Oversized tyres? - Page 4 - Honda Rebel Forum
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post #31 of 52 Old 06-06-2018, 10:46 AM
 
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Yes, like normal conversations, it seems that one thing leads to another unrelated to the original subject matter. Sorry about that...

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post #32 of 52 Old 06-06-2018, 11:31 AM
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I think the thing to remember is that the OP's question was addressed. Where the conversation goes from there isn't critical.

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post #33 of 52 Old 06-21-2018, 01:26 AM
 
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Bigger front tire

Quote:
Originally Posted by Black 250 View Post
I'm wondering whether anyone has had problems with installing oversized tyres? ie handling, fitment under guards and safety etc.

I'm proposing the idea of front 100/90-18 and rear 140/90-15 'MT 66 Route' from Pirelli. Anyone have any ideas as to not go larger and stick to OEM standards or go for the wider tyres? Pirelli do the standard tyre size for the Rebel as far as Im aware as well. Thankyou in advance to who may comment, much appreciated
I ran across a youtube series with Mike and Melly called Melly's Project. They are customizing an 87 250 and they put a 120-90-18 on the front by doing some custom work on the forks and getting a Honda Shadow front fender that fits with a little mod on the mounting holes.

I'm thinking about doing it later this year. Living in the high desert the main curves I run into are my gravel drive way and making turns.

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post #34 of 52 Old 06-22-2018, 10:40 PM
 
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I just wanted to chime in and say that as someone who's ridden for a long time and has a background in physics, Duckster is absolutely 100% correct about gyroscopic forces playing very little, if any role at all in the stabilization of a bike. If it was gyroscopic forces, you wouldn't be able to lean and bikes would not self-right.


Skip to about 25 seconds in. Notice how the ghost bike rights itself the moment it's off the grass.

The idea that gyroscopic effects have anything to do with it sounds intuitively right until you actually think about it. Gyroscopic effect can be used to prevent something from falling over, but that's because gyroscopes prevent things from leaning, and motorcycles lean quite easily.

Suffice to say there are likely many things going into it, and gyroscopic forces are why it's harder to lean a bike when you're going super fast, but they aren't the stabilizing force as demonstrated by the above ghost bike which actually stands back up on its own... That defies the way gyroscopes behave.

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post #35 of 52 Old 06-27-2018, 10:02 AM
 
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Um, I will politely disagree.
Gyroscopes do not prevent things from leaning. Through precession they translate a non-coaxial moment vectors by 90° (changes a lean into a turn).
The ghost bike's front wheel turning into the direction the bike is leaning exactly defines gyroscopic precession.
So does the penny you roll down the hallway and instead of just flopping over on its side when it leans, instead it turns into the direction of the lean and follows the natural balanced line of the curve.
Fully gimballed gyroscopes do want to maintain their orientation and are useful for non-magnetic compasses and artificial horizons in airplanes. Trapped gyroscopes with sensors at the bearings to measure the precessional forces are useful accelerometers for navigation positioning systems that do not use satelites (submarines, apollo spacecraft, aircraft backup positioning system)

Yes I agree there is more to a motorcycle's stability in a turn than just gyroscopic precession, but it is a vital part.

sorry about the rant, I'll go crawl back under my rock now....
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post #36 of 52 Old 06-27-2018, 11:20 AM
 
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Consider this conversion kit for dirt bikes... Notice there are no wheels on it, therefore no gyro effect at all, and yet the thing works in the snow much like a motorcycle works on dirt.
The gyroscopic effect of each individual wheel on a motorcycle is very much a secondary effect on motorcycle stability compared to the effect of the frame and fork geometry in linking the front and back wheels into a single track self steering unit chassis that is inherently stable.

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post #37 of 52 Old 06-27-2018, 12:56 PM
 
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Kryton, in that demonstration you're showing there, the angle the gyroscope is leaning at does not change. Thank you for proving my point. You will note that it is only turning along 2 of its 3 local axes. The third axis, which is offset from the direction of gravity, is being prevented from rotating. If you look at the animated gif and the wheel, you will note that its angle in relation to gravity is not changing at all. So your gif, which is supposed to prove me wrong, only serves to demonstrate the very point I was making.

Gyroscopic precession does NOT cause the front wheel to turn toward the bike's linear angle of motion. IF it did, you're talking about it never being able to right itself.

Gyroscopic stabilization relies on Newton's Second Law of Motion, specifically in regard to preservation of angular momentum. The faster the wheel spins, the more force it takes to change the angular momentum. Once again, this is why bikes are harder to lean at a high velocity relative to the ground than at a lower velocity relative to the ground.

Bikes leaning toward the vector of motion happens at any speed. It happens at speeds at which any gyroscopic forces applied to the system are too weak to move that much inertia. Remember, it's about momentum, not about simple s rotation. Before any gyroscopic force, precession or otherwise, can even take effect, it has to have enough kinetic energy stored in the angular momentum of the wheel that it's capable of moving the mass of the bike. Again, bikes are harder to lean at high speeds but easy at low speeds.

It is extremely easy to demonstrate that gyroscopic precession does NOT cause the wheel to turn when the bike leans. All you have to do is pick up the bike with nothing spinning and lean it. This is a lot easier to demonstrate with a bicycle, obviously. Pick up a bicycle hold it level with the ground. Now lean it. The front wheel will turn toward the lean angle.

Did you do it? If you did, you just demonstrated the wheel having that behavior occurs even WITHOUT gryoscopic precession, and you demonstrated it by having the wheel not spinning while you did it.

As you know from having enough experience to have a casual understanding of counter steering, that if the wheel tries to do that in a lean, while moving, the bike is going to right itself... and that behavior is present even when the wheels aren't turning.

Here's a video demonstration:


If the behavior occurs even when the wheel is not spinning, gyroscopic precession is completely ruled out as the cause of the behavior.
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post #38 of 52 Old 06-27-2018, 04:58 PM
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post #39 of 52 Old 06-28-2018, 08:39 AM
 
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I am enjoying this friendly discourse,

In the gif, and in the vector diagram, a rotational force (leaning) is being applied and translates to a twist force (turning)
Take the front wheel off a bicycle and roll it down the driveway and watch it turn in the direction that it leans.
Yes the rake & offset geometry will cause a non-moving bike's front wheel to flop in the direction of the lean due to gravity, all the way to the stops, which is not what is experienced when the bike is in motion. If that were the sole force in play than any lean would twist the front wheel all the way to the stops and high-side the rider into the tree limbs. it would be slower with more angular momentum in the front wheel but it would still accumulate continuously, the same way gravity curves a bullet's trajectory toward the ground no matter how fast the bullet is moving.

Bicycles and motorcycles self-straighten from a turn (if moving fast enough) due to the center of mass (center of inertial moment) being above the point of action from the wheels, moving the contact point with the road, while the center of inertial mass tries to continue forward, bringing the wheels under it.

I found a better illustration from the physics department of Georgia State University illustrating lean induces turn...

And please stop dropping my microphones, I spent a lot of money on the PA gear for my band and that vandalism just cost me $160 to replace the Shure Beta58A you just ruined (Bwahahaha)
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post #40 of 52 Old 06-28-2018, 08:52 AM
 
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a related point is why don't motorcycle tires have a flat tread like cars? (like this batmocycle...)
a 2 wheeler would have to lift up its center of gravity to lean..(as you can see in the pic).

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