Trek_geek

The 520 comes with 700x32. Wonder how wide I could go with if I plan on more dirt roads and hard packed rail-trails (like the GAP and C&O together). I've ridden with my pressure down at 60 psi with the 32s on crushed lime stone rail-trail but didn't have a load.

Trek_geek

Looking on Sheldon Brown bicycle page and researching my Bontrager 622x18 rims I found a chart that
17mm rim - tire widths of 25 28 32 35 37
19mm rim - tire widths of 28 32 35 37 40 44
So it looks like I could go all the way to 700x42s if they would fit under my Planet Bike Fenders
Might be better to try 38s first.

Jim Kukula

The tire width to rim width relationship seems to be shifting territory:

https://www.schwalbetires.com/tech_in...dimensions#rim

One important thing with fat tires on narrow rims is that you really have to keep the pressure down. I have 50mm tires on 19mm rims but I only put about 50 psi in them. A fat tire is basically pulling narrow rims apart and with enough pressure you can actually split the rim.

Barrettscv 

Just remember that Sheldon Brown uses inside rim width, not the more common outside dimension.

Jim Kukula

Yeah the outer width of my rims is 25mm, it's the inner width that is 19mm.

https://www.komponentix.de/onlineshop...dra_301629.htm

Definitely a crucial distinction!

martianone

ride on dirt roads a lot - have 37 mm Marathons on my 520, they fit fine and are comfortable at 60 psi (probably depends upon your load). not sure a wider tire would fit and allow for stone clearance.

Trek_geek

So the designator 622x18, is the 18 the inside dimension?

Barrettscv 

In this case, yes.

Most road bikes in the showroom have 17mm inside and 19mm outside. Most touring bikes and hybrids have 18-21mm inside and 20-24mm outside.

rumrunn6

isn't all that width gonna slow you down?

Jim Kukula

On a very smooth road, or a track, it's most efficient to run a skinny tire at high pressure. But as the surface gets rougher, it is actually more efficient to get some suspension from lower tire pressure. Wider tires are actually more efficient at a given pressure, but they can't handle the higher pressures that skinny tires can. But once you want suspension & therefore lower pressure, wider tires are what you want. The suspension keeps the bike and rider and any luggage from bouncing around and absorbing energy. Of course you can use arms and legs to provide suspension too and still have high pressure tires. But that gets to be a lot of work, i.e. not so efficient.

cycle_maven

The other consideration is how wide the space between your chainstays is- I like about 5 mm on each side of the tire of clearance, minimum, so a small wobble in the rim or tire won't cause the tire to rub on the chainstay.

tarwheel 

I rode on the C&O with 700x32 cross tires (Ritchey Cross Max Pro) and felt that they were perfectly suited for the terrain and surface. If your frame will fit wider tires, go for it but I don't think 32s are too narrow for the C&O or the GAP. More important, IMHO, is having some tread in case the trail gets wet or muddy.

valygrl

I rode the C&O fully loaded with 28s.

I've had Conti TravelContact 35s on my 2006 Trek 520 with fenders.

Hope those random data points help!

Thulsadoom

I had a set of Schwalbe Marathon Plus 32s on my 520 and they just barely fit with fenders, but those tires are big. I don't know if I'd want to try and go bigger than 35s in any tire though, if you are running fenders.

seeker333

Generally there is a 4-5mm difference between inside and outside width, since the rim walls are >2mm thick.

Measure a few rims, road or mtb, you'll see this is true.

trek2

I wonder if the amount of tire clearance depends on the age of the 520? Mine was a 2004, and Pasela 35s barely fit under the fenders. I would occasionally have problems with gravel getting caught on the rear.

Trek_geek

Thanks for all your insight and experience. I am going to stick with the Bontrager 700x32 that came with the bike when I ride the High Bridge Rail Trail later this fall. It's a 32 mile crushed stone surface. I'll just lower the tire pressure a bit to start and see how that works. It will be an out and back overnight with a friend so I won't to loaded to heavy.