Bicycle Mechanics - Modifying Road Bike for Trails

A friend was given a Cannondale SR400 in like-new condition. I tuned it up for him and he told me he doesn't want to ride it on the road but use it on the crushed gravel bike trails near his home. Obviously, the 25mm road tires on the bike aren't going to cut it for that use.

According to the Sheldon Brown chart, I can go up to a 32mm tire on the rim but I think that's barely adequate. I have a wider-rimmed rear wheel I can give him if he's willing to buy a front wheel to match it so he can go up to a 35- or 38mm hybrid tire. Will I run into any fit problems with this approach?

In the alternative, if I just put a wider tire on his current rim, I'm concerned with finding a matching Presta tube. Is it an absolute must to match tube size to tire size or can I use something marked for a smaller tire?

if you use a smaller tube, there is a chance the rubber will be so thin that your chances of getting flats will be increased. Remeber the tube walls stretch to fit the inside of the tire, and the more stretch, the thinner the ending tubewalls will be.

Are you sure that the frame and brakes can accept a tyre that wide?
Many road bikes have quite narrow clearance and can barely take 28mm.

Road bikes that accept wider tyres are designed around the long-drop caliper brake with a reach of 57mm. Both the brake-bridge and the fork are dimensioned to provide extra clearance.

How rough are the gravel paths? The ones I'm familiar with are very rideable on a standard road bike. As far as the tube goes, I'd use the closest one you can find to the tire size, but they don't have to be an exact match. Lastly, cannondale may have info on that bike, regarding how large a tire it can fit.

Check out the cyclocross forum.
They are the experts.

Enjoy

You may want to shop around for some cyclocross tires. I'm not sure of the clearances on the SR400, but I used to run 30mm cross tires on a Trek road bike. Clearance was pretty tight on the chain stays and I had to deflate the tires to get them past the brakes. You should also get bigger tubes to go with the bigger tire volumn.

I used this bike for the same purpose and it worked great.

I ride my Peugeot UO-8 on trails, but it has nice wide old-fashioned 27" rims and ample frame clearance for 27 x 1-3/8" knobby tyres. I have tried 700Cx28 tyres on dirt roads, and I don't have the coordination required to do this safely on gravel or loose dirt.

Are you sure that the frame and brakes can accept a tyre that wide?
Many road bikes have quite narrow clearance and can barely take 28mm.

My guess would be that is exactly the limiting factor in this case.

How rough are the trails? Crushed limestone and dirt are fine with my Paselas , 28mm. I think they run big though. Here's the description:
http://www.nashbar.com/profile.cfm?category=121&subcategory=1085&brand=&sku=4285&storetype=&estoreid=&pagename=
Wire bead.

Good luck!

Im guessing that it is a Cannondale Silk Road 400, & according to Cannondale's website the brakes on the '00 are Tiagra, If you've got road brakes, Im quite sure you're limited to a 28 or 30 tire.

A friend was given a Cannondale SR400 in like-new condition. I tuned it up for him and he told me he doesn't want to ride it on the road but use it on the crushed gravel bike trails near his home. Obviously, the 25mm road tires on the bike aren't going to cut it for that use.

According to the Sheldon Brown chart, I can go up to a 32mm tire on the rim but I think that's barely adequate. I have a wider-rimmed rear wheel I can give him if he's willing to buy a front wheel to match it so he can go up to a 35- or 38mm hybrid tire. Will I run into any fit problems with this approach?

In the alternative, if I just put a wider tire on his current rim, I'm concerned with finding a matching Presta tube. Is it an absolute must to match tube size to tire size or can I use something marked for a smaller tire?


get a V-brake (dont know if a road bike can accept them) put on bike, get wide tires (cyclocross tires) maybe new rims, go ride.

Are you sure that the frame and brakes can accept a tyre that wide?
Many road bikes have quite narrow clearance and can barely take 28mm.

The bad news is that road bike brakes are usually the factor that limits how wide a tire you can use. 28mm is about it for most road brakes. You might be able to cheat a little bit on that if you wait to inflate the tire until after you install the wheel in the frame.

The good news is that I wouldn't worry excessively about riding a 28mm tire on a crushed limestone rail-trail conversion. My wife and I do it regularly with our tandem.

Road bikes aren't as fragile as they might appear, they can do gravel just fine. Might want to get some beefy tyres (continental gatorskins?) but otherwise it should be OK.

Thanks for the input. The reason I wanted to change out the tire is that my friend hasn't ridden a road frame before and is feeling a little shaky with the body position, drop bars and down tube shifters. I left the original 25's on and he's going to try it out and see how he likes it.

The trails are hard-packed dirt covered with crushed limestone. Right now, they're hardpan but later in the season, especially after they've been repaired, they tend to accumulate loose material in curves and at the bottom of hills and they can suck the wheel out from under you if you're not careful.

His SR400 is probably mid-90's. It's a blue fade paint job with RX100 comps. I couldn't find any specs on maximum tire size but the brakes don't seem to be long-reach.

I'm trying to encourage him to ride that bike on the road where it belongs. :D

If your friend is "shaky" on a road bike, he's going to have real problems on a crushed-limestone trail...even when it is in "hardpack" state (no way when loose stuff is about!).

He should learn how to handle the bike first and get some miles under his belt before trying to ride such a path. The pavement--which the bike is designed for--is indeed the best choice for him now.

However, once he has some experience, he can try that gravel "road". 700c x 25's, while NOT ideal, are enough for a reasonably skilled cyclist to ride on--as is usual with MTB's, the bigger (and lower pressure) tire will be more useful...and comfortable...on an unpaved surface.

He should avoid really "technical" off-road trails, though...the bike won't hold up well on those!

If he finds he'd rather ride "off-road", he should look for a MTB; it might be a good idea to add one to the "stable", anyhow. Choice is always good!!

If your friend is "shaky" on a road bike, he's going to have real problems on a crushed-limestone trail...even when it is in "hardpack" state (no way when loose stuff is about!).

He should learn how to handle the bike first and get some miles under his belt before trying to ride such a path. The pavement--which the bike is designed for--is indeed the best choice for him now.

However, once he has some experience, he can try that gravel "road". 700c x 25's, while NOT ideal, are enough for a reasonably skilled cyclist to ride on--as is usual with MTB's, the bigger (and lower pressure) tire will be more useful...and comfortable...on an unpaved surface.

He should avoid really "technical" off-road trails, though...the bike won't hold up well on those!

If he finds he'd rather ride "off-road", he should look for a MTB; it might be a good idea to add one to the "stable", anyhow. Choice is always good!!

I couldn't agree with you more. He already has a hybrid that he's been riding for years, but he likes this bike because he sees it as "light and fast." I tried to explain that it's like that because it's intended for road use but he's just not interested.

I did suggest that he practice in a parking lot for a while before he takes it on the bike trail and he thought that was a good idea. I took my road bike on those trails once and I'll never do it again -- I upgraded a hybrid with flat bars, clipless pedals, a decent saddle and sort of cyclocross tires and it's perfect for the non-technical trail riding I do.

I couldn't agree with you more. He already has a hybrid that he's been riding for years, but he likes this bike because he sees it as "light and fast." I tried to explain that it's like that because it's intended for road use but he's just not interested.

I did suggest that he practice in a parking lot for a while before he takes it on the bike trail and he thought that was a good idea. I took my road bike on those trails once and I'll never do it again -- I upgraded a hybrid with flat bars, clipless pedals, a decent saddle and sort of cyclocross tires and it's perfect for the non-technical trail riding I do.

I hope he listens to you, slowpedal53: He's got a good friend in YOU!

It sounds like your "upgraded" hybrid is a do-it-yourself "Sirrus"! :lol:

BTW, I've ridden fireroads with my "Sequoia" (had 700c x 26 slicks on it at the time!) and my Trek 520...so I speak from experience that "road" bikes can be good trail bikes...on the RIGHT trail!

Your trail sounds like one, though, that needs--despite surface appearances--a "real" MTB. :D