Why can't I do this? 21" tire ?
 

New tire time again and I'm looking for something "different", no reason just bored with stock sized Dunlop, Metz & Avon.

I've read that it is not recomended to put a 120/70-21 on a stock width rim. My question is why not and what bad things can happen if one does it?


Anyone have knowledge about these two tires:

Bridgestone:
http://www.jpcycles.com/product/ZZ25167
Kenda
http://www.jpcycles.com/product/210-0235

Or any other suggestions welcomed, and Thanks,

Doug

A 120 -70- 21 Tire usually has a recommended rim width of 3.50".The minimum recommended width is 2.75 the max is 3.75". Your stock rim's width is most likely 2.15 ".The tire just wont fit properly.Hit a big pot hole or bump.The tire has a good chance of coming off the rim.Not good.This would be my guess.

Mounting a tire to a radically different rim width flexes the sidewall unnaturally. It reduces your load capacity and greatly increases your chance of sudden air loss when hitting a road defect... especially if running tubeless. It also affects the OD of your tire. In your example (wide tire on skinny rim), it pinches the tire in at the bead and makes the sidewall taller, thus increasing your OD. A skinny tire on a too wide rim would have the opposite affect. That said, it can be done and probably will hold air, but you're taking a chance. Look at it this way... tire manufacturers are in the business of selling tires, not giving you reasons not to buy their product. If a tire/wheel combination were safe, do you really think they would recommend against it? I don't. I think they are trying to avoid lawsuits.

On your FXSTC, you can go +1 tire size (to 90/90-21) without any problem at all. That's what I'm running on my FXSTB.

Good answer from both of you!! I fully agree. However, i saw the 90/100x21 kenda on a fxst awhile back. Filled the fender well and looked reasonably safe (?). Started to buy one myself but thought better of it and bought the 90mh dunlop 407. I was able to put a wrap around fender and adjust to cover the tire well. As matter of fact it looks just like your fender Mr. lucky. That's just me...

I didn't know anyone made that size, but yeah, it would be perfectly okay too. The first number is the width, and that's the important measurement. The second number is the aspect ratio, so that 90/100-21 is 100% as tall (sidewall measurement) as it is wide. FWIW, the stock FXSTx MH90-21 is the same as an 80/90-21.

Despite everything I've said, I know and ride with two guys that have done exactly what the OP proposes, and have never had a sudden air loss or been left stranded with a tire related problem. One of them has bent two front rims though, but he is lowered front and rear and was riding 2-up both times. As Clint would say, "Do you feel lucky?"

Thanks for the posts..

I was getting ready to order the 90/100-21 Kenda until I went to their site and is said: NOT for high load carrying capacity motorcycles....Can't find any actual weight information for the tire so I assume a Harley is goona be too heavy... It's a shame, nice size and I can get it delivered for about 60, probably another case of do you feel lucky....

http://www.kendausa.com/en/home/moto...allenger-.aspx

Any body know for sure?

D

I don't know the answer to your question, but I just plugged in your stock size and the Kenda 90/100-21 into the tire calculator, and it came out to a 1.35" increase in OD. Actually, I had to enter the size as 90/99-21 because the calculator won't accept a 3-digit aspect#. Half of 1.35" is .675". I know that the stock fender has a big gap to the tire, but do you have that much room to spare (also allowing for tire growth when it gets hot, and still having a small gap)?

Mr. lucky,

That's a good thought and It would be close. Hard to measure since the tire on there now is worn. If I were to give the 120/70 a try it's impossible to figure how tall that narrow rim will make the sidewall I've been thnking about one of these chrome fenders that I could modify to fit, shouldn't look too bad since the lowers are chromed. I don't want to mess with the stock fender and getting another painted to match would be more than I want to spend.

http://cgi.ebay.com/ebaymotors/CHROM...item20b5431f7e

Thanks
D