Why is the front wheel larger than the rear


15 replies to this topic
  • zwr250f

Posted June 13, 2005 - 11:29 AM

#1

A friend of mine asked me that the other day, and I really couldn't come up with a good answer. Who knows?

Is it all geometry, making the bike set level, if so why not long fork tubes to offset the difference-say between the 21" and a 19" or 18" front wheel?


Dumb question? I don't know? Anyone?

  • XR650L_Dave

Posted June 13, 2005 - 02:41 PM

#2

I think designers just went with what naturally led to more stability.
Also, the front wheel will roll over large obstacles (as opposed to crashing in to them). The back tire is being driven, so it climbs obstacles well even though its smaller.

Aren't there some street bikes out there with a smaller front than rear, to quicken up the steering?

Dave

  • zookeeper

Posted June 13, 2005 - 02:41 PM

#3

That's not a dumb question at all. I thought about it and really can't think of too many reasons why it would be. Street bikes traditionally have the same size (17" these days) although some bikes have had some weird combos, like 18" rear and 16" front, but they seem to handle fine. If I had to guess, I'd say the rear is smaller to keep seat height down with today's long travel suspension, but that doesn't make to much sense, because I remember bikes in the 70's having larger fronts, with virtually no suspension! I'd sure like to hear the answer, too.

  • zookeeper

Posted June 13, 2005 - 02:45 PM

#4

I think designers just went with what naturally led to more stability.
Also, the front wheel will roll over large obstacles (as opposed to crashing in to them). The back tire is being driven, so it climbs obstacles well even though its smaller.

Aren't there some street bikes out there with a smaller front than rear, to quicken up the steering?

Dave

I think roadracing bikes use a smaller front to make more room, since street bikes typically have the motor moved as far forward as the chassis will allow. Ever wheelie an older sportbike? They took forever to come up on their long wheelbase, then the balance point was near-vertical due to all the wieght up front. By the time it got vertical, you couldn't see where you were going, lots of fun...

  • Crf250ed

Posted June 13, 2005 - 03:07 PM

#5

I think designers just went with what naturally led to more stability.
Also, the front wheel will roll over large obstacles (as opposed to crashing in to them). The back tire is being driven, so it climbs obstacles well even though its smaller.

Aren't there some street bikes out there with a smaller front than rear, to quicken up the steering?

Dave



Thats why! :)

  • micah600

Posted June 13, 2005 - 03:17 PM

#6

actually its to make the rolling diameter the same (OD) (thats what my brother told me about 25 years ago,at least) the OD of the tires , i mean.

  • superduper

Posted June 13, 2005 - 03:29 PM

#7

actually its to make the rolling diameter the same (OD) (thats what my brother told me about 25 years ago,at least) the OD of the tires , i mean.



what? :) :) :D

  • XR650L_Dave

Posted June 13, 2005 - 03:32 PM

#8

I think he means that if you take the diameter of the rear tire at the tips of the knobbies, this will equal the diameter of the tips of the knobbies at the front tire.

D

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  • Silver Surfer

Posted June 13, 2005 - 04:20 PM

#9

the larger the front tire the more harsh terrain you can rollover...take it to the extreme and imagine a 6" wheel! you couldn't roll over anything, every rock would throw you over the bars.

i think the rear is small for two reasons:
1) keep the weight down. rubber is heavy
2) allows for more suspension travel without having to raising the ride height

  • DirtDaddy

Posted June 13, 2005 - 05:45 PM

#10

Various tire sizes have been tried many times over the last couple of decades.
As I recall, but don't remember exactly when, a 24" front (I think) wheel was
tried on full sized bikes, as well as 18" and 19" fronts. 21" has become the norm.
It rides over everything better than the other tires.

As for the rear, 18"(trail, etc) and 19"(racing) tires stand at the same diameter,
just the wheel size is different.

Hope I got that all right.


:)

  • slammmin72

Posted June 13, 2005 - 06:47 PM

#11

just ask urself this.....would u want the 18" wheel in the front and 21" in the back and constantly feel like ur gonna fall over the front bars cause the bike has a "rake"?...lol

  • zwr250f

Posted June 14, 2005 - 05:37 AM

#12

Sounds like some good theories, and probably the right answers. I like the correlation of what size will roll over terrain the easiest. That sounds right. Thanks for making me not feel so stupid now.

Any more input? I would love to hear.

I wonder how much input tire manufactures have in it. If Michelin says: Look, we can build a 21" front and a 18"/19" rear combo with so much rubber, thus making x number of tires per shift. If we go to a 20" front we can only make this many tires and have to set up a different line, etc,etc....just curious. I'm sure tire companies don't like the engineer's swapping up tires sizes every year just because, if they had it their way all tires might be 255/70-15's, no matter what you rode !!! :)

  • Neville1

Posted June 14, 2005 - 05:41 AM

#13

Various tire sizes have been tried many times over the last couple of decades.
As I recall, but don't remember exactly when, a 24" front (I think) wheel was
tried on full sized bikes, as well as 18" and 19" fronts. 21" has become the norm.
It rides over everything better than the other tires.

As for the rear, 18"(trail, etc) and 19"(racing) tires stand at the same diameter,
just the wheel size is different.

Hope I got that all right.


:)


Actually, I think the largest was Honda's CR's in the late 70's with that monster 23" front wheel. Man did that look bad. Talk about monster bikes. Those things where huge, and the giant wheel made the sterring heavy as hell. :)

  • DirtDaddy

Posted June 14, 2005 - 06:13 AM

#14

Actually, I think the largest was Honda's CR's in the late 70's with that monster 23" front wheel. Man did that look bad. Talk about monster bikes. Those things where huge, and the giant wheel made the sterring heavy as hell. :)

Yeah, I think you're right. 23". Where are all the old timers to answer this question?


:)

  • gmoss357

Posted June 14, 2005 - 06:28 AM

#15

I think that the current sizes are the result of the evolution theory. Tried others, didn't work well, so try something else. Now you have what is considered International Standard. Makes buying parts alot easier. Smaller rear allows for more rubber, less pinch flats, lower air press., and more susp. travel. Larger front allows you to roll over objects easier. I think "they" have found the happy middle for what works best. :)

  • Carioca

Posted June 14, 2005 - 08:55 PM

#16

Just to add that I believe that most bikes from MotoGP are running 16.5" on the front instead of the original 17". One rider started using it to get a quicker turning responce (less gyroscopic force) and pretty soon most riders were using the 17"rear 16.5"front combo.





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