I got a used Mongoose Excursion MTB, and it came with a 26x2.10 front tire and a 27.5 rear tire. Is this suitable? Will it ride normally? And what's the reason for different sizes?
1Don't trust the labels - try measuring the rims with a tape measure, and you should get an idea if each is closer to 584mm or 559mm across. A photo might help too.– Criggie ♦Feb 24 at 7:55
Also, a quick web search suggests the Mongoose Excursion is a 24" wheel when sold originally, but PO may have done some frankensteining.– Criggie ♦Feb 24 at 7:56
2@Criggie: I can find it with all kinds of wheel sizes on the internet. 24", 26", 27.5", 29". Seems to be some generic Walmart BSO.– MichaelFeb 24 at 8:09
1The first rule of tire sizes is that inch sizes do not represent any physical dimension of the tire or rim– ojsFeb 24 at 12:47
2It would really really help if you made a photo of your whole bike and then the detail of each tyre.– Vladimir F Героям славаFeb 24 at 15:01
Riding with different sized wheels in the front and rear
There's nothing wrong with this at all. Usually, people who ride different size wheels will put a larger wheel on the front to help roll over bumps. The only potential problem with a bigger wheel in the rear is that it'll shift your weight over the handlebars a bit, but it probably won't even be noticeable at the small size difference that you appear to have. You might have problems if you did something really weird like putting a child's tricycle wheel on the front with an adult-sized wheel in the back, but I wouldn't worry about it.
Reading your tire sizes
As has been indicated in the comments, you can't really trust the labels on mountain bike tires. The cycling industry has used a number of different sizing schemes over the years and no one really uses the ISO standard sizing scheme very well (which is the only real standard that we have).
You might see some numbers in smaller print on the tires that represent the ISO standard size, which is also the rim size measured in millimeters, probably in the 500-600 range. If it's there, it's probably the truest measure of your tire and rim size.
Typically a 26" tire mounts on a 559mm rim. 27.5" is used relatively rarely, but typically mounts on a 584mm rim. There are definitely exceptions to the 26" tire sizing and probably exceptions to the 27.5" tire sizing.
Your best bet
If you really want to know what you're riding and whether or not everything is compatible, measure the rims with a tape measure. There's a pretty good chance that one or both of your rims is either 559mm or 584mm. You won't be able to get a perfect measurement, but you'll be able to get close enough to say it's one or the other. If something seems way off, consult Sheldon Brown's tire sizing chart. It's the most definitive guide to tire sizing that you'll find.
Tire sizing is unnecessarily complicated. If you're still unsure after reading through these answers, consult with your local bike shop. They'll be able to tell you exactly what you have and what you need.
I don’t think you could mount a 584mm (27.5") tyre on a 559mm (26") rim and have it stay on. If you somehow managed to mount and inflate it, it sounds dangerous and could fail catastrophically at any moment.
Are the rims a different diameter as well?
Mullet mountain bikes (different wheel diameters in the front and rear) are a thing, but usually you’d have the larger wheel in the front.
This is a very unusual setup. There have been a few MTBs over the years that have used a bigger wheel in front, eg 29/27.5, 26/24, or even 26/20. I've never heard of a mountain bike set up with a bigger wheel in back. Looking this bike up online, it looks like it is sold with 24", 27.5", or 29" wheels front and rear, and the 24" and 29" are sold with disc-brake front, v-brake rear; the 27.5" is sold with v-brakes front and rear.
You said you bought this used. I can imagine that it originally had 27.5" wheels front and rear, the previous owner lost or wrecked the front wheel, and replaced it with a 26". Do the front brakes work? If not, I would be cautious with this bike, since your front brakes do most of the work (and who knows what else is wrong with it). Handling also will not be as intended.
As to the different tire widths, that's not a huge deal as long as everything fits. People have chosen different widths intentionally, but if my hypothesis is correct that someone replaced the front wheel with whatever was available, that doesn't apply here.
1No the front brakes don't work. The linear pull brake pads don't line up correctly to the rim as they're supposed to no matter how I adjust the pads. I'm guessing it's due to incorrect tire and rim size. The frame is meant to have 27.5 wheels so the brake mounts are relative to that size. It doesn't feel sturdy at all when riding. Very bouncy shaky and feels like it's going to fall apart lol! It was a very cheap bike. Thinking garbage can is best place for it..– AlisonFeb 25 at 8:42
2I suspected as much. I hate to hear you wound up with a lemon. You could replace the front wheel with one that's the correct size, but you might wind up spending more on the wheel than the bike is worth. Feb 25 at 14:09
It's a shame you're missing a 27.5" wheel, because 26" rim brake wheels can be obtained much more easily2nd hand– Chris HFeb 27 at 21:20
Is this suitable?
Will it ride normally?
If the tires are staying on the rim with full air pressure (correct air pressure should be indicated on the side of the tire) and holding air then it's OK.
The bike should ride normally.
what's the reason for different sizes?
There are cases where a bicycle maker will put different size wheels on a bike and claim a performance benefit. All the mountain bikes I've seen with different size wheels put a 24 inch wheel on the back and a 26 inch wheel on the front*.
I've never seen a bike maker put a 26 inch wheel on the front and a 27.5 inch wheel on the back. It's probably something a previous owner did just because a 27.5 tire on hand at the time. Or maybe a previous owner trashed the original wheel and replaced it with a different one of slightly different size.
EDIT: And, I hear from reputable sources that some bike makers have recently used other combinations of wheel sizes. All of them larger in front than in back.
I was able to find a Walmart listing for the Mongoose Excursion. One version of the bike came with 26 inch wheels and another version that came with 27.5
Nothing was listed with mixed wheel sizes.
It is possible that the factory ran out of one wheel size and substituted the other wheel size just to keep the line moving.
226 front and 24 back? you are about 10 years late with that statement. modern mullet bikes use 29 front and 27.5 back, while some freeriders like at rebull rampage prefer 27.5 front and 26 in the back Feb 24 at 15:54
1@reciprocallettuce I have edited my answer to include your information.– David DFeb 24 at 19:49