Alright all you braniacs...physics questions!!!!!

13 replies to this topic
  • sunnymex

Posted February 23, 2004 - 07:24 PM


OK, so here is the dilema. I was sitting here looking at some tires this afternoon, wondering about speed and which one would be a faster tire. I know some of you probably already know the answer so go ahead say it.

What I am trying to figure out is which tire would give me a higher top speed a Dunlop 739AT 120-100/18 or a Michelin Baja 140-80/18?

My initial guess is that the Dunlop being a taller tire would have a higher top end, however then I began thinking about rear sprockets and the smaller the faster, which threw me for a loop with the tire size.

So what is the answer? :)

  • XR250rdr

Posted February 23, 2004 - 08:47 PM


The taller tire will give you slightly taller gearing. I doubt if you would gain anything though.

Ohh and btw...if both tires are measured equally, the Dunlop is taller by 8mm. So yea I dont think you would notice much of a difference, the diameter would only be a little over 1/2" larger.

  • crashtester

Posted February 24, 2004 - 04:51 AM


xr250r is correct. Taller tire means it travels more distance per revolution. If you are simply comparing the rear sprocket and tire size (i.e. the chain traveling the same "reference" speed), a smaller sprocket causes the wheel to turn more times than when compared to the larger sprocket. An easy way to conceptulize this is if the chain goes one complete rotation (for this example 100 links), and the rear sprocket has 100 teeth, the wheel would rotate 1 time. With 50 teeth the wheel rotates 2 times, 25 4 times etc. For each rotation of the wheel, the circumference is the distance the bike moves (theoretically). The larger the diameter, the larger the circumference (if you took a flexible tape measure and wrapped it around the outside of the tire and then laid it flat.) This can be calculated diameter * pi. However, for all you experts out there who are able to keep your pigs on 2 wheels at top speed (as you can tell by my name I have issues with that), how much speed is lost to tire slippage? Also, at these rpms, do the tires grow?

  • sunnymex

Posted February 24, 2004 - 05:32 AM


OK, so that is what I thought also. So theoretically with these two different tires, the taller tire(in this case the DUnlop) at full engine rpm will give you a faster top speed then the Michelin, right?

  • TimBrp

Posted February 24, 2004 - 08:35 AM


What happens when the tire wears down a bit.. :) I mean c'mon. You've got a big bike, put on the tire that makes you happy. The bike will still screw.

  • sunnymex

Posted February 24, 2004 - 11:28 AM


The truth of the matter is a buddy of mine that I am racing against is running the Dunlop and I am running the Michelin. We got to talking about the size of the tires and speeds, etc and were trying to figure out which one was faster. I love the Michelin Baja tires, they really get some great traction. I would gladly give up a little top end for the confidence that I get in return.

What do oyu run Tim?

  • XR250rdr

Posted February 24, 2004 - 12:59 PM


Like I said before...I really doubt you could tell. Especially in the dirt. There is way too much slippage to narrow down the top speed to tire diameter.

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  • irondude

Posted February 24, 2004 - 01:42 PM


For sure im no brainiac--but you guys are forgetting some stuff!
The tires will change shape do to centrifigal forces! Just because a tire is 8mm taller while sitting still-doesnt mean it is 8 mm taller when spinning at high RPM!

ALSO: I run 2.15 rear Excell rim-as do many others: while Honda (Cambell-the-cheater) runs the 1.85--so this wider rim or narrower rim choice will widen out the tire, making it for sure less tall.

The guys with the Terrraflex can tell ya all about centrifugal effects on a tire-and while the Michelin Baja and Dunlop 739 are not radial tires-they still will feel the forces while spinning. So its important to take tire carcass issues into the equation.

Finally: You guys are just talking about top speed-which is not going to be noticably different do to tire height.

BUT- tire weight (unsprung weight), and the distance a tire will have to be from the swingarm pivot--are both going to be very noticable in the tight and choppy stuff. SO top speed isnt really gonna be much different, but overall speed will be.

One other thing:
The taller tire will have different characteristics than the less tall tire at different tire pressures. This is a big deal when it comes to traction vs. puncture probability.

Just thought i would stir it up a bit!


  • sunnymex

Posted February 24, 2004 - 01:44 PM


Yeah, I figured as much. I will let you know on Sunday who tire was faster. Hopefully my trusty Michelin will carry me quick and safely to the finish line ahead of all else.

  • irondude

Posted February 24, 2004 - 01:48 PM


Good luck at the Pacifico 300! Sounds like fun. Gimme a heads up next time there is a race downthere- i wanna try one this spring.

The 3.2 tank comes in both Dry Brake and regular. Have to order the dry brake filler parts as well. Acerbis then makes an adapter to run the regular screw in cap for everyday riding...

  • sunnymex

Posted February 24, 2004 - 02:19 PM



Yeah I am pretty excited about the race. Should be a blast. The Dos Mares 500 is the first weekend on May, I think May 1-2. It is probably the biggest race of the season as cars come over from all over the country for this race. It is going to be a good warm-up for the Baja 500 also. You should try and come down for the race. It is in and out of La Paz. It will probably run on parts of the course that the 1000 will be on.

I heard a rumor that the 1000 was coming all the way to Cabo this year. Have you heard that?

Talk to you later.

  • lapster

Posted February 24, 2004 - 03:52 PM


Just thought i would stir it up a bit!


Since we're stirring the pot... why not include the effects of inertia? Having more tire mass farther from the hub means a greater resistance to motion = the fractionally larger tires would require fractionally more of the engine's power to reach an equivalent angular velocity... leaving less to overcome things like wind resistance, etc.

Having said that, if you're basing overall speed on that small of a diameter difference, you might as well be counting the number of mosquitos you hit versus your competition... :)

  • irondude

Posted February 24, 2004 - 08:14 PM


Sunny- count me in for the Dos Mares 500. May 2 is my thats how i'm gonna celebrate it.

I have also heard that the Baja 1000 is going to Cabo- and i heard a rumor that it might be a loop race-all in B.C.S.!!! That would be a first i think...The Baja 1000 is the largest tourist draw of the year in Baja-so it would be a real coup for the govenor of B C S to get the whole thing! Maybe thats just the rumor to pressure the guys in the north...ya never know, and its always very political when its time to choose the routes...I would love to play poker with Sal Fish pres of SOCRE some time... he must be a master after 31 years of dealing with the politicos of baja!

See ya in 2 months!
hmm getting a bit off topic here.... :)

  • TimBrp

Posted February 25, 2004 - 02:48 AM


I run a Pirelli MT-16HD w/ Bridgestone HD tubes. Great grip, great wear.

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