Author Topic: Big tires, little tires and the physics of CVT trannys.....  (Read 4019 times)

Bucked

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On many occaisions I have had conversations debating what effect the overall height of a tire has on the performance of a Pilot in relation to speed, traction, and stability. Others state that a taller tire won't accelerate as fast, bite as hard in a turn and will make a Pilot, or any other buggy, more likely to roll. I'm comparing low profile 22x10 combinations to  larger 25x12 tire/rim combo.
I believe that with a CVT tranny that is torque sensitive, the only time the increased diameter comes into play it at initial take off and at top speeds. I think that at all speeds in between the nature of the CVT will adjust for the tire size and there would be no adverse effect at all. I think that a taller tire could provide a better foot print than a low profile tire and that the increased height of the buggy isn't necesarily going to make it any less stabil.  
Anyone want to discuss this from a scientific perspective?

ludedude

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Big tires, little tires and the physics of CVT trannys.....
« Reply #1 on: July 06, 2005, 04:06:25 PM »
Here's my take.

A CVT has infinite gear ratios inbetween it's lowest and highest possible (determined by pulley diameters). A CVT in principle, when clutched properly, keeps the engine in it's best power range...upshifting and downshifting as necessary based on terrain and engine speed.

Now, with that said, changing to a larger diameter tire is going to reduce the torque at the wheel, all through the CVT's range, because, well it's larger. Now if you clutch to that tire diameter, you should be able to experience near the same performance as the stock, smaller tire.

As for handling, we've run the 25x12 Spider Tracs...Stability was not adversly affected. But turning was. The pilot is light in the front, adding a wider footprint in the rear increases it's desire to plow straight ahead, regardless of what the front does.

dhjunkie

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tire size
« Reply #2 on: July 06, 2005, 05:35:58 PM »
Ok lets throw out the tire size numbers all together, generally thay are just an approximation.  the cvt has a final ratio of lets say 10:1 (it varies between modles of machines I believe)  now no matter what that 10:1 effects the amount of torque and speed.  Throw in the CVT it is basically a variable gear ratio supplement that is just a torque multiplier at WOT it still has a final ration of 10:1.  Now here comes the tire size problem.  when you increase the circumference (distance around) of a tire the Diameter (hieght) will also become larger (taller). This means that the tire will travel more ground per revolution than a smaller tire would.  big slower circle Vs. smaller faster circle.  the biggest factor that many forget about  is the mass of a larger tire  Bigger is generally heavier.  More rubber, more air (yes it does have wieght) generally a larger wheel, all of this = more weight to get spinning, hence the lack of initial responce. generally a increase in tire size also increases unsprung weight, this can effect a lot more than just the get up and go. it effects the way the suspension will want to react anf handle, it can effect your center of gravity (taller tire raises the COG).  But a taller/heavier tire will also stay rotating a bit longer on the flat stuff and maybe a little help initially up a short steep embankment.  Taller/ heavier tires are not good for hilly areas, they will rob HP.
  Now this is where the CVt is nice.  The cvt (clutches) will hold out a lower gear ratio for a longetr time until such RPM Vs. speed sensing is achieved and then it will move closer to the final ratio.  So ad you hit the final ratio at WOT and there isn't any more oomf. a and you want to go faster you can eithewr change tires sizes to a larger one and risk being slow on the steep long hill climbs (loss of power) or you can change your final ratio.  Prefferable small tires are great for groomed areas where your not going to be pounding the crap out of your chassis. taller tires are great for clearance.

hoodlum

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tire observation
« Reply #3 on: July 06, 2005, 07:08:14 PM »
When I got my pilot, the factory tires were on it. They lasted a total of one trip, and dry rot along with rocks finished them. Went to a 25-11-10 3ply. Rode several times with these.
Just got some dirt devil 23-10-10, and here is what I have found.
As far as speed difference, I haven't clocked it, but can tell a noticeable loss of top speed with the smaller tire, and did loose 1in ground clearance, but those are the only down sides.
What did it gain? Pulls much better from the hole. I could turn the big tires  over just a little on the concrete floor in my shop, but it will light up the dirt devils. The 1in drop in the rear made the seat position much more comfy, giving it more recline. This increase in recline seems to help the back out a bit after rough rides. Puts a bit more of the impact over the back instead of on the tail bone.
As far as handling, it corners much better, and has more of a tendency to slide around corners, letting the rear break loose a bit which makes for a much more fun ride. The 25's would really bite in the corners, which caused my one and only roll-over. Think these 23's would donut on asphalt, but haven't tried that yet. Just an assumption.
I think that in a situation with slow to moderate trail riding in rough rocky areas where clearance is a factor, the 25's probably would be best, but IMO, that's the only time. After running the 23's, You'll never see another set of 25's on mine
Rooky

redrider

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Tires, top speed, handling, science, forget about it!
« Reply #4 on: July 07, 2005, 02:15:04 PM »
Forget the science of it all and go with feel and experience!  Damn some number crunching anyway, half of it is just plain voodoo (or is that shocks and springs, hhhmmmm?  but anyway, back to tires)

Tire size is somewhat subjective.  I've seen identical brand and model tires of the same size be different diameters, right off the shelf.  Tire size will vary depending on the amount of pressure you run.  Brand new tread as opposed to well worn tread can make 1" worth of difference.  For this reason it can be somewhat hard to compare a 24 to a 23, or a 22 to 23, etc.  You'll see noticeable differences from 24 to 22, 25 to 22, etc. (maybe).

Top end speed?   Hmmmmmmm?  That's a good one.  Discounting all the genital insecurities out there, who really ever gets to actually use top end speed other than those who can run in WIDE OPEN terrain?  You might jump up to your top speed momentarily but are usually on and off the throttle constantly if you trail ride.  In most all trail situations, and many fairly open area riding, you should be much more concerned about handling than top end speed.  Cornering without tipping or rolling,  has as much to do and more with suspension than tires, though tires are a large contributing factor.  Tread pattern is as important as overall diameter.  A large tire might not roll in a corner at all if the tread pattern will allow for a good break away when wanting to power slide.  And conversely, if a pattern will bite immediately and keep a slide from starting at all, you may never break loose then hook, causing a roll over.  I've found that tread patterns in the front are about as important as in the rear.  In certain situations you want the front tires to go EXACTLY where you point and other times you love to have some drift or understeer.  Ply rating can assist or prevent roll in corners, front or rear, based on how stiff the sidewalls are.  A 6-ply rated 24" tire might have less roll than a 2 or 4-ply rated 22" tire.  This could go on and on and is only one individuals opinion.

For what I've experienced:

I ran 25 x 12  - 10's in the rear for a very long time.  What I experienced was good top speed (which I never needed realistically), a tendency to roll a bit in turns, good ground clearance, and a shitload of bounce!!!  The rounded shape and overall size helped in getting over a lot of rock structures but the disadvantages weren't worth it.

I tried 23 inch tires and then went to 22s.  The 22 worked best for me overall.  I came out of the hole better, could break it in a corner better, and experience much less roll in cornering.  I also have to admit that suspension changes were also taking place during the tire changes.  Tread patterns differed in the 22 inch sizes from an aggressive mud-type tread to a mx type tread.  

In the front I've gone from 21 to 22 and then to 23 inch tires.  The flat profile tires were great for hooking when wanted and going exactly where I steered.  The rounded profile was better for overall flexibility in handling control.  The 23 inch tires in the front seemed better for me in overall handling.

What I ended up with and liking the beset was 22 x 11 - 10 in rear (489 type pattern) and 23 x 7 - 10 in the front with a rounded straight tread pattern.  Specifically it was Titan 489XC (6-ply) in rear and Pure Sports Bandits XC (6-ply) in front.

Have a nice day!  :)