I think the take-home message should be that every race course has different demands. And if you think you've found an "optimal setup", then there's a race out there that will prove you wrong. Optimal equipment and pacing strategy are things you have to work hard at, if you're trying to shave seconds from your times.

---

AndyF
bike geek

I'm not contending it...the laws of physics demand it!

Let's simplify things even more: Take your 2 trucks into outer space and accelerate them to the same speed by firing a rocket motor. Then, cut the motor and tell me what happens to both...

You're confusing yourself with your centripetal acceleration example, BTW. I'm sure a "physics refresher" course somewhere might straighten things out for you ;-)

---

http://bikeblather.blogspot.com/

Due to the higher overall weight of MTBs in general, tire weight is even less of a "determiner"...for me, off-road tire selection is more about getting the right tread and psi for the course/conditions and then secondarily about rolling resistance. Once again, tire weight is way down on the decision tree.

---

http://bikeblather.blogspot.com/

Yep...if the external surfaces are identical enough so that air friction is the same, what other forces will be need to be counteracted to prevent deceleration?

Are you familiar with "free body diagrams"? Are you familiar with states of equilibrium?

---

http://bikeblather.blogspot.com/

I'm glad to hear to say this, as I've moved away from the classic "weight weenie" tire selection method over the last two seasons. I'm finding that very wide tires that roll well (with the appropriate tread and PSI for the conditions) to be the fastest in general.

Getting back to what Jack and Jordan alluded to about thicker casings likely increasing rolling resistance - would this due to the stiffness of the casing?

---

_______________________________________________

Actually, by definition, Crr (the coefficient of rolling resistance) is the ratio of retarding force to normal force...in other words: a linear relationship.

---

http://bikeblather.blogspot.com/

Si. Although there are a lot of very funky things about tire casings. Like, for instance, the common "myth" (though perhaps that's not really the right term) perpetuated by Vittoria that higher TPI = lower Crr. Assuming equally pliable fibers, you actually hit the sweet spot for Crr at somewhere between 60-120 TPI. A higher TPI tire will actually have HIGHER Crr because of inter-fiber friction.

There's a LOT that goes into making a fast tire. And I'd say that it's certainly an oversimplification to say a thicker tire = stiffer sidewall = higher Crr. It's more likely that a thicker tire is probably designed with something other than minimizing Crr as the primary goal and that for numerous reasons, Crr will be higher. Anyway...

---

"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp

I'll take that bet ;-)

I guess you missed my link in post #29: http://forum.slowtwitch.com/...cceleration;#3255798

Typical accelerations in a CX race or crit are going to be at lower levels than the "attack" acceleration I looked at in that post...AND the 200g total rim weight difference you're assuming is half what I assumed. So...if you think that an extra peak pedal force loading of somewhere in the range of an extra 1/8 - 1/4 lb. is going to "really start to add up" over the course of short races like that, I think you're drastically underestimating how much peak pedal force you're typically pushing with anyway just accelerating yourself.

---

http://bikeblather.blogspot.com/

Unless you are riding on this stuff. Then it's MUCH higher.

---

"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp

Interesting...thanks for the info.

What I was thinking about specifically was the UST (mavic tubeless type) tires. In my experience they tend to have very stiff and thick casings. As you said above, they are designed with something other than minimizing Crr as the primary goal - in this case it is for air sealing without the aid of liquid sealant. That being said, I've never flatted for slashed a side-wall on these type of tires - oh those pesky trade-offs.

---

_______________________________________________

Hmmm...that assumption of "equally pliable fibers" is a big one (and not necessarily a valid one). The reason that higher TPI casings are typically faster is because by definition the fibers themselves need to be thinner to pack more into an inch of ply. Thinner casing means lower strain through the casing for a given deflection. Lower strain in the casing means lower energy losses within the material.

Now then, if "higher TPI" is used the way that some companies use it (i.e. not the TPI of the "base" ply material, but of the TOTAL of overlapping plys), then yeah, higher "TPI" in those cases means a slower tire because they're just using thick casings and then using multiple layers.


I'm not so sure if I agree with you that it's an "oversimplification"...but, I do agree with you that there's a lot that goes into making a "fast" tire :-)

---

http://bikeblather.blogspot.com/

Yes, my mistake in framing it that way.

This, however, assumes that Crr is constant. Crr is function of normal force. I'll try and find some empirical data that confirms this...

You'll probably laugh because I don't know of anybody besides me who does this, but I run "regular" MTB tires with Michelin latex tubes.

I had experimented a bunch with DIY tubeless liquids (i.e. DIY "Stan's") a long time ago and although it works well, I'm not a big fan of having the fluid dry out in the wheels if I don't ride my MTB often, although I did like the apparently lower rolling resistance. And if a tire does get sliced or flatted, it's quite a mess IMO.

Now, the UST design tires don't necessarily require a fluid, and as you point out they have an extra layer of butyl inside of them for air sealing. This makes them seem "slow" and "sluggish" to me.

---

http://bikeblather.blogspot.com/

In the ranges of tire deflections we're talking about, it basically is constant. The only "modifications" I've seen to Crr have been related to velocity, but even then the effects are typically minimal, or being "confounded" with another effect.

---

http://bikeblather.blogspot.com/

Probably a better discussion for when we next drive to Tom's Farm... The point I was trying to make is that yes, you can make a thinner casing with thinner fibers, but those casings often have higher inter-fiber friction. Now, whether or not that ultimately results in a faster tire is open for discussion.

I had a fascinating discussion with the tire team at Specialized when I was last in Mo'Hill. You would have loved it. The senior engineer is a German guy (Wolf!) who has been doing tire design for 10years.

Even stuff just like cut angle for plies is fascinating (and rarely talked about). Same everything, but different cut angle, would result in quite different performing tires...

---

"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp

Are you calling me dirt?

---

http://bikeblather.blogspot.com/

I am ignorant to the specific situations with a bicycle, Crr may very well not change much with normal force. My comments earlier were directed at a situation with trucks.

"You'll probably laugh because I don't know of anybody besides me who does this, but I run "regular" MTB tires with Michelin latex tubes. "

No laughing here, I actually just ordered some michelin latex tubes for my mtn bike (for similar reasons you describe), but silly me ordered Schrader valves! Anyone want some 26inch latex schrader tubes?

---

_______________________________________________

Actually his test removes the most important aspect of cycling, load. A bicycle is machine used to propel a human being, without load you are negating this most important fact. His test is moronic, and so are you for thinking it gets to the heart of anything.

And since we are talking about a very small difference in acceleration due to the fact that even super badass bikers really aren't very fast moving objects and most bicycle wheel weights are comparable, one has to be very specific in how to conduct this test.

Go back to your idiotic postulating on balsa wood trucks, free spinning bicycle wheels and all the other make believe crap. You might as well use a unicorn to conduct your fantasy tests too.

I am definitely through with this topic. I don't have time to enlighten idiots. It's entirely too frustrating and futile.

If anyone with a variety of bicycle wheels (some really light and some really heavy), ones with wildly different moments of inertia and similar weight (disc versus heavy rim with light spokes), a bike, a trainer, good strong legs and a stop watch wants to conduct a real world test that will show the differences, then PM me. I'll be happy to help someone with a functioning brain and ears.

As for the Asad and rruff types that already know everything, yet are somehow getting their asses kicked exponentially by Newton's Second Law written for rotational acceleration, you can all bugger off. For a bunch of self adulating smart people, you all are monumentally dumb when it comes to simple things.

---

--------------------------------------------------------

You will remain the same person, before, during and after the race. So the result, no matter how important, will not define you. The journey is what matters. ~ Chrissie W.

I don't really have a comment to make. I'm just responding so I can have a tag on your post.

I come from a mountain bike racing background and rotating weight is huge in that scenario as there are so many hills, turns, obstacles, etc and constant accelerations. You also need to be able to stay with the lead pack, explode to overtake and respond to constant attacks. The most popular racing wheels are those built with Stans tubeless rims because they are one of the lightest and they let you run regular (lighter) tyres without the need for tubes - the total combined wheel+tyre weight (i.e. rotating weight) is the lightest on the market that's the main reason why most mountain bike racers use them.

Time trialling is at the other end of the spectrum with very little accelerations I won't enter into that debate as it is already being beaten to death and there are much more knowledgeable people than me on that topic!

Go back being a presumptuous prick and leave me out of it.

Sounds like a bad case of projection. You started off by presuming quite a lot about Tom A and jackmott that wasn't true... and the other part... let's take a vote on it?

I thought your suggestion about the trainer was a good one... or at least the way I interpreted it. If you put the bike in a trainer with no resistance and feel how hard it is(not) to spin up the wheel to a high speed, then that demonstrates just how little effect wheel inertia has on acceleration.

Agree with you Tom, but wheel weight is a more significant consideration on mountain bikes than road bikes due to the much greater number of decelarations caused by braking. The kinetic energy in the wheel is lost to heat in the brakes. This might happen many hundreds of times on a technical mountain bike course, so a small weight saving in the wheels has more effect.

Even stuff just like cut angle for plies is fascinating (and rarely talked about). Same everything, but different cut angle, would result in quite different performing tires...

Do you remember any details? I'm curious because tire plies are usually at 45 degrees, and according to Mark McMaster on weightweenies, the "neutral" angle is 52 degrees... ie the minor diameter of bike tires contracts a bit to help hold them on. But Maxxis makes a "radial" model with an angle greater than 52 degrees (and they are super hard to put on... maybe to compensate), and I have a suspicion that Hutchinson tubeless tires may have a smaller angle to make them get very tight when inflated.

Uh, I remember all the details. ;)

A wider cut angle results in more compliance but increased rolling resistance. A narrower cut angle results in less compliance but decreased rolling resistance. Of course, this doesn't necessarily consider the effect of compliance on rolling resistance. And of course, it's also dependent on what sort of specific compliance you are looking for. It was a LONG conversation. Much of it was related to an article I'm currently working on (or supposed to be working on, but I'm replying to you).

Anyway, in response to your radial question, I do have a specific answer on that. Radial tires require a circular belt to provide structure and that extra belt basically makes the tire quite poor for rolling resistance. So I'd say that the Maxxis tire is probably very compliant (though perhaps it's a bit of a tough discussion to have without actually defining what that specifically means) but probably doesn't roll very well because of a lot of hysteresis within the tire itself.

---

"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp