some people mention a wheel's lateral stiffness as an attribute. What is this for? Cornering? Isn't a wheel's loads always vertical even while cornering?
I've never felt a need for more stiffness from a handbuilt wheel unless using very thin spokes.

Is there more of a need with heavier (180lb+ people)?

some people mention a wheel's lateral stiffness as an attribute. What is this for? Cornering? Isn't a wheel's loads always vertical even while cornering?
I've never felt a need for more stiffness from a handbuilt wheel unless using very thin spokes.

Is there more of a need with heavier (180lb+ people)?

Yes, you're right, the primary loads on the wheels are radial (in plane with the wheel). Although cornering requires lateral loads with respect to the ground, because we lean into corners, the net direction of load on the wheels is still in plane with the wheels.

However, there are (small) lateral loads occurring all the time. They can occur due to rapid steering motions, due to leaning our bodies out of plane with the bike, and sometimes due to the size and shape of road obstructions. Wheels are tremendously stiff to radial loads (typicall 10,000 - 20,000 lb/in of stiffness), but have much lower stiffness to lateral loads (typically only 200 - 400 lb/in of stiffness). So, it doesn't take much lateral load to cause a small amount of vertical flex.

When does lateral stiffness matter? There are a few situations. If you sprint out of the saddle and rock the bike side to side, the rapid up and down loading on the wheels with the bike leaned over can make the wheels flex enough to interfere with good handling. I'm not so sure there are any real power or energy losses in these cases, but it definitely can interfere with the handling of the bike.

In rapid manuevering, a rapid steering motion can generate a relatively high instantaneous lateral force on the front wheel. Although the flex is relatively small, it can result in a certain amount of imprecise handling.

In cornering, the more you are leaning over, the more perfect bike balance is important in holding to a holding a a line. In addition, as you lean over more, the greater affect on small steering angles on the tracking of the bicycle. Also, as lean over, bumps and surface irregularites start to contribute to lateral loads. Therefore, the more we are leaned over into a corner, the more lateral flex affects our ability to hold a precice line.

But, all that being said, I think the importance of lateral stiffness is a bit overplayed. Lateral forces are small enough that lateral flex will be quite small in most riding. A smooth and skilled rider is able to manuever and corner without rapid steering motions, and is able to smoothly compensate small variances in tracking due to the (small amount) of lateral flex when leaned over into a corner. I've got quite a few wheels, and I personally rarely notice a difference in lateral stiffness between them. Even in rapid manuevers, I can notice a difference feel in my flexiest wheels, but it doesn't really interfere with my ability to handle the bike.

i have recently changed my opinion re the importance of lateral stiffness.

i have three sets of wheels i've been riding lately. one set is an older velomax orion with 24/28 15 ga spoke count and a shallow rim. another is velomax/easton tempest with 18/20 14 ga spokes and a 30mm rim. th other wheelset is a 16/20 15 ga spoke count 46mm carbon tubular set-up (stratus dv). the stiffest feeling wheelset is the tempest followed closely be the stratus dv and then the noticably less stiff orion.

i don't know if what i'm feeling is differences in lateral or radial stiffness or a combination

i used to think the wheel stiffness was a function mainly of the total spoke cross sectional area but apparently rim stiffness plays a bigger roll. it makes sense i guess when you think about a shallow rim deflecting on the compression (bottom) side. i don't know....but i will be using taller profile rims from here on out.

just a thought - those deeper rims may be providing a more harsh ride, which to the rider, feels like a stiffer wheel, but the the higher spoke count lower profile wheels may actually be laterally stiffer.

similar reasoning to the pros using 32 spoke low profile handbuilts for the cobbles in PR and Tour of Flanders..

I am interested to hear Mark's comments on how exactly the box section rim provides a smoother ride. Obviously it's related to the mechanical properties (bending moment? - been a while since i took a strength of materials course) )of the low-cross section, and perhaps allows for more flexion of the rim itself at the contact point of the ground??

from what i've read, low profile/box-section rims are more vertically compliant than deep section rims, which makes them more comfortable to ride. this doesn't necessarily make box section rims less efficient. dougn's experience, i think, is more related to vertical compliance than lateral stiffness.

boon

just a thought - those deeper rims may be providing a more harsh ride, which to the rider, feels like a stiffer wheel, but the the higher spoke count lower profile wheels may actually be laterally stiffer.

similar reasoning to the pros using 32 spoke low profile handbuilts for the cobbles in PR and Tour of Flanders..

I am interested to hear Mark's comments on how exactly the box section rim provides a smoother ride. Obviously it's related to the mechanical properties (bending moment? - been a while since i took a strength of materials course) )of the low-cross section, and perhaps allows for more flexion of the rim itself at the contact point of the ground??

There is a common expectation that a box section rim provides a more "compliant" ride. Afterall, a shallow rim will flex more radially than a deeper rim, right? Well, if we rode bare, unsupported rims, that might be the case. But we don't - we ride wheels with the rims rigidly supported by the spokes. Individual spokes have stiffnesses between 5,000 lb/in (the thinnest spokes) to 10,000 lb/in (the thickest spokes). And that's just for a single spoke - when you have a whole set of spokes supporting a rim, the stiffness of the wheel assembly will be higher - typically betwn 10,000 lb/in to 20,000 lb/in. This is stiffer even than many deep section rims. For most wheels, the total stiffness is governed mostly by the spokes (number and thickness) not the rims.

As a demonstration of this, take a look at the wheel vertical stiffness data in this test: Francois Grignon wheel test (http://www.sheldonbrown.com/rinard/wheel/grignon.htm). In particular, look at the first and last wheel in the data table. The Campagnolo Shamal (original model for the '90s) has a 40mm deep aluminum rim, but only has a stiffness of 12,600 lb/in. The Mavic GEL 330 wheel has a 350 gram 13mm deep aluminum rim (i.e very light and shallow), yet has a stiffness of 20,200 lb/in (60% more than the deep rim). Why? Because the Shamal only has 12 spokes, whereas the GEL 330 has 36 2.0 straight spokes. For these wheels, the number and thickness of the spokes far outweighs the contribution of the rim in determining wheel stiffness.

But this is really all academic. In reality, any wire tension spoke wheel is so much stiffer than the tires that the wheel stiffness has little real significance. Consider when you hit a pot hole, you can pinch flat a tire (i.e. flatten it out against the rim), which means compressing it about 1", with only a few hundred pounds of force. At the same time, the wheel will only flex 0.04" or less under the same load that compresses the tire by an 1" - i.e. the flex in the tire is about 25 times greater than the flex in the wheel. The difference in vertical flex between two different wheels may be less than the difference in tire flex when you change the air pressure by a few psi. Can you tell the difference between 100 and 104 psi in the tires? Its pretty doubtful.

"There is a common expectation that a box section rim provides a more "compliant" ride. Afterall, a shallow rim will flex more radially than a deeper rim, right? Well, if we rode bare, unsupported rims, that might be the case. But we don't - we ride wheels with the rims rigidly supported by the spokes."


is the shallow rim deforming around the contact patch and not transferring as much load to the spokes 90 degrees from the contact patch?

if you considered an extreme case where the rim was just a strip of aluminum, it's easier to "see" what would happen

so why all the fuss about box-section rims for the cobbles?

I know that the number of spokes has more impact on lateral stiffness then the rim depth. What about radial vs. 2x vs. 3x lacing patterns? Specifically the front rim?

(My curiousity about this stems from the set of Easton Circuits I own. They are the first wheels I have ever owned with radial front lacing. Any out of the saddle climbing or sprinting will cause brake rub as the rim flexes back and forth).

so why all the fuss about box-section rims for the cobbles?

Just because the pros do it doesn't mean there's a real good reason. They're as susceptable to and as gulty of perpetuating myth as anyone.

I know that the number of spokes has more impact on lateral stiffness then the rim depth. What about radial vs. 2x vs. 3x lacing patterns? Specifically the front rim?

Lateral stiffness is greatly influenced by the bracing angle of the spokes. On the front, heads in radial is the stiffest... and the greater the number of crosses the less stiff the wheel will be because the effective bracing angle is lower as the spokes get longer. Heads out radial is usually about the same as 3x.

Front wheels on regular road bikes are always stiffer than the rear, though. I've been riding wheels with 18 CX-Rays in the front with a Niobium 30 rim, and I think the handling is fine. Did a fast technical descent in a race last Fri and there were no issues with the wheels at all. I think that generally stem, bar, and fork flex are bigger issues on the front of the bike.

BTW Mark McM... very good posts... as usual.

[QUOTE=dougnis the shallow rim deforming around the contact patch and not transferring as much load to the spokes 90 degrees from the contact patch?[/QUOTE]

Well, yes. But since any rim (even a deep rim) is so much more flexible than the spokes, no rim will transfer much load around to the rest of the wheel.

When the wheel is pressed against the ground, the rim is loaded in bending, which is it's most flexible loading mode. However, the spokes which are in line between the hub and the ground are loaded in tension/compression, which is the least flexible loading mode. A rim will would have to be incredibly deep and heavy to be stiffer radially than the spokes. So even a deep rim will deform around the contact patch, compressing (unloading) the spokes near the contact patch, and this change in spoke tension is enough to support the load at the hub.

The spokes each have a longitudinal stiffness of 5,000 - 10,000 lb/in. That means any change in length of the spoke (whether it is elongating or contracting) requires 5,000 - 10,000 lb for each inch of length change. The spokes in a wheel are typically pre-stressed to about 100 kgf or more (220 lb), so as long as they aren't de-tensioned by more than the static pre-tension, they will retain their longitudinal stiffness. "Compressing" a pre-stressed spoke by 100 lb. will only change its length by 0.010" - 0.020". And if you have several spokes between the hub and the Load Affected Area (LAZ) near the ground contact point, the combined stiffness will be greater, and hte wheel may only deflect 0.010" - 0.005" (this is the typical stiffness of actual measured wheels). How much force does it take to bend a rim (even a deep rim) by that much ? Not much. Therefore, with a just a small wheel deflection, the spokes take up all the load, and there is little load left to be distributed around the rim.

So, yes, the rim does "deform" at the ground contact patch. But even for a shallow rim, this deformation is very, very small, and is measured in thousandths of an inch. Don't believe me? Take the tire off a wheel, stand it up on very hard surface, grab the axles, and push down as hard as you can. Can you detect any flex? No, you can't - it's too small. Vertical flex of a wheel is a insignificant, and non-issue when compared to the vertical flex in many other components in the load path between the ground and the rider.

so why all the fuss about box-section rims for the cobbles?

As Forrest Root says, a large part of it is tradition and mis-perception. But there are some practical reasons for using wheels with a large number of spokes in cobbled races. There is a greater likelihood of wheel damage in races with rough, cobbled roads, and a wheel with many spokes will stay true better in case of a broken spoke. Since cobbled roads tend to be narrow, they clog up easily with racers and support vehicles, so in case of wheel damage, it could take a relatively long time for a support vehicle to reach a rider for a wheel replacement. So, wheels with many spokes can be advantageous in these races.

There aren't many wheels with deep rims and lots of spokes - afterall, deep rimmed wheels are generally designed with aerodynamics in mind, and deeper rims tend to need fewer spokes to support them. So, the most commonly available wheels with many spokes tend to have shallow rims.