Tire diameter - rollout and its effect on gearing

[ASC6000]

Lots of threads talk about gearing, for a certain car - motor - etc. But especially in off road rarely does anyone mention tire diameter and it's affect on rollout. Final Drive Ratio is mentioned frequently but unless you take into effect the tire diameter it's kind of pointless. For example a mounted set of Excalibers is larger in diameter than a mounted set of Enduros and even more of a differnce to Barcodes. Maybe it's time to start using it as a part of the discussion, what are your thoughts? check this simple calculator http://www.dallas-music.com/rollout.html

[Coelacanth]

Rollout ratio and tire diameter go hand-in-hand. I brought this up in discussion not too long ago and didn't get much of a response. I got a lot of help from a few "gear geeks" over on RCTech, mostly via PM's.

http://www.rc10talk.com/viewtopic.php?f=40&t=21525

I'll have to look around but I even found a very useful website that has an online calculator taking into account tire diameter for calculating an estimated top speed. My original question arose due to the fact I custom-built an onroad car out of an offroad chassis, and the onroad tires were significantly smaller than the offroad ones I was using, and that offroad buggy was going well--and I wanted something with about the same performance characteristics.

EDIT: Here's the link to the calculator, it was in the thread I linked above. As you can see, one of the variables is tire diameter, so the two are needed to make a valid calculation.

Rollout ratio calculator:
http://www.scriptasylum.com/rc_speed/rollout_ratio.html

[teman]

This is the calculator I started using recently. It does gear ratio, roll-out and top speed
http://www.gearchart.com/index.cfm?fuseaction=chart.create

[Coelacanth]

Also, since rollout ratio is a measure of the distance the car travels in relation to the rotation of the motor, tire diameter must be one of the variables. With bigger tires, the car travels more distance, all else being equal.

@teman: The link I provided also has calculations for top speed and other stuff, look at the top of the page.

[teman]

Your link is better, haha

[Coelacanth]

Your link is better, haha

I think so too, I ran across the link from gearchart.com too and the results I got didn't seem valid. It only seems to work properly if you can choose a car from their list--which isn't the case with an old Turbo Optima.

[THEYTOOKMYTHUMB]

I grew up running on a small VERY tight and long oval made of a tennis court type material indoor. It was awesome! There weren't many rules. Guys use to try and glue only the edges of the foam tires on their 10Ls so that they could accelerate hard out of the very narrow corners, but fly down the long straights when their tires ballooned. It was basically 2 drag strips connected by a couple of hairpins. It was a bitch to tweak, but fun as hell(and this was in the late 80's early 90's when power didn't come too easy). This tire gluing method was obviously great when it worked(about 1% of the time ). Mostly it ended in a few hot laps and then disaster. Not sure how relevant this post is here, but that's what it made me think of. Reunited with most of those guys at our current dirt oval track throughout the last couple of years. Those were great times!

[Jay Dub]

In off-road at least, roll-outs are a good ball park #. But because of vareing insert densities, and tire tread thickneses, on track testing is the best scenario. -Jeff