21” Tube on an 18” Wheel – Can it be done?

[OD Cleaver]

YES.

I have seen this question in more than one tire thread, but it received no response. The asker appears to be thinking of carrying only the front tube as his spare.

I took a used 21” standard thickness front tube and put it on my 18” rear wheel. I ran it this way for ~50 miles at speeds up to 60 mph. So, based on this one case, it is possible to do this, but probably not recommended.

This experiment was done by mistake. My spare 21” tube was neatly folded on my workbench. The new 18” tube for my new rear tire was neatly folded on the shelf. I grabbed the wrong tube by mistake. Later when cleaning up the garage I found my mistake and corrected it.

[Chas_M]

Many riders carry just a single spare 21" tube for trail-side tire repairs, and find that it works just fine in an 18" rear tire with no problems at all. Some say that it should be replaced with a proper 18" tube ASAP, however it won't hurt anything to leave the 21" tube in place until the next rear tire change.

[npm]

Yes it can be done. Many years ago I was riding the Trask two day qualifier and got a flat on the back tire on the first day. I put the 21" tube in from my tool kit (changing tires on the trail is a real pain) and rode the rest of the event.

[ptgarcia]

I'm one of those people that only carry a 21" spare tube on my rides. While not ideal it will get you where you need to go.

[Noble]

Very common as an emergency measure. BUT it can blow out suddenly. Does not fall into the category as "no problem at all". If you do it, change it ASAP. One blowout at speed and you will not do it again.

[MandoThumper]

+1

I only carry the 21" spare and figured it would be good enough to get me back to the truck. I would definitely replace it with an 18" once back to camp, or home.

[gonefishing]

Myself, I carry both tubes and leave them at the truck. Yes we go on long rides in technical terrain and we will carry extra fuel but I have never seen the need to carry a tube and fix a tire on the trail. Thats not to say that I haven't seen a flat tire while out on the trails - bead locks are a wonderful thing. As a side note in the 06 Vegas to Reno race one of our teams bikes got a flat rear tire, the rider had some large zip ties and wrapped them around the tire and rim in about 6 places and still road 40 miles to the next check point.