Correct me if Im wrong but won't larger tires increase the speed of the bike?

 

No it slows you down because of the larger tire the longer time it hast to spin around. The smaller the tire the faster it can spin around thus making it go faster.

 

OK here it is. First of all, there is no power loss by changing tires, on any machine, to any size tire. Now wait a minute, before you start thinking about how your going to tell me I'm an idiot, think about it. You don't lose any power to the wheels, but you may lose some power to the ground. Larger tire diameter and heavier tires require more HP to travel the same distance. That's just the way it is. Larger tire diameter will also move your bike further per tire revolution, hence higher top end speed. The reason that you may have to choose a lower gear to enter the same mud hole that you are used to going through in 2nd gear is, now you have traction! I can take off in 5th gear on my Foreman and pin it, on ice. If your not spinning there are one of two problems, too much traction and, or not enough power. If the bike had enough power before you changed the tires, so you must be getting traction now. Well, isnt that why you bought the tires in the first place?

 

Yes, assuming the quad has enough power winding out to increase the speed. On my dads 450 with the stock tires, the quad would top at about 48mph.(according to the speedometer) Once the blackwaters were put on, the speedometer only tops out at around 43mph. With the larger tires, the quad is still probably really going around 48mph but it isn't able to wind up like it did before. This is another reason to spend extra money on aluminum wheels. You could probably get the speedometer to read closer to 48 again. (meaning you'd be closer to 53 or so.)

Now if you take a quad like the Wolverine which only gets 23" tires to begin with, putting 25" tires on it DOES increase top end because the motor has plenty of power left. But it still loses low end power like any other quad would.

 

 

I agree with you about the traction part, that ones obvious. But I still don't totally agree about the power loss. (If theres any mechanical engineers or physics majors out there, help us out a little) By changing tire size, its just like changing gear ratios. Take a 10 speed bike for instance. Lets look at the front sprocket. By moving up to the bigger sprocket, it takes more power from your legs to get the bike moving because the tires want to start out faster. When your on the smaller sprocket in the front, It takes much less power from your legs to get moving. Or am I looking at this wrong? I don't think I am but it wouldn't be the first time. (It would be the second. LOL)

 

OK, here it is. I am a mechanical engineer. Please bear with me, this is very hard to explain over this forum type situation.
Horsepower, to the ground, does not change. Period. There are, however, other factors that make you, the rider, feel a difference in the performance of the machine. Static weight, rotational weight, and motor rpm to ground speed ratio all change. I think that we can all agree on that?
Static weight.
If you are carring a load, does it change the HP? No. It changes the performance, but does not change the HP to the ground. The same with pulling a load. Now you have increased the total weight of the load (quad +rider+load=Total Load) now you feel the difference, but there's no change in HP to the ground. If you are simply going up a hill, nothing has changed on the quad, but now you have more load on the motor and you feel the change.
Rotational weight.
To increase the performance of ANY motor vehical, one thing that can be done, without changing motor or transmission, is lower the rotational weight. eg. lighter wheels and tires, hollow axles, lighten rotors, lighten drive shaft, lighten flywheel. All of these adjustments will increase throttle response, and make them rev quicker but do not change HP.
RPM to ground speed.
The way I figure it, going from a 24" tire to a 27" tire increases RPM to ground speed by 12.5%. This doesn't mean that you lose 12.5% HP. In racing, you decrease gear ratio to run lower RPM's at the same speed, or go faster at the same RPM's, same difference. You try to match the peak HP of the motor, say 7500 RPM's, to reach the highest expected MPH of each track. You can fine tune this with tire size. It's the same with sprocket tuning. Bigger rear, or smaller front higher RPM's lower ground speed. Bigger front, or smaller rear, more top end speed, less lo end. NO change in power. Every motor has an RPM where it developes it's peak HP. Finding it, and keeping it in that range is the trick. Naturally, if your gear ratio is higher, more top end, you will have lower RPM's at the same ground speed; therefore, not less HP, just not enough RPM's to develope your machines peak HP. For instance, say your machine developes it's peak HP at 5000 RPM's, and @ 5000 you could do 50 MPH. With 12.5% increase in "gear Ratio" or RPM to ground speed ratio, now, you will only be turning 4325 RPM's at 50 MPH, which may be so much under your machines "power band" that you may not be able to go any faster. I remember seeing an artical on the '98 Corvettes, that said it would do 160 in 5th gear but only 150 in 6th because the motor couldn't reach it's peak HP at that RPM.
So to summerize; You don't LOSE power, you shift it not from it's original RPM, to a new corrosponding wheel speed. I hope that this makes sense, and I hope that I didn't ramble. BTW, mecanical engineers don't have to be good spellers.

 

Thanks man, I think I'm all better now! LOL.
Now I see the problem, You were already thinking like an engineer the whole time! OK, I get what your saying now. The motor isn't actually losing power to the wheels but with the increased tire size, the quad will be more sluggish right? Meaning, if you changed the gearing in the tranny to compensate for the larger tires, the only performance difference would be the extra weight of the tires and the increased traction. So with the bigger tires and stock tranny, your putting less power to the ground right. So, isn't that then a loss of power?

 

Before you get upset with me, let me make an amend to my last post. If I have a foreman 450 with stock tires, side by side with a foreman 450 with larger tires and they both had equal traction, would the stock 450 be able to pull more weight?

 

Good point. By changing tire size, we have efectively changed the RPM to ground speed ratio. We can agree on that? So, if you took 2, 450's, one with 24" tires and 1 with 27" tires, "out of the box" hooked them up, side by side to 2 sleds of equal drag, took off in the same gear, gave them the same amount of fuel, the one with smaller tires would pull the sled with less effort. This does not mean that the 24" machine has any more power. It simply means that it is essentually "geared" lower with smaller tires. But again, your only moving the peak HP to a different corrosponding ground speed. Once they have both reached their peak HP RPM, they will pull the same. The 27" machine will be traveling faster though. If you took off in 2nd gear on the 24" machine, and 1st gear on the 27" machine, which would pull better? Obviously the 27" machine, but does that mean that it has more power? No.
I am having a hard time explaining my point, and I think that we are saying the same thing here. You will FEEL a change in performance with no loss in HP. That's the bottom line.