Wednesday, August 05, 2009

Trail Braking.

Recently I did a post dealing with using the rear brake for speed control in low speed turns. There is often a bit of confusion between this technique and trail braking. There is a significant difference in the two techniques. While dragging the rear brake is used in low speed turns, trail braking is the opposite. Trail braking usually involves just the front brake. It's also used in cornering at higher speeds.

I'll explain trail braking so everyone understands what it is. However, I again repeat my urgings for riders not to use trail braking in street riding. It really belongs on the track. Some folks may argue that point with me. We'll come back to that later.

Essentially, trail braking involves staying on the brakes past the point where the bike begins to lean. Typically the brakes are gradually released ( or trailed off ) up to the point of apexing. Thus, the term "trail" braking. The lean angle of the bike is increasing during this time period.

The reasoning behind this technique involves accomplishing several objectives. Cornering quickly on a racetrack demands a lot of the front tire. Trail braking puts weight forward onto the front tire, increasing available traction. The weight transfer also compresses the front forks. This changes the steering geometry. The downside is that the bike becomes less stable. The upside is that the bike is more willing to make rapid directional changes. It's part of what riders call "flickability". Scrubbing speed in a corner means the bike can make a tighter turn. Quite useful if you're trying to stay tight in the corner and ride a defensive line.

Trail braking, when done correctly, can help make more precise, smooth, adjustments in corner entry speeds and line selection. Notice closely the words "when done correctly".


Trail braking is an advanced technique with a steep learning curve. In other words, there's an extremely small margin for error. That's why I strongly advise against the use of trail braking by street riders. Let's take a closer look at it.

The longer a rider stays on the brakes, the longer it is before the throttle can be applied. Remember, the idea of trail braking is to keep the weight forward. There is some overlap between braking and throttle. However, the amount of throttle application has to stay small or the weight will come off the front wheel again. The bike is more unstable which enables quicker changes of direction. So, in trail braking, the throttle really isn't applied until the apex is reached. It's the throttle application that gives the bike real stability. In racing it's a risk versus reward proposition.

One the street, however, the risks far outweigh the rewards. Applying the throttle before the bike leans is the best choice for a couple of reasons.

Firstly, rolling on the throttle lifts the bike and extends the suspension. Particularly with the front forks. This is important to street riders because the roadway conditions aren't as predictable as on a racetrack. In the real world we are going to encounter a lot of pavement irregularities, rocks, etc. If the suspension is extended, it can absorb the bumps without throwing our lines off. It's a critical aspect that's often overlooked.

Secondly, this lifting of the bike gives us maximum ground clearance. Ever feel a peg scrape? What do untrained riders do? They roll off the throttle. If rolling on lifts the bike, guess what rolling off does? You got it. Rolling off the throttle drops the bike. The scraping sound has already told us we are getting close to the extent of our ground clearance. Dropping the bike by rolling off the throttle puts the rider into deficit spending. That's when hard parts like mufflers and centerstands impact the roadway, thus lifting the rear wheel off the road and shooting the bike into the weeds, or worse.

Much, much, preferable to be on the throttle before we lean. If we're worried because we scraped, keep steady throttle while easing off the press. Eyes need to focus on the solution, not the problem. Plainly stated, keep looking where you need the bike to go!

This photo came to me via e-mail a while back. It is not of my originality. The picture was so cool I had to share it. Credit goes to the party listed in the bottom left corner. I apologize in advance if they're offended that I shared it!

Back to trail braking. Probably the biggest hazard with trail braking is that the front brake is being applied while the bike is at a serious lean angle. There's only so much traction available. It has to be shared between braking and leaning. That's why so much finesse is required. It's rarely possible to tell the exact moment when the demand suddenly outweighs the supply. It only takes a tiny bit too much pressure on the front brake lever to spell disaster. At the same time, there's another factor involved.

Remember the need to quickly change direction? Here's how it works. A motorcycle will want to stand up as the front brake is applied. This "standing up" gives the bike a head start preparing to turn in the opposite direction. The rider, though, has to maintain pressure on the handlebar to keep the bike turning until the precise moment they're ready to let the bike quickly flick the other way. That conflict of interest adds to the hazard level. Like I said, trail braking is a very advanced technique that is really best left to the track. There's really too much risk if the rider gets it wrong. And the chances of getting it wrong are huge. The rewards are pretty small in comparison.

Take a moment and think of the crashes in Moto GP racing. Arguably these are the best riders in the world on the most high tech bikes available. The two main causes of crashes are a high side caused by rear tire grip problems and washing out the front wheel. Why does the front wheel slide out? Too much pressure on the front brake lever while leaned over. Yes, while trail braking. It happens a lot. If the best riders in the world often get it wrong then the average street rider shouldn't be doing it.

I'm not saying there isn't a place for trail braking in the real world. Certain circumstances may dictate its use. The rider needs a very high skill level to use this tool, though. No matter the skill level, trail braking shouldn't be a regular habit for a street rider. A rider needs to have all transitions done before leaning the bike. During the corner, all the rider should be doing is keeping their eyes on the target, keeping steady or slightly increasing throttle, and pressing to keep the bike in the lean.

Rather than try to develop skill at trail braking, better to gain precision in cornering. The better the rider is at being precise in corner entry speeds and line selection, the less actual need there will be for things like trail braking.

Confusion is often generated because some riding schools will say trail braking should always be used. Just remember that, while track instruction has benefits for the streets, the very line between street riding and track riding can become easily blurred in these schools. It's a vital point to be cognizant of.

On the one hand, some very top notch riders run these schools. On the other, everyone will agree that Valentino Rossi is a very top notch rider himself. Interestingly, a lot of high level racers don't ride on the streets much. Rossi has, and does. Rossi himself says trail braking is for the race track and not the street.

This is probably more than you ever wanted to know. Bear in mind that a key to staying on top of things and preventing premature mental degradation is exercising the brain!

Miles and smiles,

Dan



6 comments:

Steve Williams said...

I can always count on you Dan to explain technique and put it in a perspective I can understand. Thank you for all you do in this area.

I won't be practicing or putting trail braking in my rider toolbox. I consider myself an enthusiastic amateur with good basic skills and a healthy dose of common sense and a natural inclination to not push limits. At least speed limits. So no need for trail braking.

I'll follow your advice and the advice of Mr. Rossi. How can I go wrong?

Steve Williams
Scooter in the Sticks

bobskoot said...

Irondad:

You are such a good teacher, and explainer. I now understand the theory but, like Steve, I do not push the limits and I know better not to overuse the front brakes while cornering. I am in the habit of slowing down and gearing down before the corner and gently powering out.

bob
bobskoot: wet coast scootin

kz1000st said...

"Firstly, rolling on the throttle lifts the bike and extends the suspension. Particularly with the front forks. This is important to street riders because the roadway conditions aren't as predictable as on a racetrack. In the real world we are going to encounter a lot of pavement irregularities, rocks, etc. If the suspension is extended, it can absorb the bumps without throwing our lines off. It's a critical aspect that's often overlooked."

Dan, are you sure about that? My ancient drive theory says that Shaft drive bikes lift the rear when accelerated and chain drive bikes squat in back. It seems to me that backing off slightly on the throttle on a chain drive bike gives you more ground clearance and suspension.

irondad said...

Steve Williams,

Thank you, I try. The more we can understand the better we can take care of ourselves. Your second paragraph is golden advice.

Bobskoot,

Between you and Steve, I'm blushing. Your corner entry method is spot on!

Take care,

Dan

irondad said...

kz1000st,

Thank you for the question. It shows you are capable of intelligent reasoning. I find that extremely refreshing in today's society!

You have a correct, but partial picture. Let's zoom out and look at the whole bike. Keeping that picture in mind, zoom back in, but this time focusing on pivot points.

For side to side motion, the bike pivots up at the steering head. When a bike "wags its tail" it's actually most of the bike that moves. The front tire stays pointed in the original direction. The whole rest of the bike is what actually moves.

For up and down motion, the bike pivots where the swing arm bolts to the engine casing.

Thus, rolling on the throttle puts upwards pressure at this point. As a whole, the bike is lifted. It's not like it magically floats straight up in the air. Rather, it's more of an angle. The closer you get to the front of the bike the greater the angle. This is where the bike gains its overall ground clearance. At the same time, the front forks are extended. This is really the critical component in absorbing bumps that might otherwise move the bike to another line. The rear suspension is important, but not as critical as the front in this regard. The whole bike follows the front tire, as it were.

Since there is no intermediate pivot point along the swing arm on a chain drive bike, the dynamics of the swing arm geometry and the action of the chain around the sprockets do create a squatting effect on the rear tire, pushing down onto the roadway. Interestingly, it's this pressing down at the back that exerts leverage to lift the bike at the pivot point.

Shaft drive bikes have an intermediate pivot point where the drive shaft meets the pinion gear in the final drive. It acts like a transfer case in a four wheel drive and changes the direction of force.

Either way, despite differing actions at the rear, the net effect of rolling on the throttle is to lift the middle of the bike which creates the condition we're after.

Suspension dynamics is an extremely complicated field. I'll be the first to admit that I am not the final expert on the subject. I do not have an engineering degree. My goal is to just try to translate the science into things that help the real world street rider survive!

Take care,

Dan

Conchscooter said...

I never heard of trail braking until a little while ago when it became an internet fad and I doubt I will apply the front brake in a curve until I am ready to leave this vale of tears. I rememeber first learning the radical notion of applying pressure to the handlebars and putting weight on the footpegs and that made an enormous difference to my cornering. That was from a magazine interview with Giacomo Agostini which dates me a bit. Besides he didn't explain his techniques near as well as you, O master.