pros & cons of a 21" front wheel vs. large 16" wheel/ tire

Conventional wisdom is dirt bikes have a 21" front wheel with small tire with a meatier rear tire, and large touring bikes have 16-18" wheels with meaty tires on both ends.

Why?

Wouldn't the meaty front tire float as well over dirt and mud as the 21" with smaller rubber? Does it, or is it a weight issue, or a side-loading issue on unstable terrain, or tradition, or something else altogether?

Taller tires go over obstacles easier. Thinner front rims are stronger when you hit stuff.

You don't want to float on top sometimes because the traction is deeper down. Thinner tires track better in mud in my experience

Handling off road is different than highway. A street bike generally has 17/17" rims and they corner great and stick great. The best handling though is NOT a fat front, ride a TW 200 Yami and see. The fast street bikes run a reasonably narrow front tire. Not exactly sure of the physics of it all but they roll into a turn better and have less flex in the sidewalls than a fat tire does. A fat tire like what you see on Harleys and the like are usually 15" or so and they are mushy handling and sloppy feeling.

A tire like that in the dirt does not track well at all and gets very little feel and feed back. I have done it many times and it is not impossible, but not that fun either.

A comprimise is a 19"/17" frt/rr combo like many ADVenture bikes run like on a Super Tenere or BMW Gs/GSA. The fat front on those still limit TRUE off road ability IMHO, but they are better than the smaller diameter on a sportier or tourer style bike.

As for the 21" front wheel you can not get any better in the DIRT but less traction and control on the pavement. The front 21" inch in soft sand works more like a rudder on a ship, It does dig in a bit to sand but it manuvers through the sand much easier that a 19" or smaller wheel does. It dopes still wander in soft sand but goes where it is pointed overal whereas a 19 or smaller with a fatter tire also digs in to soft sand but it PLOWS the sand and if it catches any edge under the sand it just hooks and burys in and you corkscrew in.

I have owned and DO own many bikes, If I was 100% pavement or hard pack dirt I would not hesitate to run a supermoto style 17" set like a Ducati Multi Strada. If I am 80/20% highway with some gravel & dirt mix then I am 19" frt, 17 or 18" rear. If I am any soft sand or rocky dirt with 65/35% or greater tar to dirt ratio it is 21" front and 17-19" rear, period.

So the key is picking the right bike for your riding style and places and use it. In a perfect world we could all have many bikes and choose what we were doing today and use that weapon.

I hope that helped a little. Your premise is not without merit, but until you get into REALLY fat tire like a quad or 3 Wheeler style tire it will not track on top off sand, like a Honda Fat Cat, but then you have NO street useability for all intensit purposes. It is ALL a comprimise of some sort,

I'm running Shinko 705 17x120 on the front of my SV650 and don't find it that bad for most DS riding. Even ran up a sand ditch for awhile and did not have any problems. I plan to get some knobs this spring and expect it to be even better. I agree with you premise that a little floatation helps with a heavy dual sport.

On the other side of the argument are the older Dakar bikes that run more sand than any sane person would and they all seem to use normal 21 inches wheels.

What I have found is that I really don't push hard enough on dirt or street t really care much about the perfect tire or rim diameter. All tires seem better on pavement and suck in sand. Horsepower and speed really are the only things that make soft sand any fun.

sailah says it--think about a scooter tire, it would get stuck in every hole and wouldn't want to roll over anything, where a monster truck tire wouldn't even notice (speaking of diameter here, not width).

And he's right on regarding width too, you have to cut through the soft stuff and get to the traction underneath.

Changing from 110x19 front / 120x18 rear to 100 front 110 rear on my airhead dualsport increased my dirt/gravel traction considerably, with no real loss in pavement performance--I'm just using all of the tire more often on the road now, where before I rarely rode it to the edge of the tread.

Hi there,
perhaps I can shed a bit of light on it, too. Not in every aspect though (look at the line-up of our bikes):
Thinking of diameter and it's influence you can run a simple test. Get yourself two bicycle-wheels, one from the usual bike for the grown-ups and a small one from a child's bike. Grab the axle with one hand and spin that wheel. Try to tip the turning wheel over with your hand. You will realize, that you have to try really hard with the big wheel and that it's much easier with the small one. So a bigger wheel offers more direction-stability than a small one. Put aside the up-to-date Chopper and Cruiser wheels of up to 30 inches (they're just for show!), Honda tried to do one better by putting 23''-wheels in the front of their 1980-ies XLs. Found out, that 21'' was the best compromise ...
I made similiar experiences as chollo when putting just slightly fatter tires on our Suzuki GN 400 single to get more modern rubber on the bike. Just like on chollo's bike the difference might be 10 mm more width per wheel. Up to that day, the bike was great on sand, 60 mph, even 2-up, was never a problem. Put on the thicker tires and some months later I hit a short stretch of gravel during a journey to France. I couldn't recognize the bike again, the wheels wondered in every direction with a very unstable feeling in the bars.
But it's not as easy as just summing-up the points that the other guys and me mentioned before. It also depends on steering-angle, wheel-base, weight of the bike, center of gravity, power, the kind of 'road' under your wheels and and and ...
I'm from Europe and here most off-road-bikes are ridden with 21'' fronts and 18, now 17'' on the back. My amateur-experience tells me so far: The front-wheel should be 2 - 4'' bigger in diameter than the rear one. There should be a difference of 20 - 40 mms in width, the front being the slimmer one. Rear-wheel up to 130 mm width. And I must say that I'm talking about classic bikes of small- and mid-size, all off-roaders or derivations of these. From a 175 cc 7-speed Hercules over some 250cc MZs, those Suzuki GN 400 singles, a Triumph Tiger Trail to our biggest, heaviest, youngest and most powerful bike, a Suzuki DR Big 750 from 1988.
I often wondered how the US-Boys could ride these big brit-bikes in the sand with those chunky front-tires in 18 and 19''-size. I would expect these fronts to weave all over the place ... still hope this thread gives the answer to this (motorcycle-)life-long question of mine ...
I'm working at that bespoke Suzuki 400 to put on a 17''-rear-wheel so I could use a stickier tire(Bridgestone BT 45 120 or 130). I'll also have a go with a 19'' front-wheel on it so I can try to build some kind of dual-sport for my wife out of it. I'm expecting many new experiences while doing so.
Kind regards, Bambi

Inmates, I am pleased and impressed with your thorough replies. Thank you.

I'm gathering information for an upcoming Adventure Touring motorcycle build, something I can ride all day on the Interstate Highway to reach a destination, and then ride it off pavement once there. I expect the finished dry weight to be around 550+ pounds, or about the weight of an 1150GS. This will be initially for the UCC (Key West Florida to Prud Hoe Bay), so 10,000 miles of pavement, and 1000 miles of gravel. But, I could see trying the Continental Divide ride in the future.

As I don't see this motorcycle being used for racing in the canyons, I now see no downside to going with a 21" front wheel. If there are other comments others want to share, I certainly welcome them.

Thanks again, and if you all don't mind I'll pepper you with more ? as my planning comes along.

Ask away...but please write a build report with LOTS of pics of all the custom work,
Not that I am any real help, but I will chime in with some OFF CAMBER info.

What bike are you thinking of using as a platform for this build? There are probably dozens of decent-to-good candidates that could fit the bill.

A late model model rubber mount fuel injected Sportster 1200.


There are others factory-built and able to do this chore already, but I simply do not enjoy them enough to ride 500+ miles day after day after day. I even bought one which I thought would allow these kinds of trips for me, but no. Behavior and characteristics show up at the end of a long day which don't show up on a test ride. I really enjoy the low RPM torquey non-buzzy behavior of Harley's. And I think this conversion will be fun to do.

FWIW: The rationale I recall for narrow, 21-inch fronts was that they were steerable in deep wheelruts left behind by other motorcycles participating in off-road competitions.

The rut will be at least the width of a typical motorcycle rear tyre, and so a narrow front tyre will be steerable in the rut for balance and to steer out where possible.

Go narrow and you need a big diameter for a decent footprint area, hence the 21.

Other advantages of the big diameter include a smaller tendency to high-centre when attempting to negotiate big steps, fallen logs, etc.

On soft surfaces where you want to sit on top (e.g. sand), it is a question of flotation, and of how quickly the trail measurement changes when you start to sink.

Bigger wheel diameter would be better on both counts, but flotation would be better with wider rather than narrower rubber.

Rokon uses wide front rubber on its go-anywhere 2WD Trailbreaker and while I have no first-hand experience I believe they do pretty well.

<iframe width="420" height="315" src="http://www.youtube.com/embed/ZW8v6y1yErk?rel=0" frameborder="0" allowfullscreen></iframe>

It is not a competition bike, so no strong need to handle others' wheelruts with speed efficiency.

That Rokon does NOTHING well. Have you ridden one?

I stated earlier and I stand by it, That the floatation effect does not kick in until you get quite large like a quad or atc tire.
Going to a 140x19 or so tire from a 100/90 or 90/90 x 21 will not gain you enough floatation on sand or mud to help.

In fact if you were to go to any real sand dune park like Pismo or Glamis where the sand id deep and heavy virtually all of the true sand buggies and sand quads run a smallish front tire with a big rib around the center to sink into the sand and steer more like a rudder on a boat. Bikes still use a traditional 21" front with a paddle rear tire.

In any real dirt, sand, mud or other off road conditions I only want a 21" front. If I am on any real tarmac I want a 17" front. If I want a good comprimise of tar and gravel or firm dirt, I want a 19". Just need to know what you want to do the most and then settle for subpar performance everywhere else.

I think of it as carving through snow. Like skiing or snowboarding. The motorcycle counter-steers, its geometry makes it lean into the turn, the front wheel is not "floating" but is carving, slightly digging into the soft terrain.

The Rokon is intended to be a low speed machine, (it doesn't even have a suspension system ffs) you can see in the video the riders just "steer" left and right, like it was a quad or a snowmobile.

Marco Moto and others have the right notion in my view. No question, the 21 front carves dirt where shorter and fatter tires don't. We are talking higher performance I think. Probably the heaviest bike with a 21" front wheel is the KTM LC8 series of dual sports. I rode one for seven years, and I can attest that you want a STRONG 21 front hoop on that bike if you expect to do forests service roads. Hitting a washout at speed is puckering, but not much drama. For a 550 lb beast, it probably doesn't matter. Just make sure it is a steel spoked wheel rather than a cast job.

In the Southern California Desert when big desert sleds were raced (Triumph, BSA, Matchless 650cc) they all used 19 inch front wheels with big wide trials tires on them. This combination was used specifically because it doesn't knife through the sand but rather floats on top. 21" front wheels were the choice of Motocross bikes and lightweight 2 strokes where the front end could be lifted at will. With the big sleds you couldn't do that and you plowed through everything. The knifing/plowing of a heavy bike in the sand gets pretty darn tiring in a hurry. So to me, if the bike is heavy and has a tendency to plow then a 19" with a nice wide tire is the way to go. Additionally, if the bike is used primarily on fire roads then think of what the scrambles/TT/flat track racers use and again that is a nice big wide tire.

The larger in diameter the front tire is the better, to a point, for all the reasons mentioned above. Back in the early '80s, Honda used a 23" front tire on their XL500 and 250. I've never ridden one, so I don't know how well it worked, but they did go back to 21". With mountain bikes, 26" has been standard forever. But now many of the high dollar mountain bikes have gone to 29" wheels, and have used them for some time. I have never ridden a 29er, but most who have switched to them claim they are better. Anytime you have something turning, centrifugal force comes into play, and the more the weight, the higher the centrifugal force. This doesn't matter nearly as much on street bikes, but on dirt bikes, you want wheels that can stop and start turning with relative ease. Those big monster "adventure bikes" are meant mostly for road use, and occasional dirt road use. They are not trail bikes or MX bikes. I tend to think of them more like a pack mule, capable of carrying a lot of stuff over rough terrain at slow speeds, but no match for a race horse. You should choose your bike for the type of riding you intend to do. I have a cruiser for road riding, and an XT225 for off road riding.

Many cruisers also have a 21" front wheel, but it is only for looks. It actually hurts the handling, making the front end floppy. Mine has a 19" front wheel.

Very insightful additional information. Kind of messes up the consensus we were getting for the 21" front wheel, but good to have nonetheless.

29" wheels on MTB's are actually used because they 'span over' dips and bumps that would possibly swallow a 26" wheel. There is little if any centrifugal advantage to using a slightly bigger diameter at the rotation speeds turned on these bikes. It's more about increasing your contact patch.

There is a downside to making spoked wheels bigger, they become weaker and are less able to hold their true when subjected to abuse.

I still think the 21" wheel is a product of Motocross and most pure offroad bikes are sold as Motocross bikes. (Trials bikes could be the one exception.) The push for high performance Dual Sport bikes causes them to be compared to MX bikes and made as close to a MX race bike that can still be made street legal. Back in the late 60's and early 70's the "Dual Sport" bikes were "Scramblers" and the light two stroke bikes were "Enduros". The scramblers were all 19" wheeled.

Scramblers are gone for a reason Strong Bad. 19" wheels were NOT better in the desert. Not even close. It is what was used back in the day of those pics. Once DIRT bikes of all types went to 21" they never went back. NO ONE uses a 19" in the desert. If they were better they would use it. Johnny Campbell would be using one, Mouse McCoy....

19" wheels on your bigger ADV bikes are pure there as a comprimise and NOT because they are BETTER in the dirt.

The comments about picking the right tool for your job is exactly right. If you are gonna ride alot of hiway and some dirt gravel roads a 19 is BEST...for that. If you are gonna ride alot of dirt, sand, mud, and loose stuff and 21" is it.

there is NO floating effect on a 19" wheel. As I stated before, until you go to a really fat tire it is not effective. Look at all sand toys. they use a ribbed front not to float but to rudder basically.

21" suck on long hiway stuff. Not great wear, not that that is impossible but no one I know make a long lasting 21" knobby. very little contact patch because of its narrow width and generally knobbies minimize contact patches as well.

As for the 23" on the Hondas back when, they ran them on the CR125 in 1979 I think as well as the early 80s xl500s. I had one...It was that TOO A POINT thing. The 23" was a bit too far. Not neccesary and made the steering heavy. It never took off and tire were then and still tough to find and thus expensive.

In the early 2000s Mike Larocco tried running a 20" front tire for SX that was a bit wider thinking it would stick better and there are no big rocks in SX but it just did not work either. It did not carve the same in ruts.....

21" in dirt. 17" on tar, 19" if tar dominated but occasional dirt.