rjm3u

I commute on a Surly Crosscheck 13 mi. one way over some big hills. For the past several months, I've been riding on 32mm cyclocross tires. Looking for some extra speed, I switched over to skinny Kevlar tires. I also went to thorn-proof tubes to avoid flats on the commute. Now I'm pretty sure I'm much slower. My max speed on a given descent is 39 mph instead of 41. In my highest gear on a downslope I used to break 30 mph at 100rpm, but now the same gear and cadence on the same hill puts me about 28.5mph. I'm pretty sure that my climbs have gotten harder--my cadence/gearing isn't generating the same mph it used to. And I'm crossing the finish line about 2 min. slower on a commute that used to take about 38 min.

My old tire/tube combo weighed a total of 918g. The new combo, with 25mm slicks, weighs in at 1140g (b/c of the monstrous thorn-proof tubes). So I'm riding with 222g of extra rolling weight.

What gives? Does the weight of my new tube/tire combo really make that much diff? Or should I be looking for other explanations (like mental exhaustion)? Whatever the case, I think I'm getting rid of the thorn-proof tubes.

Thanks,
Ryan

within

https://www.analyticcycling.com/

There are so many variables! Maybe play around on this site and see if you can figure something out on your own. The question you ask, imo, is impossible to answer.

merlinextraligh

your speed went down for the same gearing and cadence because the circumphrence of the tire is smaller.

waterrockets 

Yep. You also have a mental block now. Just go faster. Sheesh

kendall

I notice the same thing, I'm faster on my old 27x1-1/8" bike then I am when riding the 700x25, nearly the same gears on both bikes, just less actual travel per pedal rev on the 700

Ken.

rjm3u

I adjusted for circumference on my cyclocomputer.

Perhaps if I boil the question down:

In percentages, how much should getting skinnier, slicker tires improve my speed? (I had been expecting something like 5%, putting me in 2-3 min. faster on my previous 40-min. commute.)

In percentages, how much should 300g extra rolling weight slow me down? (somebody on this site said that it should be negligible: 10g on the wheel = 10.01g on the frame.)

rjm3u

A friend of mine swears that 32mm 622cc slicks are mechanically the most efficient wheels. He says that as you get skinner, the rolling resistance does decrease, but the higher tire pressure correspondingly increases the friction with the ground. I had completely distrusted this theory, until I actually tried the dreamed of skinnies. My experience might confirm fat-tire theory, since you could interpret the data to say that the only advantage of skinnies is weight. Since I've eschewed that advantage with my gargantuan thorn-proof tubes, I'm actually slower. Hmmm . . . I think that other fellow is probably right: it's all psychological now. Part of my commuting philosophy is not to get wrapped up in Gear and Buying stuff. Perhaps I need to be reminded of that philosophy. And quit frequenting Bike Forums.

waterrockets 

^^^^^
Here's a graph relating some tires of varying sizes to pressure. This is for tubulars, but translates to clincher fine for relative comparisons on size and pressure. This is from Jobst Brandt:

merlinextraligh

I'm not saying that your cyclocomputer is reading incorrectly. I'm saying that if you pedal a smaller tire at the same cadence, your mph will be lower.

Effectively, you've lowered your gear ratio. Thus to maintain the same speed, you have to either increase your cadence, or go up a gear.

michaelmc

First, they're not saying that you're computer is wrong, they're saying that you're actually slower because the tires are narrower. Because the circumference of the tire is smaller, each revolution of the wheel moves you a shorter distance.

Second, there are so many factors that change when changing tires that it's impossible for any of us, or likely even any engineer or physicist, to calculate such a percentage of difference in such a real-world way that it's meaningful to you.

UT_Dude

It's possible you're slower because of the smaller tires, but in that case you just push a harder gear and it's all good. How big are these climbs you're going up?

Although, there's no reason you should be going slower down the hill, save maybe the tires breaking in a bit...

registered usar

similar thing happened to me. turns out i forgot to take my epo for several days

rjm3u

Ah, I see. That makes sense. Time to start working harder on the cadence.

Psydotek

Those thorn proof tubes are real anchors. I have them on my commuter bike with gatorskins AND Mr. Tuffy liners.

But hot dang, I can fly when i get back on my other bikes.

bbgobie

I mean, if you can't increase your cadence slightly, go to the next gear.

In effect your pushing an easier gear with a smaller tire.

+ extra weight = slower acceleration if your stopping a lot on a commute.
+ probably worse rolling resistance.
+ possibly different air pressure

Maybe even your rim drags a bit in certain places after removing your wheel. I've seen it happen to people...

I'm sure those 3 factors are enough to change your commute.

ggg300

yup...10t cog for small wheels...

rjm3u

Final report: the general advice was right on. I adjusted my riding style today and it made up all the time I'd been losing. For example, a faster cadence actually felt easier to keep, so I was riding an easier gear at 100rpm rather than one tougher at 90. On the flats, that seemed to give me about the same average speed as before, maybe even a little faster. Downhill I could still use an extra gear (I have a mtn bike triple up front for pulling two kids in the bike trailer), but I found that my tucked-in coasting gets me going a good 2mph faster than with the knobblies. Still working on the right gearing/cadence for the climbs, but again, 100rpm is feeling comfortable and fast.

Now I'm gonna drop the thorn-proof tubes and really fly!

waterrockets 

Glad you figured it out. Dropping the tubes would only help measurably on a looong climb or maybe in a sprint series.