Hi Peter!
I'm still shaking my head over your temperature extremes Down Under at present, but I believe I can offer at least some insight to how narrower tires might affect your Club Tour's handling. Remember, bike tires typically have a 1.0 aspect ratio/profile, meaning the profile (sidewall and casing height) is as high as the tire is wide (section width). To restate the obvious, going narrower also means going lower or smaller in diameter and radius. Diameter will affect your computer, while radius will affect handling (it changes trail to a greater or lesser degree, making the bike handle differently).
With that in mind, I'll refer you to my article on how tire section width and profile affect handling in my Sherpa-oriented article here:
http://www.thorncycles.co.uk/forums/index.php?topic=4245.msg19567#msg19567With the Sherpa's wide range of available 26in tires, we're talking about potentially big differences in tire size. With your Club Tour (and most 700C-wheeled road bikes), the difference isn't so great...and so the difference in resultant trail and handling is also less.
Going down from 28mm tires to 25s, you'll be reducing the profile and effective radius by 3mm -- same as the width. This is close to the 4mm you dropped going from 32s to 28s. To predict the effects, think of the difference between 32s and 28s and effectively double it, so "more" of whatever you experienced in the last switch. In each case, going with the smaller tires results in less (lower) trail. If I can take the liberty of quoting myself (it is not ego, but laziness, as I don't want to re-type it)...
[A] LOW TRAIL [bike will have the following handling characteristics:]
Generally speaking, a low-trail bike will will require a deliberate effort to lean into a corner and to stand back up again on exiting corners. As speeds increase, the bike will tend to rise out of a corner on its own. At low speeds, the bike will tend to go straight and will do so with little rider input. This makes low-trail frames highly prized by fatigued randonneurs and by those who also tend to carry weighty handlebar bags low over the front wheel; low trail bikes don't need a lot of minding at speeds below 30mph/48kph and tolerate weight well in that location. Unfortunately, there's a downside -- at higher speeds, the stability goes away along with steering feel and there is a greater tendency to shimmy with or without a load.
That last, underlined part
may be behind Andy's warnings you referenced (I can't speak for him and don't have the Club Tour geometry to say so conclusively), but if so, any problems would tend to emerge at high(er) speeds rather than low(er). Unfortunately, without the specific geometry, I can't calculate the original and resultant pneumatically-affected trail for you, so I can only speak in generalities, Peter.
Again speaking in generalities (since I do not have specific figures to go by), I believe you would have better success running 25s F/R rather than running a 25 in front and a 28 in the rear, which would have the effect of reducing trail even further. If you wished to affect handling the least, then a 28 on the front and 25 in the rear (Sheldon Brown often advocated running the wider tire up-front for shock absorption) would tend to maximize the trail more than a 25/25 or even 28/28 combo. It'd make for a harsher ride in the rear, however, and the handling could be a bit unpredictable due to the difference in profile/aspect ratio and section width. I think you'd be happier with both tires the same size, but it would be fun to experiment by mix-matching (once more for the old rallying cry, "For Science!")
If it was me (not you, and my needs might differ from yours), I would probably stay with the 28/28 combo unless it has some decided disadvantage for you and your use. Remember, the narrower tires will require higher pressure and will ride harder, but won't necessarily roll any better;
if properly inflated for load and use, the wider, lower-pressure tire will have lower rolling resistance on anything but billiard table-smooth roads, though it sounds counter-intuitive. The narrower tires will have an aerodynamic advantage, tough it will be slight. The narrower tires will also be lighter and their reduced mass will make the wheels feel a bit livelier. If lighter-feeling, more responsive wheels are your goal, then it might me better met with lighter 28s and really light tubes. After all, the further weight is carried from the hub, the more it affects acceleration. If you do a lot of steady-state riding, you won't feel the difference as much. If you start and stop frequently or make explosive charges through the peloton then the difference is not only apparent, but important; it can offer a real advantage that offsets the increase in rolling resistance. I'd like to hear more about why you're leaning toward 25mm tires, Peter, if only for interest sake.
Though I've never owned a lovely Club Tour, I own a 1983/84 Centurion ProTour15 touring bike I use primarily for my randonneuring -- really long, reasonably fast day rides in mixed or mountainous terrain over 200-400km distance with a smallish load. It is very close in design, purpose, and intent to a Club Tour, but likely with lighter wheels. I built the wheels for it over 28,000 miles ago -- 15g, 3x DT unbutted spokes in 36-hole Phil Wood freewheel hubs and Mavic MA-2 rims. My all-time favorite tire was a Cheng Shin 917 (a 32mm tire with a 35mm tread cap), but I've toured with a load and offroad on 23mm and 25mm tires, have run 28s and am currently running Bontrager SelectK road slicks in 32mm width (close to 30mm mounted on the MA-2 rims). The bike has wonderfully sweet, neutral handling and did well with each tire size, but I am back to 32mm tires for general use including day rides for the reasons mentioned above. I find *I* (the engine) can go farther and more efficiently if I am subject to less road shock, and the difference between 25mm and 32mm tires is really apparent to me at the end of 17-24 hours in the saddle, especially if I am riding hard and spend very little time off the bike. Because I am more comfortable, I am more efficient, less tired, and actually go farther and faster than I do on skinny tires run at the higher pressures required to prevent pinch flats. Best of all, I don't ache all over and feel like I was hit by a truck afterwards. All this comes from someone who used to ride 22mm tires at 8.6bar/125psi; the workout came *before* the ride, when I hammered all that air into the tires!
I hope this helps. If nothing else, it is another perspective with some thoughts to consider in light of your own needs and experience, which you'll know best. No doubt, others on the Forum will weigh in with helpful information as well -- hopefully some that is firsthand with a Club Tour -- even better if it is the same size as yours, so the geometry would be identical.
Best,
Dan. (...who is never ehm, "tired' of the subject!)
