I saw something the other day that made my mind go WTF.
I stopped at the grocery store, I noticed this bike locked to the bike rack. I was admiring it and then I noticed that the front tire had a Presta valve, and that the rear tire had a Schrader valve.
I have never seen that before, I can't fathom why anyone would do that. As an aside there wasn't an airpump on the bike either.
Any thoughts?

I''d guess as a contingency. Maybe the owner got a flat and used a spare wheel so he or she could make it to the store.

I'm not 1 million percent sure, but I figure you could probably get away with fitting a presta valve in a rim with a hole for a schrader valve. I know that if you just did that and nothing else you'd get a flat pretty quickly, but there's probably some simple hack to make it work.

As to why one would do that... Lazyness or necessity are most likely. They got a flat and only had a presta tube available...

I confess that my daily summer commuter has a 700c tire with a presta valve. Rear tire is a 27 inch Shraeder. Reason is that I just built up the front wheel and am test riding it. Problem is, I liked it so much, I didn't take it off. Currently, I seem to have more wheels than bikes... not a bad situation...

One thing I love about my old Fuji commuter's down tube friction shifter is that I can put any wheel on the back, from a 126mm 5 speed to a 130mm 9 speed and it shifts perfectly.

Great for checking wheel builds.

You either walk home, or you install the tube you have.

Ha ha, that's what I've got right now. I got a flat and the only spare available was a presta. I was planning to get rid of the Shraeders eventually anyway. My pump handles both - neither very well, though - so it's not a problem at the moment.

Sure, you can put a grommet in the rim to convert the Presta hole to Schrader (there are both metal and rubber options). But I disagree about necessarily getting a flat quickly if you didn't do that. When I got a new rim for our tandem years ago I assumed the hole was sized for Presta since that's what the LBS told me. Used it for over a year without any problems, but then I decided to convert it to Schrader so it would match one of our other bikes. Well, it turned out that no modification was needed - the Schrader valve fit ok in the rim already and I'd been using Presta valve tubes for the last year just thinking the valve hole was a bit oversize. It was a very snug fit with Schrader, but that was clearly the design goal once I examined it closely.

And I've ridden bikes with a mix of Presta and Schrader a number of times - usually because there's a problem with one of the wheels/tires and I grab one off another bike temporarily. And my Bike Friday came with Presta valves on the bike and Schrader valves on the trailer so I need to carry a pump that converts easily.

Ah I see. I did that on a friends bike (put a presta valve in a schrader rim), and he got a flat after about 20 miles, right at the valve. Somehow I didn't even think about the presta vs schrader, but once he got the flat I assumed that was the reason. Maybe his valve hole wasn't as snug a fit...

Over the years I have slowly moved most of the family bikes to presta, but there always seems to be one around with at least one schrader. As a tube becomes useless, I will change it over. Since pumps, both frame and floor, that handle both valve types are easy to come by, why would I throw away a perfectly good tube just to make the valves match?

Heh
I have a commuter that has both. Schrader on front and Presta on the rear.
The origional rear wheel died and the cheapest replacement I could find was drilled for Presta...I just haven't gotten around to either drilling it out or getting a grommet for the front wheel. (I keep hearing about the grommet option but haven't found out where to get one. Any suggestions?)

I have both valves on my bike. I have a cheap wheel on the rear (it actually came off the imfamous Denali), it is drilled for, and has, a schrader-valve tube, it isn't even quick release, so I carry a little wrench in my supply bag. My front wheel is a basic Alex rim with quick release, made for presta-valved tubes. My floor-pump and frame pump both have dual-heads (two different holes to accept either valve), when I move from one valve to the other I don't have to fiddle with the head (unscrew or turn something over, etc.). I do carry a presta-to-shrader adapter, just in case air from a gas-station might become an option of choice, but I've never had a reason to use it.

I came to this set-up months ago because it was my best option at the moment and have stayed with it because there isn't a compelling reason to change it. I do see a slight potential problem if I have a flat on the schrader tube, can't repair it on the road with my repair kit, and am left with a presta tube (I carry a spare presta tube with me) as my only next option, but I could probably get home with a presta tube (the low PSI from the mini-pump would probably be a good thing in this case), and there would be no issue if I got the widget to make the stem-hole adaptation.

As a side note I think using both valves on the same bike gives you greater insight when the schrader-verses-presta debates break out.

The guy probably had a flat and used whatever tube he had laying around. Not everyone obsesses over valve stems. It won't hurt anything to run a presta stem in a schrader rim.

I don't obsess about pumps either. Maybe he lived close enough to just walk if something went wrong.

I have both on my Miyata. The bike came in(ebay sale) with a 27" rear wheel and a 700c front(both Araya rims). It's an '81, so I assume there was a problem with the front rim at some time, and a replacement was ordered in 700c and was drilled for a presta valve. I just carry an adapter for the presta in my under seat bag. Besides, the 27" rear is a bit taller than the 700c front rim, so I get the boost of always pedalling downhill!

I did the same thing once. I got a flat on my mountain bike and I had to get a new tube. I wanted to get a Schrader valve, but the shop was out of tubes in that size, so I got the presta valve one. I have a pump that can pump both types of tubes so it's no problem for me.

I've had that setup on quite a few occasions. 1 mile walk to the schraeder tube store, 10 mile walk to the presta tube store? Tube emergency on a Sunday?

I drill out all the presta MTB rims I get so I can fit them with schraeder in a pinch.

Oh, I thought I'd add that when I'm in Portland or downtown Vancouver, there's NEVER a pump on my bike when it's locked up outside. Sometimes I'll get lazy and leave my pump on the bike up here in the land of milk and honey, however.

He had a flat near WalMart.

The one here only carries 700C in Presta. Everything else comes in Schrader, and they don't carry anything to air up (or check pressure) a Presta with, but luckily I had an adapter with me. One of these days, I have to remember to make the 30mi drive to the "L"BS for a couple more Schrader spares, since I know I'll lose the adapter right around the time I have another flat.

I did have that for a while, when I replaced a wheel with a factory built wheel, and I still had some presta inner tubes sitting around that I figured I might as well use up. After about a couple of years when I'd patched them and got sick of flipping my pump, I finally drilled the rim, tossed the remaining Presta tube on the "scrap rubber" pile and put a schrader tube in there.

Even the cheapo Bell pump that I looked at at a WalMart a couple of years ago had a flippable pump nozzle. It's not always obvious but so far I haven't found even a $10 cheapass pump that doesn't flip.

I run schrader because I don't see any reason not to given that I run mid-width rims and 32mm tires, and I have an air compressor in my garage. With Schrader valves I can top off all 3 cars, all the bikes and my Airzound in about 5 minutes without messing with anything.

I'm pretty sure every bike I've ever owned came originally with Schrader valves (I've never paid more than $300 new for a bike so far).

One of my bikes has a presta rim and a schrader rim. I did go the extra mile and got a grommet to put into the schrader rim so I could run 2 presta tubes. But I could have as easily had mixed tubes to go w/ the mixed rims.

I have a coworker who runs presta tubes in schrader rims w/ no ill effects (yet), and another who had a big explosion when the base of a presta tube pushed through the hole for a schrader rim.

I run presta on a MTB with schrader drilled rims, no flat problems. I can see why there might be, but I think it is unlikely unless you have burrs or sharp edges on the valve hole (in which case, you should fix them regardless of your valve type).

My old road bike (27" wheels) has schrader valves on it, but I usually carry a presta tube. Makes it easier to help someone else out if I see a flat on a road (or if one of my friends is unprepared). I have not had a flat in a very long time on that bike, so I still have matching valves, but I imagine I will end up with mismatched valves eventually.

I have a Schwinn pump that only does Schrader (from Walmart nonetheless) >.<
It came with one of those adapter things - which was promptly lost.

I put a topeak 'smarthead' on the floor pump at home so it doesn't matter. The 1st one I got didn't work but they sent me a replacement asap and all is well.

https://www.topeak.com/products/Floor...HeadUpgradeKit

I have that pump too. Both my bikes have presta valves at present. I guard that adapter thing carefully

I just want to give a thumbs-up to the above idea. Topeak's smarthead will fit any reasonable pump. I replaced a Specialized-brand pump's head with Topeak's Smarthead. In doing so, I retained most of the original hose, using the Topeak-supplied adapter to simply add their hose and head to what I already had. The resulting pump is now my favorite.

The job took all of five minutes. Snip the Specialized head and bin it. Press the hose ends onto the adapter thingy. Tighten a hose clamp. Done.

who cares? i do this all the time, sometimes with a rubber grommet, sometimes without. I'd rather ride my bike than worry about what kind of valve stems it has. I also don't carry a pump, i carry a small flat kit with a few CO2 cartridges, a presta/schrader adapter, and a patch kit of a few sizes. I've only had to walk home (closer than the nearest LBS) once, knock on wood.