Why Are Funny Car Front Tires Thin

I'm wondering why such a narrow front tire on a dirt or DS bike.
Wouldn't a wider tire have more grip in gravel, dirt and sand?
I'm just pondering the question.
Yes it would. But it won't turn as well on asphalt. Nor would it track in-line as well. Ride a newer Goldwing and then a H-D touring bike and you'll see why a narrow tire is used. The H-D won't want to turn at all compared to the GW.
It's a compromise.
You would actually have less grip in most circumstances and it would wonder all over the place. You'd also suffer increased weight and slower turning. Slightly wider tires can be useful on hard pack (hence some folks running slightly wider 20" fronts at tracks like carlsbad), but even then its a small amount wider and it still noticeably detracts from stability in soft stuff and is of course a bit more of a handful in ruts.
A shorter, wider tire would actually be better on asphalt, not worse as MB said (ever hear of a supermoto?), but the dynamics there are quite different from a soft surface that you dig into rather than stick onto.
Dig into dirt instead of stick on it.
That makes a lot of sense.
Thanks for the info.
Dig into rather than stick onto? Oh please.
Please don't put words in my mouth. I never said anything about the height of the tire. He/she was asking about width, mkay? Supermoto bikes run the tires they do because no one makes a slick in 19 or 21 inch. Supermoto setups (real race setups) are a compromise between dirt and asphalt. Bringing up a Supermoto setup is meaningless.

For effective off-road use you want tall and narrow compared to a road bike on asphalt. However, too wide on asphalt has its downsides as well. There are a number of reasons for this, but it all boils down to handling and terrain. Heck, a 19 inch itself is a compromise... How many good road tires are available for a 21 inch wheel? I think the answer is zero.

Not mention your grammar sucks for a college student.
Didn't mean to imply that you said anything about height, just pointing out that it sounded a lot like you were sayign that goign wider than a stock 21" dirt bike setup would be bad on the street and that's simply not true. Just saying that a wider tire is exactly what you need (to a point) on the street and what you don't want in the dirt. SM tires are shorter to reduce gyroscopic effects of tall tires and wider to provide additional road contact area. Anyhow, if 21/19 slicks were a good idea it would be easy enough for them to be made (and you can get street tires in 21" front thanks to the cruiser crowd - comes in handy for sportsman class SM btw).
I totally agree that too wide is bad anywhere. As an example I switched out from a 6" rim and 190 section tire on my sport bike for a 5.5" rim and 180 section rear tire and it greatly improved turn-in (and I'm sure reduced the amount of power/heat the rear tire can absorb, but my measly 120rwhp isn't enough to overpower a 180 anyway, at least not with me twisting the right grip:)
Careful now, because wide is not the entire picture. There is also tire profile. Dunlop is now making a 200mm wide DOT race tire in 17 inch (211 NTEC) that works fine on a 5.5 inch width. It looks like a monster on a 6 inch wheel but completely different on a 5.5. Still, it turns just fine. And it puts down a patch at least as big as the 16.5 wheels. Clearly, they are trying to kill the 16.5 tires.
Tires keep changing (thank goodness) and the problem with 180 vs 190 width rear tires and how well they turn is pretty much history.

Oh yeah, call Dunlop and ask them to build a 19 or 21 inch slick and they will tell you pound sand.
A nice narrow front tire can also thread its way through obstacles a little better.
Profile is important - I have just fitted 4.00x18 Dunlop K70 tyres front and rear to my R65 for gravel road use,and it's the best setup I've ever used on gravel.The profile puts the whole tread down into the gravel.
Define "good".
Avon, Dunlop, Metzeler make nice road tires in 90/90-21.
Thank you, I stand corrected.
Sorry if this goes off-topic and/or is a stupid question, but can someone explain to this n00b what the various numbers mean on tire specs?
For example, my DR650SE uses:

90/90-21 on front
and
120/90-17 on rear.

I understand that the 21 and 17 refer to the diameters, but what do 90/90 (front) and 120/90 (rear) numbers refer to?

Also, the 21 and 17 are (apparently) English measurements (inches) ... so are the 90/90 and 120/90 numbers expressed in metric or English units?
XX/XX refers to tire width and the relationship of the carcass height to the width, first number is metric, the second is a percentage.
For example 90/90

90 is the tire width in millimeters

/ 90 means the carcass height is 90% of the width.

90/90 would be a high profile tire

120/70 is lower, since it's 70% of the width

Auto tires, not uncommon to see something like a 265/40 these days, which is a very low profile.
I'll add that the combination of tire and wheel sizes a manufacturer chooses for a vehicle is a whole series of compromises. Design, handling, and use would normally come first. Then add in looks. This is very important to many people, especially with autos. A good example are SUV's with large wheels and low profile tires. That equals good looks but a lousy ride and for sure, lousy off-road, and just asking for wheel damage.
Lastly, they must take into consideration what tire companies are building, or are willing to build for the market.

A tall profile tire generally rides betters but handling can be vague because the carcass will tend to flex more while cornering. Whereas a low profile will offer crisper handling (shorter, stiffer side walls) and a less comfy ride.
Thanks, MB. Good explanation.
Pantah Jiggy Dog Fan Supporter

The reason for a 21 inch rim is to allow the machine to roll over the bumps better because of its larger diameter.
The reason 21 inch tires are narrower is because they carve better on dirt surfaces. Wide tires wash out like they were on marbles.

The reason supermoto and sportbikes have short wide wheels is to achieve a bigger contact patch, allowing more grip on pavement. Profile is also an important design variable for improving grip on the side of the tire, particularly the front.

Pirelli makes excellent road tires in 21" sizes as well.
How about wide front tires in regards to riding in sand?
A ride on a Fatboy and TW type bike is in order.
Remember trail and rake, too . A Fat Boy turns like a turd. But then, a Fat Boy is only supposed to look good, not feel good. ;-)
My sense is a wider tire in sand will do better in a straight line, up to a point. And prolly not turn as well, all other things being equal. ?
markk53 jack of all trades... Super Supporter

They're just screwing with ya!
Actually a fatter tire is better in deep sand, like desert racing. But the skinny tire knifes down through the dirt/gravel/mud to get to solid ground where the fat one rides on top. Ever see the tires used on a rally car in the snow? About 1/3 the width of the normal tire, to cut down into the snow, then studs grip the ice. Would you want your tire to ride on top of the gravel where it can slide around or cut through it and get some solid grip on the underlying dirt?

Same reason for the coarse tread, cut through the dirt/gravel/mud to grip something harder.