Chassis Geometry 101, and why using the rear brake on the racetrack can be ok. - Page 3 - ZX6R Forum
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post #31 of 39 Old 11-17-2014, 11:50 AM Thread Starter
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Next question:

Why would a rider want to slide the rear tire?

We covered some of the technical info on this above but we really didn't discuss why would we want to do it in the first place.

We have all seen the pro's do it on TV. Rear ends drifting effortlessly around nearly every corner while the rear wheel is spinning. Other than some truly impressive slow-motion videos, why is that desirable?

Well, this would be a good time to post some bike pr0n from this last seasons MotoGP races. The opening sequence you can see Marquez drifting around a corner like it was nothing.

MotoGP? Indianapolis 2014 ? Best slow motion - YouTube

To begin, we need to make sure that everyone understands that when you are at the edge of your tire, your traction is at its lowest point. This means we probably cannot accelerate as hard was we would like for fear of losing the rear end. This seems like common sense but it needs to be said.

Consequently, it would make sense that in order to make full use of the horsepower of our machines, we want to get off the edge as quickly as possible and into the 'meat' of the tire so that we can give the bike full throttle.

Well, how do we control that? Isn't a corner a set radius which requires a certain lean angle to get through? Well, yes and no.

If you have managed to get through all of the crap that I have written here you probably have an understanding that racing motorcycles well has a lot to do with bending the laws of physics. This is another such case.

Behold my awesome MSPaint skills!

The yellow line is the traditional path through a turn that a motorcyclist might take. The rider tips in and his bike follows the same lean angle until he exits the turn and gets back on the gas. The line is a perfect arc and both the front and rear tires are on the yellow line throughout the entire turn.

Now, check out this next example.

Someone way smarter than me came up with the magic number of 5%. That is supposedly the optimal amount of rear wheel spin to have, meaning your rear tire is spinning 5% faster than you are traveling.

By intentionally forcing the rear into a controlled spin, it will cause the rear tire to step OFF that yellow line (red line in this example). Doing so effectively turns your bike in the direction you want to go. You are 'cutting the angle' of the turn. Once your bike is pointed in the right direction, you can stand the bike up a lot sooner than if the rear tire were still on the line. You are essentially pivoting the bike on the front tire by allowing the rear to step off the line.

Meaning that by spinning the rear a rider shortens the amount of time they will need to be on the edge of the tire, and maximizes the amount of time they will be on the meat of the tire, and thus accelerating as hard as possible.
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post #32 of 39 Old 05-14-2015, 11:23 AM
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In the conversation of rear braking, it must be mentioned that engine braking is a huge variable often ignored in the equation. As a very fast rider enters a corner he is applying huge amounts of front braking force which shifts all weight and traction off the rear tire while feverishly downshifting. If your engine braking is very strong, the rear tire won't have much grip on corner entrance and will be sliding around quite a bit. You can see a lot of riders "backing it in" on corner entrance because their engine braking from downshifting (this is exaggerated if they don't know how to rev'match and rely on the slipper). If your engine braking is very strong, you are ALREADY applying "rear braking". Since engine braking is adjustable via engine tuning, some riders prefer to not have much and instead use rear brake. Some corners you cannot downshift into a high rpm to produce strong engine braking, so in those corners it may be advantageous to use more rear brake. LOTS of variables involved, and riding style and preference. Lots of ways to go fast, plenty of stupid fast riders out there (including gp level racers) that never touched the rear brake. Hell in a few years combined ABS will be standard and the art of rear braking will pretty much be dead (computer will do it all for you). For those of you who know anything about 2 strokes, you will know they don't produce hardly ANY engine braking, so the art of rear braking really stems from the good 'ol days of 2 stroke motorcycle road racing where this skill was an absolutely vital technique.

CCS #958

Last edited by sonicnofadz; 05-14-2015 at 11:52 AM.
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post #33 of 39 Old 09-30-2015, 07:13 AM Thread Starter
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BUMP to answer some recent PM questions...
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post #34 of 39 Old 09-30-2015, 07:48 AM
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Wow great write up Michael!

I never saw this one before.... I cant wait for you to collectively put all these together
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post #35 of 39 Old 09-30-2015, 08:02 AM
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Superb work blending in not just the technical details along with the science behind everything but also including real world applications and visual examples.

Damn good work

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post #36 of 39 Old 10-07-2015, 02:58 PM
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So awesome man.... should help when I try to explain rear braking to a new rider. The guy doesn't listen so much... lol.
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post #37 of 39 Old 04-22-2016, 10:46 AM Thread Starter
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BUMP! Tons of suspension questions lately. This should be a good primer for someone looking to delve a little deeper into it.
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post #38 of 39 Old 04-22-2016, 12:29 PM
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To add in: my previous opinion was 'Don't use the rear brake on the street'. This is because, in stopping situations, I know I can stop with the rear wheel in the air, and using the front instead of the rear means I'm not going to slide the rear by overapplication.

While I still use the front essentially exclusively for coming to a stop or bleeding speed in a straight line, I've found I really like the rear brake for speed adjustments entering a corner.

When using the front brake, it compresses the suspension, decreasing rake and thus decreasing stability. I found that I could fine-tune my speed entering a corner WITHOUT compressing the front forks by keeping it in a higher gear and using engine braking. This keeps the suspension more stable so that I turn more confidently and smoothly.

That's great when you've approached the corner at the right engine speed/gear, but if not, the rear brake behaves in a very similar manner. It bleeds speed from the back by 'pulling', avoiding the heavy weight transfer and fork compression associated with the front brake. This makes for a nice, smooth corner entry.

So, while I stand by Front Brake Only for emergency braking, stopping, and other inline applications, I have found that the rear brake can be helpful in some scenarios.

Never ride faster than you can stop
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post #39 of 39 Old 04-22-2016, 01:09 PM
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I just read through this again. Awesome stuff. Thanks for such an awesome contribution to this place.
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