Oversized tyres and teaching my son to measure!

Question

So my son has been trying to customize his bike and seems to have never heard of a tape measure!! I need some help to get him on the road! He has purchased 2 oversized tyres with a size 26 x 2.50. The exsisting tyres are 27.5 x 2.10 so will these tyres fit onto that size rim?
The tyres that are currently on the rims are a brand called kenda and seem to be stuck to the rim somehow, I cant seem to take them off the rim? If anyone can help me I would be so grateful cause at the moment I'm getting grief from all angles and it's not even my fault!!!!

Adam Rice · Answer

Regarding diameter: You can't put 26" tires on a 27.5" rim, which it sounds like your son wants to do. I can guarantee with absolute certainty that the new tires will not work because they're the wrong diameter.

Regarding width: We don't know the width of the rims on your son's bike.
Also, rims can handle tires across a range of widths. It is possible that the 2.1" tires on the rims now are at the outer limit of the rims' range, but we have no way of knowing. I'd look at the rim and look up the manufacturer's guidelines online.
Regarding removing the existing tires: do you have tire levers? There are numerous videos showing how to remove a tire. Here's one.

Argenti Apparatus · Answer

Regarding wheel rim diameter compatibility:
On mountain bikes, wheel and tire diameters are typically referred to in inch sizes: 26", 27.5" and 29". Those measurements are nominally the diameter of the inflated tire mounted on the rim. However, what they really mean is the tires fit wheel rim bead-seat diameters of 559mm, 584mm and 622mm respectively.
Regarding tire width compatibility:
Unlike car tires bicycle tires can fit on a range of rim widths. More critically, the tires have to fit between the fork and rear triangle frame tubes. There is an existing good question and set of answers that addresses both these issues. What is the maximum or minimum tire width I can fit on my bicycle.
Sometimes tires are marked with diameter and width in inches, sometimes either the standardized 'ETRTO' marking which gives bead-seat diameter and width in millimetres. Often you'll see both markings.
Don't bother taking the existing tires off the rims, the new ones will not fit.
The tire beads do form a pretty tight seal against the rim bead seats and can be a pain to dislodge. Often the tire can be pushed off with pressure from thumbs, sometimes a tire lever is needed. You also need to know if the tires have tubes or are set up 'tubeless' in which case they will have a quantity of liquid sealant inside. Guides for changing both types of tires are easy to find on YouTube.

Oversized tyres and teaching my son to measure!

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