My current setup is a Giant TCR w/ 23 mm Michelin Axial Pros. For some reason, my butt has been getting sore after about 3 hours of riding. I've got a carbon seatpost and a comfortable saddle. I know my wheels are stiff (Velomax Orions), but I can't afford to change them (nor do I want to). So, I figure that going from 23s to 25s will provide a softer ride and create less rolling resistance. The only drawback is that the wider profile will theoretically create more air resistance.

By my calculations, an extra 2 mm in width for a 700 mm tire will add about 14 cm of surface area facing the wind. This seems to be the equivalent of raising just my head 1 cm.

My question is: will I be noticeably slower with 25s? I do fast 35 to 50 mile group rides pulled by Cat 2s and 3s. Some days, I'm barely holding on, and occasionly get dropped (I'm a relatively strong Cat 5). Because I'm a marginal rider in my group, even the slightest drop in performance is an issue.

I'm also thinking about running a 25 in back and a 23 in front, because (1) it's my ass that's hurting, and (2) a wider rear tire should have only a negligible effect on the aero profile. I can't imagine that the handling characteristics would be harmed by such a setup, but I've never tried it. Has anyone experimented with this. If not, I'll probably give it a go since it'll cost me only $40 for a rear tire.

My current setup is a Giant TCR w/ 23 mm Michelin Axial Pros. For some reason, my butt has been getting sore after about 3 hours of riding. I've got a carbon seatpost and a comfortable saddle. I know my wheels are stiff (Velomax Orions), but I can't afford to change them (nor do I want to). So, I figure that going from 23s to 25s will provide a softer ride and create less rolling resistance. The only drawback is that the wider profile will theoretically create more air resistance.

By my calculations, an extra 2 mm in width for a 700 mm tire will add about 14 cm of surface area facing the wind. This seems to be the equivalent of raising just my head 1 cm.

My question is: will I be noticeably slower with 25s? I do fast 35 to 50 mile group rides pulled by Cat 2s and 3s. Some days, I'm barely holding on, and occasionly get dropped (I'm a relatively strong Cat 5). Because I'm a marginal rider in my group, even the slightest drop in performance is an issue.

I'm also thinking about running a 25 in back and a 23 in front, because (1) it's my ass that's hurting, and (2) a wider rear tire should have only a negligible effect on the aero profile. I can't imagine that the handling characteristics would be harmed by such a setup, but I've never tried it. Has anyone experimented with this. If not, I'll probably give it a go since it'll cost me only $40 for a rear tire.

Assuming that you've already lowered the pressure on your rear tyres a bit, guess you could put a 25 on the back. Few things first: I thought increasing the width of the tyre increased it's rolling resistance, not reduced it. Think of it this way. How much rolling resistance would there be on my motorcyle tyres which are 110mm wide on the front and 140 on the back? Lots more than any skinny bike tyre, I can tell you that! This is also the reason why they use 19 or even 18mm width tyres on the velodrome.

It will create more additional SA than that, as the tyre is round :rolleyes: Remember circumference = Pi x D, so at least 50% more than what you have stated. Then again, that is masked by the frame etc. :confused: Ummm how much do you weigh? And BTW I think a lot of people's ass's hurt after 3 hours in the saddle! You just need an adequate saddle-butt acclimation period. :D

The rear tire is blocked by the seat tube, so the aerodynamic effect is virtually non-existant. The larger tire will only improve comfort if you also lower the pressure a bit. Even then, it's unlikely to be the source of your sore butt after 3 hours. More likely a marginally padded saddle is at fault.

There is a simple solution................................ride more.....If you can't ride more, try different saddles, until you find the right one for you.

This can't (apparently) be repeated enough: AT THE SAME PRESSURE, A WIDER TIRE WILL HAVE LOWER ROLLING RESISTANCE. This assumes the same model of tire, just in different widths. Bike tire rolling resistance is largely a matter of casing flex, and a wider tire will flex less than a narrow tire at the same pressure. Simple as that. In practice, narrower tires are pumped to higher pressures, so there is precious little difference in rolling resistance across the "normal" width range for bike tires.

There may be some usefulness to the notion of a narrower front tire. Take for example Continental's GP Attack/Force combo. Conti's ads state:

22mm wide for secure cornering while reducing wind resistance

...for the Attack front tire and,

23mm wide for efficient power transmission, low rolling resistance, and high comfort

...for the Force rear tire. Sure, some of it could be a marketing ploy but I think the idea has some merit.

A change of tires is unlikely to affect your speed or comfort. Unless your running the 23s at some unusually high pressure your unlikely to detect much difference between 23's and 25's. I use both and couldn't tell you which I was on with my eyes closed. I run 23's at 100 and 105psi and the 25's at 95 and 100psi. I weight 157# and that feels plenty firm to me. When I did race I used higher pressures, but in retrospect I don't think there was any difference in the performance. I only bounced around more.

As you described your perfromance challange, I would suggest that aerodynamics has nearly nothing to do with your problem. You probably drafting 90% of the time and wind resistance just isn't a factor under those circumstances. Even if it was, the difference between 23s and 25s is so minute that all you need to do is lean down to grab a water bottle once every 10 minutes, and you've probably negated the advantage.

If you're looking for a way to get faster that doesn't involve either; a) losing weight, or b) gaining power, I'd look into some aero wheels. They do make a difference.

Pro Race is the new version of the Axial Pro, and the 25mm is MUCH bigger than the 23mm (visually at least, have not measured them). I made the switch awhile back (front and rear), am much more comfortable and the bike doesn't feel any slower...but I'm 195 lbs, the difference might not be as noticeable for a smaller person.

I am running 23 in front and 25 in back right now. Mostly because I cut a sidewall in my 23mm rear tire and am using a 25mm I had at the house. No scientific data to back it up but I really dont feel that am any slower or faster but I do notice a difference in the ride. It is definitely smoother, probably partly because I run the 25 at a lower pressure.

My suggestion is get a 25 and try it. If you dont like it use it for the trainer or as a back up and go back to 23 when it is time to replace.

i could be wrong but i remeber a 23 being real tight...

Given the advice I've seen, I think I'll first lower the pressure from th 110 psi max to 100 on both tires (I'm 160 pounds), hoping this won't slow me down too much. After that, I'll change to a 25 in back.

Mr. Grumpy: As for riding more, I already do 6 - 8 hours a week (150 to 200 miles) and don't have more time. :)

Synchronicity: As Kerry Irons said, I think that it's been well established that a wider tire has a lower rolling resistance, even though it's counter intuitive. I've seen several articles/threads on this issue, but can't now cite to them.