braking in a turn

[MustangGuy]

Well, actually it's more like "braking in a curve"...
My noobness is showing here, I'm embarressed to say. I returned to riding last year after almost 20 years. I wasn't a great rider back then, just lucky... I didn't have any knowlegable teachers, just my brother who wasn't much better of a rider than myself. Anyway, since I took the MSF course, I've probably logged about 5,000 miles of riding. In the course they taught us "No braking in a turn" but never really explained braking in a curve. Most of the time, I don't, but there are a couple of roads that I often travel that are curvy (not really really twisty though) and the have lights or stop signs that come up while the road is still curved - i.e., no straight stretch before having to stop. I slow down by letting off the throttle, but I still have to brake before my bike is fully upright. Of course, I do this smoothly and gently and I'm going slow by this time and I have not had a problem. Does this seem like the right thing to do?

 

[GOduc]

braking while the bike is leaned over is something you should only do while riding agressively, like on the track for instance. or in an emergency situation and is absolutly your last line of defense. your first line should always be to get the bike upright then grab brake.

and if it comes right down to it, feather the front and leave the rear alone

 

[Bossanova]

Its called "trail braking" at the track. [cool]

 

[MustangGuy]

Hmmm... this is more of a routine stopping situation. Maybe I'm not leaned as much as I think. I do know to get the bike upright in an "unplanned" stop. They made us do it in the course from 30 mph which may not be fast to an experienced rider, but you can still wipe out if you don't do it right. I think I'll analyze those intersections and what I'm actually doing when I come to them a little more closely. If I really do find that I'm braking before I'm upright, maybe I just need to slow down a little! Thanks!

 

[iabueller]

im completely the oppisite. and i have done track days and had instuctors tell me that they cant believe the control i have with it. If im in a corner carrying to much speed i trail brake with the rear. Alot of people are going to say not to but i can ease on the rear while still giving it some throttle. With doing that i dont lock the rear up. I know it sounds wrong but i have been doing it since the first day i stepped on a street bike. But for me by doing that i have a little more control on how much brake i apply.

 

[stellio78]

Great DVD that helped my riding http://www.twistofthewristdvd.com/home.asp

 

[Kid1620]

Do this get the book too. Memorize it and live it it'll save your life one day. ^^^^^^^^^^^^ highly highly recommend "Twist of the Wrist" to every rider new or life long rider it'll make everyone a better rider.

 

[MustangGuy]

I will check them out.

 

[LightningLevi]

If you think about it in a physics kind of way, each tire has a coefficient of friction, or certain amount of force that is applied to it before it breaks loose and slides. Braking slows you down because it increases force on the tire. When you lock up a wheel and slide a tire, you've put too much force on the tire and exceeded this coefficient of friction. Turning also puts force on the tire, putting you a little closer to the CoF. If you brake while turning, you get even closer to the CoF, and if you brake too hard while turning, you break the CoF and go for a skid. In other words, there's a limited amount of force you can put on a tire before it slides. When you're turning and you apply the brakes, you get a lot closer to this limit a lot faster. Same goes for cars too. Brake before the turn and accelerate in the turn. I probably suck at explaining it, but hopefully you catch my drift.

 

[christianmb]

Levi, not trying to argue by any means, more of an honest question. You are talking about the Cf and you mention that the Cf of a material is constant, but also thinking about it as a physics problem, does the Cf rely on the direction the force is being applied (i.e. if one force is applied in a direction, 0 degrees, and another force is applied at 90 degrees like in this example, would your argument hold absolute?

 

[victormonopoly]

Twist of the Wrist is a fantastic video. I brake in a lean if I HAVE to. Braking in a turn stands the bike up all on its own. Using the front while in a lean creates drag on that side of the tire causing it to turn towards that same side. This creates a counter steering effect and stands the bike up.

 

[LightningLevi]

Well I'm not an expert by any means lol. I think I know what you're saying about directions. I think that if you're in a turn and you brake, the directions more or less add the force in a 45 degree ish direction, right? If they oppose each other, they cancel out, but they don't in this case, so the forces still add to a bigger amount of force on the tire, pushing it closer to it's CoF. But it's been a long time since I took a physics class and I'm a compsci major, not a physics major lol. Please correct me if I'm wrong.

 

[ConVICK Pitbull]

BTW, if you up/down shift in a turn it will also have an effect on the CoF , as it were.

 

[OldGeezer]

The COF remains the same regardless of the direction the forces are applied, with the exception of forces which press the tire down or lift it up, which is why spoilers work to increase traction.

For a given weight on a tire the resultant of the forces applied to the tire must be less than the COF of the tire on that particular surface (COF is a function of the tire and the surface it runs on). So if you have two forces at right angles to each other working on the tire, ad they are equal, the resultant is at 45 degrees to each of the forces and equal the the square root of the sum of the squares of each of the forces and that resultant force must be less than the COF otherwise the tire starts to slip (squeal) or if it exceeds the COF by too much the tire just slides out from under the bike.

The other factor to consider in braking is the weight transfer. When you brake you transfer weight onto the front whell, increasing its COF as well as the loads on it, but decreasing the COF of the back tire thereby making it more likely the back tire will slip or skid. This is why trail braking works in a car, it unloads the back tires enough so the the centrifugal forces working on the tire exceed the COF of the rear tires and the back end slides outward 'pointing' the car further into the turn. The trick is not oversteering or reducing the COF too much.

Same basic physics apply to a bike, problem with a bike is that it is much less stable and if the back end drifts out while you are in a turn and leaned over at or near the limit, you probably go down because the back end going out increases your lean angle…

Similarly accelerating transfers weight onto the back wheel or wheels giving them more traction, which is why in rear wheel drive, rear engine vehicles you accelerate through a turn (gently so as not to overcome the COF) it also unloads the front tires some.

On a bike braking or accelerating in a turn has to be approached very carefully and gently and the results depends on a number of factors, the main one is how close to the limit of adhesion of the front and back tire you are at.

Another parameter is the type and shape of your tire. In most modern sportbike tire the size of contact patch increases as you lean the bike over - up to a point. The maximum area of the contact patch is usually when the bike is leaned over close to the maximum possible - so if you are riding that patch you are probably very close to the COF of that tire in those conditions and have very little margin for braking or accelerating.

There are some interesting slow motion videos of bike crashes on Mulholland which show exactly what happens when the rider braked too hard with the front or rear brake, or accelerated too hard, or lost traction because of gravel or water crossing the road or a tire crossing onto the middle double line (paint has a lower COF than pavement).

Riding the edge of control on the streets is a bad idea and likely to get you and the bike and bystanders hurt. There are too many unpredictable things that can happen. The track is the place for that kind of riding.

However that doesn't mean we have to drive like grannies going to church, our bikes are extremely capable and as long as we train, practice and know what we are doing, and ride with a decent margin to spare, you can brake in a turn or take other evasive actions which you'd never contemplate riding on a track.

Ideally, you want to get all your braking done and the car or bike settled before you turn into the curve. In club racing the motto was "slow in is fast out" and the guys that could repeatedly enter a corner at the optimum speed (not necessarily the fastest speed) always had the best lap times consistently.

 

[LightningLevi]

So.... I was right then? At least mostly, anyway?

 

[OldGeezer]

Yes Lightning, you were right, I just had to get long-winded about it and talk about the weight transfer, which is a very important factor in cornering

 

[coolhandgarreth]

Made sense to me. Got a chance to apply some of that force today, the longest amount of time spent just riding to ride. 6 hrs on twisty roads with some interstate to get me there and back. Awesome day for this newb.

 

[christianmb]

Thank you 'Geezer. That actually did make a lot of sense!

 

[OldGeezer]

Thanks guys!

You're welcome and thanks christianmb, I've always found the theory easy, doing it well with rubber on the road is much harder - and more fun

Sounds like a great day coolhandgarreth!