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Login to post Forums » Off-topic discussion » help me think this through

July 16, 2011 12:40 a.m. alex SuperDork

There's a trend in current motorcycle custom builds to use a big, fat front tire - in fact, a tire that would normally be used on the rear.

Aesthetically I think it's a little weird (though it appeals to me since it reminds me of Big Wheels and just looks like FUN! But I digress...), but instinctively I feel like it poses a dynamic advantage. Looking at this bike, it should steer pretty well, as opposed to these raked out skinny tire 'choppers,' which I can verify, can't handle for E36 M3.

(That's a 300 rear tire, by the way.)

But I can't explain why. Obviously, the rake has a lot to do with it. But given similar rake figures, I feel like the bike with nearly-identical tire widths should steer better than the one with a big disparity. (Of course, that logic is refuted by the fact that modern superbikes sport pretty disparate sizes front-to-rear (common sizes are 190 - 200 rear and 120 front).)

So, help me think this through, or tell me why my head's up my ass.
July 16, 2011 1:52 a.m. fritzsch New Reader

Well the way i see it, the larger tire in back helps with the acceleration so you want a large cross section. the front tire is primarily concerned with braking and cornering. since braking is just another form of acceleration it might help to have a larger front tire for that but its possible that the cornering demands outweight that. you want a skinnier tire for cornering since that equates to lighter and less rotational inertia giving you quicker turn in.
July 16, 2011 1:53 a.m. fritzsch New Reader

but i suppose going to the extremes is also a bad thing
July 16, 2011 2:36 a.m. donalson SuperDork

I personalty would never look to the custom cruiser bike trends to consider anything of good handling or comfort...
July 16, 2011 7:24 a.m. 1988RedT2 Dork

For guidance in terms of pure handling, I'd look to see what they're running on road race bikes. I doubt you'll see many fat tires with mud cleats on those bikes.

Like everything else, the first question is "What do you want to do with the bike?"
July 16, 2011 7:47 a.m. 914Driver SuperDork

The gyroscopic effect of a spinning mass makes it difficult to change direction; I would think a fattie up front would make you apply more muscle to turn at speed.

Maybe not ....
July 16, 2011 8:10 a.m. z31maniac SuperDork

^It would.

And the front tire size on a sportbike doesn't really matter when braking, even with the smalling 120s, you can easily flip yourself with 2 fingers on the lever.

However, that big sticky gumball in teh back helps put down the power. COnsidering most sportbikes are putting as much power, or many times more, than modern small cars.
July 16, 2011 8:38 a.m. SVreX SuperDork

The E36 M3ty handling of the chopper is primarily due to the geometry of the fork extensions, not the tire size.
July 16, 2011 10:22 a.m. Curmudgeon SuperDork

The geometry of the chopper makes for a lot of 'trail'. A little trail is a good thing, a lot is not. That makes the thing handle really weird to start with because instead of the bars wanting to stay reasonably straight with minimal operator input, they want to 'fall' to either side and the long forks flex like they are made of chewing gum. Also, once the tire is turned either way from straight ahead it leans a lot at the top and turns at a large angle to the rest of the bike, bringing the tire's slip angle into the picture in a big way. Slip angle is not a real big deal in a car but on a bike oh yes it is. Bikes need to be much more precise in their positioning if the operator is to continue unscathed.

So we move to the fat front tire. Sure, the fat tire can handle more weight being put on it but at what cost? It's heavier and will have more of a gyroscopic effect at high speeds than a smaller tire, meaning the bike will be more difficult to turn, it's going to want to stay vertical. Gyro effect on a motorcycle is a big deal. I learned that first hand on a CR500, at ~7k RPM the big ol' heavy crank spinning made for the weirdest feeling when trying to turn; when I put body English into it I had to exaggerate my movements to get it to go the direction I wanted and I had to start all that a lot earlier. If the clutch was disengaged and the engine allowed to idle down the feeling pretty much went away. Strangely, my XR600 didn't do that and I put it down to lower RPM.

When i mentioned it, long time CR500 guys always looked at me funny and said 'I don't know what you are talking about'. Guess it's all in what you get used to.

So the racers and manufacturers have gone to smaller diameter tires (less gyro effect) which still have enough load capacity to keep slip angles at a minimum.
July 16, 2011 10:25 a.m. z31maniac SuperDork

SVreX wrote:

The E36 M3ty handling of the chopper is primarily due to the geometry of the fork extensions, not the tire size.

True.

But the large rear tires, have a crappy flat profile, that makes turn-in much less than optimal.

That's why I was always a big fan of the Pilot Powers, nice steep profile off-center, vs Metzeler/Dunlop and their tendency to be more rounded.
July 16, 2011 1:50 p.m. novaderrik Dork

alex wrote:

There's a trend in current motorcycle custom builds to use a big, fat front tire - in fact, a tire that would normally be used on the rear.

i think it's supposed to look like an old school flat track racer.
July 16, 2011 2:37 p.m. carzan HalfDork

From a looks standpoint, I kinda like it. I'm just afraid the trend will continue into the "mine's bigger" territory not unlike another (unfortunate) trend:

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