim from compressing i.e. moving upward in response to the stress. But don't spokes further away from the place where the stress is applied provide a dimensional limitation? If you think of a half-circle arc of the rim, when the bottom on the ground tries to move inward/up, it pushes the ends of the half-circle outward. but that outward movement IS constrained by the spokes in tension.

Such geometric constraint is a commonly used feature of many constructions. In fact it is just what makes the strength to mass ratio of the I-beam you reference possible. Another example is the complex shaping of 16 oz plastic juice bottles. All the curves and creases strengthen the bottle to crushing forces much better than a perfectly smooth surface would without the need for the bottle to be heavier. An even more significant example is all those decorative creases on the sheet metal of modern automobiles. Sure they achieve a certain look, but they also strengthen the body to deflection under impact without adding weight.

Similarly the OTHER spokes in the wheel, not the ones right near the point of stress are what help to keep the rim from flexing inward by limiting the motion outward of the rim sections they affect. And one would expect that more spokes, shorter spokes, and/or heavier gauge spokes would be more effective in accomplishing that.

I don't think it is just the characteristics of the rim itself that keep it from deflecting.

Psimet2001 

It's tensioned structure for sure, but when impacted - the localized area of impact deflects inward. Been looking for a great video that I have seen that shows this in slow motion - drastic movement on a newer carbon rim shape that a company was testing. They were touting how well it bounced back without damage, but it easily demonstrated how the spoke unloaded and the rim moved vertically inward.

It's a huge effect - having built enough wheels - you know. Take any hub and any spokes - lace them to any rim. Them move them to a different rim of a different depth, etc. Over the different wheels you end up with you will have drastically different rides and feel. That's 100% the rims.

rpenmanparker 

I agree with everything you said. But now what happens when you do the alternate experiment and move the same rim between two different sets of hubs and spokes, say very low, narrow flange hubs with 2.0/1.5/2.0 3X spokes and very high, wide flange hubs with 2.0 straight gauge, radial spokes? Will a discriminating rider not be able to feel the difference? Especially when you are testing the rim from the first experiment that gave the greatest deflection?

Psimet2001 

That spacing will have the same effect as under-spoking - giving the wheel a more flexible feel laterally.

emveezee

Trying to understand the some of the physics of this discussion. When talking about "localized" forces or deflections, that's only a situation where the tire is out of the equation, and there's a direct impact to the rim, right? Short of bottoming out, the air pressure of the tire distributes force over the whole wheel. So, any compliance or comfort or deflection in that situation is essentially on the scale of top versus bottom half of the wheel and not localized, yes?

rpenmanparker 

No doubt. But that is only one of the effects, besides differences in vertical compliance. These are just the principles of the Roman arch or how an arched bridge (with a keystone type structure) is held up in the middle by fixing its ends in the river banks. The whole idea is that the forces on the arch (corresponding to the curvature of the rim) are transferred from the point of application to the ends of the arch where they are supported by being buried in the bank (corresponding to the spokes in tension). This is completely analogous to the bicycle wheel. You are talking about how stoutly the arch or bridge is constructed throughout its length, and I am talking about how firmly it is fixed at its ends. I fully accept what you are saying about the sturdiness of the rim itself, but strongly believe you are short changing the importance of the spokes and hub in limiting vertical compliance of the wheel. I'm happy to specify that the sturdiness of the rim is the first and foremost influence. I just contend that you can't ignore the effect of the rest of the wheel construction.

Thanks for the good discussion. I appreciate it.

juanebici

Psimet2001
"Don't mix ideas about why we use carbon fiber in wheels. Carbon fiber is a HORRIBLE wheel material when you step back and look at it. It prefers to break/crack instead of bend when it hits obstacles. The resins used tend to experience problems when under high heat - such as when braking. Carbon is a thermal insulator instead of a thermal conductor like aluminum.

So why do we use carbon? Ever try and make an aerodynamic foil shape out of aluminum? It gets REALLY heavy REALLY quickly. Carbon drops that weight out of the equation and makes deeper aero sections possible at a weight that's acceptable for the market."

So with that said, what do you make of the carbon/aluminum Shimano Dura-Ace WH 9000 C24? I am interested on getting this wheels. It seems to me that the bonding of aluminum and carbon makes it for a great combination for stiffness, weight savings, and comfort. Do you agree?

rpenmanparker 

The key word is "bonding". It hasn't always been what buyers hoped it would be. Some nasty crashes have been caused by detachment of the aluminum section from the carbon.

LesterOfPuppets

I've had soft wheels before. In 1990 I got a bike with 105 hubs, double butted 15 gauge spokes, Wolber GTX rims. Soft, plush and yes, kinda vague when cornering. Also rubbed brakes a bit when stomping on the pedals out of the saddle. Killed a rim on a pothole in the mid 90s. CXP-22 and new 15 gauge double butted spokes, but same tires and hubs. No more brake rubbing, felt stiff, kinda dead at first but tracked better in hard corners. Definitely a difference.

I reckon there's a happy medium but I'm not gonna go out of my way to chase it.

Psimet2001 

+1 - Carbon and aluminum don't really like each other and a lot of attempts at bonding the two have been extremely problematic in the past.