m/c tire tread, speed and bike weight

[wolbrink471]

Hello all!

Can anyone offer some shade tree explaining...

is there a relationship between the weight of the bike and the chance of chunking tire tread or is this an issue that only becomes a problem over a certain speed?

on a related subject, what are the issues that most affect wheel spin at speed?

Mark

[Seldom Seen Slim]

Wheelspin at speed -- I presume you imply wheelspin as in tracton loss?

The amount of horsepower that it takes to push a vehicle through the air is pretty well fixed by aerodynamics.  The amount of horsepower you can put to the ground is determined by traction -- contact patch, bouncing due to poor suspension compliance, stuff like that.

If the amount of HP needed to push you through the air is greater than the amount of horsepower you can put to the ground and you've got excess available hp to spin that tire -- your drive wheel will spin up.  Also, if the amount of power you've got available to spin the tire is greater than what you can put to the ground through the contact patch -- your drive wheel will spin up.

I think that's how it goes.  Add ballast to increase the amount of power you can put to the ground and you can go faster.  Add aer drag (a wing, for instance) to get more downforce -- and you likely lose speed 'cause the loss in aero efficiency in greater than the gain in traction.  I think.

As for chunking a tire -- (correct me if I'm wrong) -- doesn't that happen when the tire blisters due to localised extreme heating -- from a tire spinning, maybe?  That's how I hurt a tire -- too much wheelspin coupled with a too-new/too cold tire = blisters and rubber chunks a-flyin'.  Keep things under  control and chunking will be less of a problem.  Cure your tires with a few runs or some time on the dyno.  Warm them before a run.  Don't stay hard in the throttle when the tire spins up.

Okay, that's my two cents' worth.  Now let those who know what they're talking about show you the truth.

[wolbrink471]

ok SSS

i get the relationship between the force needed to drive the m/c through the air and the grip of the tire, but doesn't the force required to seperate the air increases exponentially with speed?

what speed does the rear start spinning? anybody even heard of a rear tire on a rs125 spinning at a speed of 138 or so?

is there a general rule of thumb or benchmark that is the starting point for this problem? i am not going to be anywhere near 200 mph and i wonder if 'wheelspin' is even going to be a realistic problem for me.

am i 'making up' problems?

Mark

[Dean Los Angeles]

Spinning the tire is horsepower and friction limited. A 1000 hp motorcycle is going to spin the tire any time the rider wants to dial it at any speed.
An RS125 can spin the tire right off the start, but at some point the power to the rear wheel through gear changes and aero drag will limit the engine to less power than is available to spin the tire. The other guy with an identical bike that weighs 10 pounds more/less than you can't give you valid info either. I guess you will find out! I doubt at 138 mph that you will have tire problems.

You didn't mention the track. Asphalt, alkaline lake beds and salt have very different coefficient of friction, and they all change with atmospheric conditions.

Tire chunking happens when the force on the tire exceeds the structural strength of the tire. Spinning the tire heats it up. Side wall flexing is the majority of the heat the tire sees. I've watched overloaded test tires go from ambient to 160 F in 30 seconds at the sidewall. Centrifugal force increases with speed and at some point the tire will come apart even if it isnt in contact with anything. The combination of load, heat, speed and inflation pressure determines when the tire will come apart.
At 200 mph the tire is loading and unloading at the contact patch 50 times per second. The DOT speed ratings have a safety factor, but with non-DOT racing tires it may be difficult to determine what the maximum speed would be.

[wheelspin]

At 130-140 MPH I dont think you will have a problem at speed, meaning you may get minimal slip at takeoff and low speed but when you get rolling it shouldnt be a problem.
Tires chunk at Bonneville because either they have been overheated and blister from excessive wheelspin, which weakens the tire structure ( molecular bonding) and centrifugal reaction throws the rubber off, or the tire was not designed to operate at the speeds intended and the rubber is thrown off because of the centrifugal force. This is why you shave the tires at Bonneville, you remove the unnecessary weight from the tire therefore reducing the centrifugal effects and the force the tire structure has to resist.
There are many complicated ways to predict when and how wheelspin will occur but unless you have an imbalance in the Hp vs drag scenrio ( like massive drag and a lot of power) you shouldnt need to worry about it. I rode my 200hp motorcycle at 178MPH and spun the rear tire up to about 140 then it settled down and went straight.
The rest of it I think Slim explained well.

[Stainless1]

You will always have some wheel spin at Bonneville, you will see it as slip.  When you calculate your tire size, gearing and RPM, your will expect to be traveling a speed.  Typically if you get really good traction, your slip will be 3-7% on the salt.  When you start approaching 25%, you will probably end up with some blisters and tire problems.  We saw 19% at 180ish, the tire was hot but no visible problems.  Weight reduced slip, increased speed and lowered tire temp enough to notice. 
Will you have a problem at 140?  I think that will depend on the tire and the amount of slip.

[wolbrink471]

thanks for all the great info...

but now i am wondering what effect the tread pattern has on traction on the salt at race speed?

it looks like most of the light bikes run gp slicks, anybody grooving them?

mark

[firemanjim]

Mark,you don't really see too many slicks,I would say that a treaded tire would work to your advantage--ala knobbies on a dirtbike.

[Loose Goose-Terry#1]

  Mark, I?m not an expert but I suggest using treaded tires for traction at Bonneville. You can count on about 10% tire slip (spin) when you approach 100 mph. Tire sidewall flex will be a factor in heating your tires also. Keep a close eye on tire pressure to limit the sidewall flex. Your tires shouldn?t be a problem running between 100 and 150 mph. Be careful about tire temperatures. You should keep your tires between 130 deg. And 150 deg. If they sizzle, they are tooooo hot! I have had my tires up to 184 deg. with no chunking but I don?t recommend running them that hot.

[Larry Forstall]

Run any tire that has the proper speed rating for your intended goal. Tire spin is a function of torque and a 125 does not have enough to cause a problem. Your challenges will be with aero and mechanical drag. The thinking man's racer. Good Luck and have fun.

[bak189]

I like to add me 2 cents worth......if you go 138mph on a RS125c.c.  "sit-on bike" at Bonneville....."you are the MAN" 

[Freud]

I can't see the possibility of much grooving on a shaved tire.
FREUD

[wolbrink471]

Thanks for the input and all the err uhmm encouragement!

When a tire is shaved, how much of the tread is left behind? I 'assumed' a shaved tire was cut down to a profile that resembles a slick.

also, is the problem with slicks because of a lack of tread or does the race compound grease out on the salt?

Are 'salt' tires used at Maxton?

Thanks
Mark