Skinny vs fatso tire cornering

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I was shocked that my spiffed up Commando could run circles around my spiffed up SV650. I am pleased with the ease to guide a Commando, but displeased with the efforts on fat tire appliances i've tried. I'm fascinated on what gives one such a distinct advantage. Tires are a major factor, but how much? A lot of interesting concepts in a couple forum pages below. I sniped out a quote that struck home most with me. Its Commando related in the tire and bike sizes covered.

http://www.sportbikes.net/forums/track/ ... speed.html
My last trackday, there was a guy out there on a RS125 and he was flying through all the corners taking the boys on the bigger 600s on the outside!! There is something extraordinary about flogging those little bikes around the track, it just looks like so much fun. I was watching the dude from the Paddock and could just imagine the frustration the rider was having when all the other guys would pass him on the straights. But my..... how quickly he would pass them in the corners. It was a delight to watch.

hobot -
even when past Peel got passed in the long runs the fun was all hers.
 
Don't think that's anything to do with tyres is it? The lighter bike needs less lean angle to go round a corner at the same speed as the heavier bike. Therefore when the heavier bike is at full lean the lighter bike can still lean further which equals faster corner speed. Friction and tyres are beyond me as well...
 
Since a motorcycle is two casters, the first being the trail of the front end and the second being the rear wheel following the front, many things can effect how these two casters work together to keep the machine stable. As the back tire gets larger in section, the front end has a more difficult time getting the rear to follow. One of the comments made about the Hinckley Rocket III was that the rear tire tended to steer the front, which is more or less meant to say the rear was so large, it resisted the fronts efforts to lead the steering. The new "touring" model has a much smaller section rear tire to facilitate easier handling. If you compare the steering geometry of a modern machine, with its larger disparity between front and rear tire sizes, to an older one, you will note they are a few degrees steeper. This steepening of the neck angle quickens the steering a bit to make up for the reluctance of the rear tire to turn as quickly as the front. Trail has been added back in by decreasing offset of the centerline of the fork tubes from the neck pivot. Harley touring models have their necks set in front of the fork tubes to create enough trail to make them stable yet steerable. A somewhat larger rear tire is required to cope with increased horsepower, but going overboard for looks, such as in the case of the original Hinckley Rocket, can have negative consequences.
 
FastFred said:
....The lighter bike needs less lean angle to go round a corner at the same speed as the heavier bike. ...

Sorry, but that is simply wrong. The mass is present and equal in both forces, lateral and vertical, and therefore has absolutely no influence at all on the angle of lean of a motorcycle. None whatsoever.

Tyre width in conjunction with CoG height on the other hand has because the angle of lean refers to the actual line through the tyre contact patch and the CoG. The contact patch "travels" laterally on the tyre and obviously this effect is bigger the wider the tyre is. Therefore a wide tyre and low CoG means high angle (choppers...) and narrow tyre and high CoG means low angle (race bicycle...) for the same speed and corner radius.


Tim
 
Moderator alert: someone has stolen hobot's password and posted in disguise.

You can tell because the post is the full width of the screen, something hobot would NEVER do.
 
Further to Tim's post, the shorter the wheelbase the less lean angle needed to corner at the same speed, all else being equal.

Hobot, I see where you are coming from with a lot of your stuff about tyres and cornering. What we must remember is that current tyre technology is based on what is best for racing which is then transferred to road bikes. The syle of cornering has changed much over the years with the tyres built to suit a certain style. In the older times, lines were smooth with more time spent at maximum lean. For many years now the plan is to charge up to a corner, brake hard all the way to the apex, throw the bike into the corner and then give it the berries to pick it up and get down the next straight. If you ride a modern sports bike, they don't necessarily feel "nice" while cornering. The width and compounds are to handle the braking and acceleration stresses on the tyre whilst leaned over. When I corner on the Norton, I smile a lot. Having said that, if you try classic cornering lines on a race track today, you'll have five riders up the inside who will then totally ruin your speed through the corner. Such is life. The other reason the Norton feels great at any lean angle is............you guessed it, same tyre profile and size front and rear. Ride one of those hideous things with a trendy 240/300 whatever profile and tell me you like the way it handles. Don't think so. If you want a good laugh, watch a Motard race at club level. Backing it in, all the tricks, but they have no idea what they are doing, but the professionals are doing it so it must be faster. Or, the boy racers on the road, getting their arses off the seat and dropping the knee down which is still two feet off the road. It's all about style folks.

Also, as modern wide low profile tyres wear, the feel turns completely to crap and becomes the equivalent of driving a two wheeled bus, especially the front. My Buell is like that at the moment. The front tyre is deceased. My Norton still feels great even with none of that nasty tread stuff left. I rode a mate's Hayabusa a few years ago with a worn front tyre and it was almost unrideable. Still had legal tread depth but felt truly awful.

Keep at it Hobot, I like your style.
 
Sorry, but that is simply wrong. The mass is present and equal in both forces, lateral and vertical, and therefore has absolutely no influence at all on the angle of lean of a motorcycle. None whatsoever.

The mass is greater in the heavier bike because it's larger. Centripetal cornering force equals mass x velocity squared divided by radius (constant). The c of g is also higher on ther heavier bike. It's 30 yrs since I studied the free body diagrams of a motorbike. Please expand on your theory...
 
FastFred said:
Sorry, but that is simply wrong. The mass is present and equal in both forces, lateral and vertical, and therefore has absolutely no influence at all on the angle of lean of a motorcycle. None whatsoever.

The mass is greater in the heavier bike because it's larger. Centripetal cornering force equals mass x velocity squared divided by radius (constant). The c of g is also higher on ther heavier bike. It's 30 yrs since I studied the free body diagrams of a motorbike. Please expand on your theory...

As per usual there are laws of physics in the Southern Hemisphere that are different from normal parts of the world. :mrgreen:
 
FastFred said:
The mass is greater in the heavier bike because it's larger. Centripetal cornering force equals mass x velocity squared divided by radius (constant).

Wrong. The radius is depending of the CoG height and thus not constant. The lateral forces are fed into the mc at the contact patches but they depend on the cornering radius of the CoG. Just imagine an infinitely high CoG, the bike would corner with no lean at all.

r_eff = R * (1-sin_alpha)

The lateral force at the CoG needs to be counteracted by the lateral force fed into the contact patches. The sum of forces in lateral direction is zero.

F_horizontal = 0 = m * a_horizontal - F_tyre,lateral

The moment the lateral force at the CoG has around the contact patches (which is the roll axis of the bike unless we're considering heavily sliding race bikes.....) needs to be counteracted by the gravity.

M_tot = 0 = m * a_vertical * s_horizontal - m * a_horizontal * s_vertical = m * g * h_CoG*sin(alpha) - m * a_lat * h_CoG*cos(alpha)

The sum of the moments around the roll axis is zero and the horizontal or vertical distance obviously depends on CoG height and angle of lean, sinus for one and cosinus for the other (depending on how you do the reference).

And finally the sum of the forces in vertical direction must be zero as well. This means that the vertical force at the contact patches must equal the weight aka m * g.

As you can see the mass is present in both lateral (what you call centripetal) and vertical force and thus has no influence on the system in total. It is simply reduced from the equation system.

The c of g is also higher on ther heavier bike.

Wrong again. The CoG height depends on the mass distribution and not the mass itself. A chopper has a lower CoG than a race bicycle (apart from bikes like my recumbent, that is....;-) )

It's 30 yrs since I studied the free body diagrams of a motorbike. Please expand on your theory...

It's not my theory....


Tim
 
Thanks Tim and sorry for misleading everyone so badly. There are some great people out there to deal with your attitude problem... :mrgreen:
 
Well, Fullauto, my usual ride is a 167BHP k1200s, which has a 190x50 rear tyre. This thing handles superbly.
But, on corners up to around 80mph, I can get the Commando around them faster. It actually, as you say, feels nicer.
I do have conversations with other people about this, & of coarse, they don't believe me.
Just wish I could get rid of my high speed weave, which is the only thing that lets the Commando down.
 
Flo said:
Well, Fullauto, my usual ride is a 167BHP k1200s, which has a 190x50 rear tyre. This thing handles superbly.
But, on corners up to around 80mph, I can get the Commando around them faster. It actually, as you say, feels nicer.
I do have conversations with other people about this, & of coarse, they don't believe me.
Just wish I could get rid of my high speed weave, which is the only thing that lets the Commando down.

Some of what I've read says that it's not the size of the rear tire as much as the difference between the front and rear tire size which lends to the "feel".
 
FastFred said:
Don't think that's anything to do with tyres is it? The lighter bike needs less lean angle to go round a corner at the same speed as the heavier bike. Therefore when the heavier bike is at full lean the lighter bike can still lean further which equals faster corner speed. Friction and tyres are beyond me as well...

An RS125 (Aprlia or Honda) are 2-stroke race bikes. On track days I've noticed that most bikes are street bikes with lights etc. A decent RS125 will weigh about 200 lbs and will put out at about 40HP. A 600 street bike will usually weigh over 400 lbs and put out 120 or so HP. Depending on the rider's weight, the lb/HP may not be all that different.
 
Just from my own experience of riding a lot of modern sports bikes, including my GSXR 750 K6, they handle superbly well and feel great if you have the right choice of tyres, if you inflate them to the correct pressure and if the rear does not have a flat on it. A slightly narrower section tyre seems to work better on these bikes, especially for the track. My Suzook has a 180 section rear, which speeds up the steering a bit. They are fussy, but then look at the difference a set of Avon Roadriders makes to a Commando instead of the TT100s and Roadrunners they used to be shod with.

Hobot — I was lucky enough to ride an RS125 Honda (if that's the machine I think you mean). As you know, these are proper, thoroughbred GP bikes, designed to do only one thing — circulate round a track as fast as possible. And they are very good at it. At first, you have to re-calibrate your braking and peel-in, otherwise you brake and then have to accelerate again to the peel-in point! You can ride around the outside or underneath almost any road-based bike. Those and the 250 racing strokers ruin you for anything else on a track.

Dave
 
Flo said:
Well, Fullauto, my usual ride is a 167BHP k1200s, which has a 190x50 rear tyre. This thing handles superbly.
But, on corners up to around 80mph, I can get the Commando around them faster. It actually, as you say, feels nicer.
I do have conversations with other people about this, & of coarse, they don't believe me.
Just wish I could get rid of my high speed weave, which is the only thing that lets the Commando down.


Fork Brace
 
Danno said:
Flo said:
Well, Fullauto, my usual ride is a 167BHP k1200s, which has a 190x50 rear tyre. This thing handles superbly.
But, on corners up to around 80mph, I can get the Commando around them faster. It actually, as you say, feels nicer.
I do have conversations with other people about this, & of coarse, they don't believe me.
Just wish I could get rid of my high speed weave, which is the only thing that lets the Commando down.


Fork Brace

More like rear isolastics and swingarm bushes I would think.
 
When Kenny Cummings teased me and my buddy to watch him at Barbers. I was awed by the whole show but those 125's blew me away watching them spank everything through the turns - it took almost half the straight for the hp bikes to over take em - again. There is no doubt in my mind that if they had the power they would have plenty of traction to not be caught up with but lap the others.
I'm only hear-say familiar with 125 race bikes behavior, mostly from fella that rode em when the triangle profile tires were the rage. He said that he ran right at the apex then flung bike over to max and as sharp as max. Most turns could have many lines of attack with such on rail hook up.

Hobot — I was lucky enough to ride an RS125 Honda (if that's the machine I think you mean). As you know, these are proper, thoroughbred GP bikes, designed to do only one thing — circulate round a track as fast as possible. And they are very good at it. At first, you have to re-calibrate your braking and peel-in, otherwise you brake and then have to accelerate again to the peel-in point! You can ride around the outside or underneath almost any road-based bike. Those and the 250 racing strokers ruin you for anything else on a track.
Dave

Man oh man Dave your 125 description sure matches the fun I have on Peel.
Had to recalibrate my braking earlier or non at all to be on max acceleration by peel in. Could wait till very outside edge to fling a turn or hug the inside curb.
Could just wait for others to commit to an apex line so no chance to collide then pick any clear path out of there and be gone by time they started exits.
So is Peels advantage turning below the ton tire size or dampened compensating chassis twist? Peel's mass is almost twice a 125 but don't know about tire size differences yet. Hope to find out what 3X's power means to my tire philosophy.

Anywho like 125's GPs, Peel did not need much power to get bored with 1200's of various brands, as long as there is more time spent leaned a bit than straight up straight ahead. Ask those 1200's riders where they can use that power and they'll tell ya -any where they want, can zip up 150+ in seconds, as long as straight ahead and mostly upright. I know Peel could hook up more power on leans, though not when she was still a factory isolastic, nor Trixie Combat which stays a factory isolastic pleasant museum ride noy a sports bike spanker.
Will see how much more power can be hooked up on narrow rear someday.
Will see if 125's spank Ms Peel or not in their element too some track day.

hobot
 
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