So why the small tire?

Started by djsather, Jun 25 2009 07:20 AM
12 replies to this topic

  • djsather

Posted 25 June 2009 - 07:20 AM

#1


i'm just curious as to why most off-road bikes, i.e. DS, enduro, trail bikes... all have a 18" wheel out back and motocross bikes have a bigger 19" wheel in the rear.

why wouldn't they have the same size wheel? i'm sure there's a reason for it i just don't know what it is.

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  • XR650L_Dave

Posted 25 June 2009 - 07:50 AM

#2

Since MX bikes don't have to handle 'real world' obtsacles, and since MX bikes have higher loading on the tires while turning, MX tires benefit from having a shorter rim-to-knob-tip profile (less tire deflection), while general off-road use benefits more from a squishier tire that will conform to obstacles (roots, rocks, that guy that won't let you by during the hare-scrambles and you have to knock him down in the mud section and use him for traction).


Dave

  • djsather

Posted 25 June 2009 - 07:57 AM

#3

ah, that makes perfect sense. thanks.

oh, and if we ever end up in the same hare scramble, remind me to let you by before we hit the mud section.

  • XR650L_Dave

Posted 25 June 2009 - 08:15 AM

#4

djsather said:

ah, that makes perfect sense. thanks.

oh, and if we ever end up in the same hare scramble, remind me to let you by before we hit the mud section.


I just use the horn and turnsigs...


Dave

  • rumplefester

Posted 25 June 2009 - 04:10 PM

#5

XR650L_Dave said:

Since MX bikes don't have to handle 'real world' obtsacles, and since MX bikes have higher loading on the tires while turning, MX tires benefit from having a shorter rim-to-knob-tip profile (less tire deflection), while general off-road use benefits more from a squishier tire that will conform to obstacles (roots, rocks, that guy that won't let you by during the hare-scrambles and you have to knock him down in the mud section and use him for traction).


Dave

OK, that explains the tire profile, however if your sag is set up correctly, the rim/tire profile is irrelevant IMO. I still don't understand why the rim size difference.

Just trying to learn...

- KeVin

  • mx730md

Posted 25 June 2009 - 04:17 PM

#6

rumplefester said:

OK, that explains the tire profile, however if your sag is set up correctly, the rim/tire profile is irrelevant IMO. I still don't understand why the rim size difference.

Just trying to learn...

- KeVin
huh read what he said it 100% true. both 19 and 18 tires are just about the same overall size but the 18in tire has a bit more side wall so the tire has more meat to take the shape of the terrain also they are a little bit more resistance to pinch flats. and you get a wide verity of offroad tires with the 18in

Quote

conform to obstacles (roots, rocks,...)

  • djsather

Posted 26 June 2009 - 04:35 AM

#7

rumplefester said:

OK, that explains the tire profile, however if your sag is set up correctly, the rim/tire profile is irrelevant IMO. I still don't understand why the rim size difference.

Just trying to learn...

- KeVin


what he's saying if i understand correctly is that the outside diameter of the two tires are the same. so the 18'' rim gives that tire a deeper sidewall, which in turn allows for more sidewall flex.

it is like putting low profile tires on a car. you will hook up better and corner a little better, but you will feel bumps much easier.

at leaste that's how i understand it.

  • XR650L_Dave

Posted 26 June 2009 - 07:31 AM

#8

Yah, I should have pointed out for the most part the 18 inch and 19 inch tires are about the same outside diameter overall.

It's my guess above, anyway.

Dave

  • Ruffus

Posted 26 June 2009 - 08:03 AM

#9

Just a guess, but having more rim & less tire makes for a lighter package for quicker acceleration & braking, along with less unsprung weight that'll help the suspension react quicker.

  • dirtmill

Posted 13 September 2009 - 03:42 PM

#10

I have a KTM 300. I recently borrowed a buddy's 19" rear rim and tire. We went to a friends farm were we have an enduro loop complete with logs, rocks, hills, and single track. the parts I had difficulty with when I had my 18" rim mounted I found to be easier with the 19" rim. I measured the diameter of the tire, and the 19" is 1.25" larger than the 18". Is there any technical info to explain this, or was i just having a good day?

  • brentktm

Posted 13 September 2009 - 04:11 PM

#11

was it the same tire just diffent size? The heigh would be the same if it was the same like 100/110-18 or 100/110-19

  • ptgarcia

Posted 13 September 2009 - 07:42 PM

#12

brentktm said:

was it the same tire just diffent size? The heigh would be the same if it was the same like 100/110-18 or 100/110-19



Actually, the 100/110-19 would be 1" larger in overall diameter than the 100/110-18.

  • fender bender 59

Posted 13 September 2009 - 07:48 PM

#13

djsather said:

i'm just curious as to why most off-road bikes, i.e. DS, enduro, trail bikes... all have a 18" wheel out back and motocross bikes have a bigger 19" wheel in the rear.

why wouldn't they have the same size wheel? i'm sure there's a reason for it i just don't know what it is.

Have you ever put a 18" and a 19" tire on the rim back to back????
they are the same hight. the 18" has more sidewall which gives you more room to hit those rock and such without pinching the tube.



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