Natenat138

McBTC

If you have rim brakes they'll have to accommodate wheels with 3/4" smaller diameter when going to 650B...

tangerineowl

A 700x23 diameter is around the same as a 650b x 42. You'd be able to work out if a 42 fits taking some measurements within your frame.
I pushed the limits on my road bike using a tubed 700x32 for a while. Gave up in the end and purchased a frame with the clearance I was chasing.

dtheory

Bump...
I'll be doing some experimentation with a set of 650b wheels on a 2018 Roubaix soon..
will let you know how it goes.. I have a friend who has stated 38mm rear is the max with 650b
(42mm on the front)
Will verify that..

gus6464

The diameter might be the same but the width will not be. There is no way on earth a Roubaix will have enough clearance for a 42mm 650b tire. If you go with a smaller tire the frame is not designed for 650b so the BB drop will be too much.

McBTC

Good catch... there's a market for this-- I'd swap out my rear wheel and enjoy a 28mm tire tomorrow if all I had to do is spin on my current HG700 freewheel... currently, I like my bike a lot but cannot run a larger tire than 25 and the brakes are at the top of the calipers so, that's all it'd take to give me more options.

09box

I have a Fuji Sportif disc version and run 700x32 Ritchey Crossmax knobby tires with room to spare. They work great on gravel and singletrack.

Elvo

Roubaix sport has a high bb drop @ 76mm. A 650x38 tire has a diameter of 660mm, radius of 330mm. Your bb height will be 330mm - 76mm = 254mm = 10". 10.5" is typically the minimum for "gravel" to avoid pedal strike.

I would only switch if you can use 650x38 minimum

shoota 

I think I would skip all this and get a real gravel bike.

Caliper

It will depend if diameter or width is limiting your tire size. A 650b helps on the diameter side by using a smaller rim diameter, but if your chainstays only have the width for a 32mm tire then you'd still be stuck. This is a situation where you'll have to measure or experiment.

gus6464

I just can't see even a 650x38 fitting on a roubaix. I would cut my loses and get an actual gravel bike.

Mr_Crankypants

I'll join the chorus recommending that you look at purpose-built gravel bikes. I tried converting a 2018 Giant Defy to a gravel bike with 650b wheels and 38mm tires. It was great fun to ride, but when I went to swap my road wheels back in recently, I noticed a good deal of scratching in the fork and stays. I didn't do any real damage to the frame other than the paint job, but it was enough to convince me that a road bike is not an ideal gravel bike. I'm selling that bike now and planning to get a dedicated gravel bike. That way I'll have plenty of clearance for big tires, and can always put road tires on it if I want to. I also wanted a "Swiss Army Bike," but determined that it's a lot easier to go gravel -> road than road -> gravel.

dvn_85

Nice project mate!

gus6464

Huh? The Roubaix is an endurance road bike.

Hunterdog

Besides using tires as wide as possible (I think there is clearance for 32 mm tires on 2017 and 2018 models), what else could OP do?

gus6464

Get another bike.

shoota 

Agreed.

rosefarts

I rode 28mm on a road bike all summer. Went over more crazy washboards than you'd think a bike like that could do.

I got a CX bike with 38s and capacity for 45 in Sept. I was instantly blown away at just how much more capable it is off pavement. It's a big enough difference that I will be putting 25's back on the road bike and am only riding it for completely paved road.

yellow130

At the risk of bringing up an old thread,

I did just this. My 2017 Specialized Roubaix is now a gravel bike! Specialized and some LBS's will tell you that this voids the frame warranty, but as the 2nd owner, I'm not eligible for it anyways. Soooooo why not?

I ended up with the FSA FS Wheelset, and 1.35" Schwalbe G-one Allround tires. I also added the Specialized Hover Bars w/ Flare, for added knee clearance.
The bike shop put them together as a tubeless setup, and moved the 11sp Cogset and Rotors from the old wheels.

Don't even try using the 1.5" tires, as they're too wide for the rear. Even with the 1.35" tires, there's only 4mm side-side rear tire clearance on both sides... But since I'm not planning to blaze through rock gardens, blaze new trails, or hit awesome gaps, I'm not really worried about this.

I've maxed out the tires to 70 PSI, and the bike has taken on a new personality on the local gravel MUP, and is quick as it used to be on the paved road.

Pros:

• My wife is happy that my timeline to purchase a new mountain bike has been placed on hold.
• The squish.... Mmmm that squish is pure loveliness. I thought the free floating seat tube and future shock on the Roubaix were awesome... well now the whole bike is ever so much MORE awesome.

Cons:

• These tires can't hold enough air pressure to run on my rollers. Winter training in the garage has been put on indefinite hold until i can get new cogs and rotors.
• I need to purchase lightweight fenders to protect the bike from pebbles and water.

** Apologies, BF says I don't have enough posts to link to the products. maybe I'll update at a later date...

Natenat138

Actually, thanks for digging this up,. I've been meaning to respond with my results of this experiment, as I did finally take the plunge and grabbed a pair of 27.5 rims and 27.5 tires (their branding, not mine).


.Giant PTRX-1 27.5 wheelset (I don't recall the exact specs)

.160mm rotors, the stock model that comes with the original Roubaix wheelset

.11-32 Shimano Ultegra cassette.

.Maxxis 27.5 x 1.5 Ramblers


I ended up doing this sometime mid-summer so I had a little bit of time to test it a bit. First things first, the clearance on the rear wheel is UNREAL tight and is something I'll address soon. Front wheel is just fine for clearance but the rear is something else and I have to keep pressures on the lower side to keep it from rubbing the frame. I'll be buying much more narrow tires before I do any more extensive gravel riding.


I live in Central PA, primarily riding anything from fine, hard-packed gravel to the chunkiest stuff of fire roads in State Forests. While this was a great setup for the former, it got sketchy at times for the latter. Climbing and flat riding is fine but descending gets to be a little sketchy/squirrely. That could be a number of things, but on the road wheelset I feel incredibly confident in descending but as soon as the other set is on, I'm not laying off my brakes.


This is experiment absolutely spurred my love for gravel riding, so I'll likely be trying a more narrow tire and if that doesn't help, I'll be building a gravel bike from the frame up with this wheelset in its current iteration.

mkarikom

This is a cool project. I've often wondered how my Roubaix would fare over some of the fire roads in the hills around Southern California. Maybe I'll give this a try!

Takis

Cool.

Which fire roads do you ride? I am in Orange County.

chas58

Cool project, and not unlike what i did before I just bough a good gravel bike (Well, back when I started there really weren't any gravel bikes).

FYI, the steep head tube angle and short wheelbase make downhills (or any turn at speed on loose gravel) a bit sketchy. Other than that - its a matter of how stiff or compliant the ride is.

dpicare26

I also live in central PA, and an event that you may be familiar with, called UnPAved, has spurred my interest in getting into more gravel riding. I have an older Specialized Tricross comp (07) with cantilevers, that aren't great (i've read many reviews about this particular bike, fork, and brakeset). So i don;t feel comfortable on the descents. While I was reading about solutions to that problem, I figured in the mean time, I could just throw some knobbies on a seldom used roadie that I have.

Will have to look into clearances further.

insideimoutside

Try to trade for a Diverge locally or another gravel bike.