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Top tubes and front brakes are dangerous. Myth or Reality?

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Top tubes and front brakes are dangerous. Myth or Reality?

Old 12-22-09, 04:54 PM
  #26  
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Top tube crashes, never; brake endo...ohhhh, yeah.

Mine was like this: I had a 203mm Avid BB7 on the front, and hadn't replaced the stock fork spring with a stiffer one. New winter gloves, as I found out a second too late, reduced feedback through the lever; they were the secret ingredient in this witches' brew.

I grabbed lever to stop short of colliding with traffic on a residential street (visibility on that corner wasn't the greatest) -- the next thing I knew, the exploding THUMP! of hitting the pavement, and white-hot pain in my right shoulder, preventing me from getting up. Right collarbone broken in five places.

I have since replaced the spring, and have never had the problem again.

Uh, Rensho, if back brakes were that dangerous, they'd be left off bikes.
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Old 12-22-09, 05:11 PM
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Originally Posted by Pscyclepath View Post
I rarely cease to be amused by the ignorance that gets perpetrated in these forums.

The braking power of a wheel is affected by the amount of weight it bears. When you're JRA, the weight is fairly evenly distributed, and the braking power of each wheel is about the same. When you apply brakes and the bike slows, momentum carries the rider's weight forward, placing more weight on the front wheel and greatly increasing its braking force. In a hard braking maneuver such as a panic or emergency stop, enough weight can be transferred forward that the rear wheel loses traction (skids) and can actually come up off the ground. This is the phenomenon that throws a rider over the handlebars.

I never recommend that a rider use the front brake alone. Use both front and rear brakes together to come to a smooth, controlled stop. To prevent losing traction with the rear tire, shift your weight to the rear to keep traction back there.

For an emergency stop, squeeze the front brake lever two to three times as hard as you do the rear brake lever, because as you slow down, the front wheel becomes your strongest brake. Level the pedals, get your butt off the seat. and throw it back to get as much weight as you can over the back wheel. A good MTB'er can just about lay his chest or belly-button on the saddle this way, and keep control. If you feel the back wheel start to slide or skid, let off the front brake lever until you're back in control.

Jeez, guys... this is very basic mountain-biking technique for going down a slope.
When teaching this in bike handling classes, I set up a short, narrow course and have the rider practice stopping with just the rear brake, then both the front and rear, and then practice a time or two just getting out of the saddle and shifting weight to the rear. Then you practice stopping using both brakes and the weight shift. Once they have that down, then we try the emergency stop. For MTB's once they have this technique down on a flat surface, then we go looking for a hill or slope to practice on.

As for the top tube business, the usual cause is letting your foot/feet slip off the pedals. With the trend toward sloping top bars on MTBs this happens more to roadies than to mountain bikers. Some may recall it happening to this fellow named Armstrong during the Tour de France stage on Luz d'Ardiden back in 2003.

Clipless pedals, or good grips on your flat pedals can help prevent this problem
Exactly. On a mountain bike, you are in the slippery stuff all the time, so you have to rely on the back brake to avoid locking the front tire and losing it. My post concerned riding on the road, and you will note that I said that whenever it gets slippery, but you should avoid using the front brake.
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Old 12-22-09, 05:18 PM
  #28  
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Originally Posted by cyccommute View Post
Sorry, but using the front brake only is yet another myth that is perpetrated in bicycling...particularly road cycling. (No mountain bike rider who likes their teeth would ever advocate front braking only)

It comes from not understanding the physics and dynamics of braking. While it is true that you reach maximum braking power when the rear wheel leaves the ground, it is not true that this is the only way to reach maximum braking power. The contribution from the rear wheel is only about 20% of the front and it does decrease rapidly as weight is shifted to the front wheel, however while still in contact with the ground, the rear wheel is contributing to the deceleration of the bicycle. Moving the body back and down, i.e. adjusting the load to a lower center of gravity, will make the rear wheel even more effective and reduce the tendency of the rear wheel to slide. Mountain bike riders have been doing this for ages. In fact, mountain bike riders are told to release the front brake when the rear starts to slide and push off the back of the saddle. This brings the rear wheel back into contact with the ground, stops the slide and makes overall braking more effective.

Road riders would be wise to train themselves to know how to deal with a sliding rear wheel. A sliding wheel doesn't mean that you are going to crash...any 10 year old kid will tell you that...but it does mean you should do something about it. Release the front brake, move backwards on the saddle and move your center of gravity downward. I do this just about every time I stop and not just in panic stops.
If you read my post again, you will see that I was talking about riding on the road in dry conditions. I even said that when it gets wet, that the front brake is not the brake to use.

As far as a sliding rear wheel goes, should never even put yourself in that situation in the drive. A locked rear wheel eliminates all of your steering and people have a tendency to let go of the rear brake when the rear is sliding, which is exactly what you should not do because of the bike is in any way sideways, the rear wheel will then get traction and lead to a high side.

By training yourself not to use the rear brake, to use only the front, you will never put yourself in the dangerous situation where you walk the rear wheel and lose all steering control, and at worst crash.

I agree with you, in mountain biking when it's slippery, it's exactly the opposite, the rear brake is used much more than the front.
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Old 12-22-09, 05:23 PM
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Originally Posted by DX-MAN View Post
Top tube crashes, never; brake endo...ohhhh, yeah.

Mine was like this: I had a 203mm Avid BB7 on the front, and hadn't replaced the stock fork spring with a stiffer one. New winter gloves, as I found out a second too late, reduced feedback through the lever; they were the secret ingredient in this witches' brew.

I grabbed lever to stop short of colliding with traffic on a residential street (visibility on that corner wasn't the greatest) -- the next thing I knew, the exploding THUMP! of hitting the pavement, and white-hot pain in my right shoulder, preventing me from getting up. Right collarbone broken in five places.

I have since replaced the spring, and have never had the problem again.

Uh, Rensho, if back brakes were that dangerous, they'd be left off bikes.
They should be. I have mine adjusted so that I cannot lock it up at around 15 mph.
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Old 12-22-09, 06:04 PM
  #30  
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Originally Posted by San Rensho View Post
If you read my post again, you will see that I was talking about riding on the road in dry conditions. I even said that when it gets wet, that the front brake is not the brake to use.

As far as a sliding rear wheel goes, should never even put yourself in that situation in the drive. A locked rear wheel eliminates all of your steering and people have a tendency to let go of the rear brake when the rear is sliding, which is exactly what you should not do because of the bike is in any way sideways, the rear wheel will then get traction and lead to a high side.

By training yourself not to use the rear brake, to use only the front, you will never put yourself in the dangerous situation where you walk the rear wheel and lose all steering control, and at worst crash.

I agree with you, in mountain biking when it's slippery, it's exactly the opposite, the rear brake is used much more than the front.
Even in dry conditions, the same principles apply. Until you have fully lifted the rear wheel off the ground, i.e. a nose wheelie, the rear wheel is contributing to the overall deceleration of the bike. Once the rear wheel is disengaged from the ground, then the absolute maximum deceleration has been reached. However, that only happens when the rear wheel is no longer in contact with the ground.

A sliding wheel does not mean that you are going to suddenly gain traction when you release the rear brake and go over the high side. Weight is already shifted to the front wheel...that's why the rear wheel is sliding...and the bike will be leaning away from the slide. A sliding wheel is also indicative of too much front brake. Training should be to release pressure on the front brake to regain braking on the rear wheel. Gaining traction will only result in the bike straightening out. It's easy to do and we do it almost instinctively.

In mountain biking, the front brake is still responsible for most of the deceleration of the bike. We just use the rear wheel to a bit better effect because of the angles of descent that we encounter, not necessarily because of the surface. If the techniques work well on poor surfaces and higher angles of attack, think of how well those same techniques work when applied to a level surface with good traction.
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Old 12-22-09, 06:05 PM
  #31  
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The 3" clearance for sizing is really for mountain bikes that people actually intend on using for mountain biking. A 1" clearance is a more appropriate start for sizing a bicycle that will be used on the road.
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Old 12-22-09, 06:27 PM
  #32  
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MSF courses and any practical test you'd like to try demonstrate that the shortest stopping distance requires using both brakes, bleeding off the rear as it gets close to slipping. I practice full braking on both bicycle and motorcycle. The effectiveness of both together proves very clear if you start breaking at the same point and mark where you stop.

I would certainly be concerned about a bicycle I couldn't lift the rear wheel on without much effort. On my 600 lb motorcycle I can get the rear wheel completely unloaded in substantially less than 2 seconds, but it's clearly helped scrub some speed. A second is a long time. On both machines the rear brake proves very handy in the mountains and on gravel or other loose material.

I'm surprised there's any debate - the braking tests and charts are discussed at every motorcycle basic course.

As for the top tube, I've not seen anyone hit it. High incentive operating environment. I have seen over the bars wrecks, but only from impact, not from braking. I would think either type of problem must be rare or I'd have seen it more often.
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Old 12-22-09, 06:48 PM
  #33  
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Originally Posted by MrCjolsen View Post
Two common fears of cyclists - crashing down on a too-high top tube and going over the bars as a result of applying the front brake - have never even come close to reality for me in over 40 years of cycling. I jam on my front brake all the time, and my rear wheel stays firmly attached to the ground, and my body stays on the bike without any problems. And I've never worried about somehow slipping off my seat and injuring myself by hitting the top tube. And all of my bikes are on the large side.

I find this interesting considering the most common determiner of whether a bike is "too big" is the inability to lift it 3" while straddling the top tube. If I can do that, the bike is too small. Yet hundreds of thousands of bikes are purchased at department stores with this as the main sizing factor.

And worse that that, I actually saw a kid riding a bike at the school where I teach and his front brake had been disconnected. He said his dad did it because he didn't want him to go over the bars.

Are these two types of accidents myths or do they actually happen to people in real life?
When you begin braking, the weight is shifted forward, putting more weight on the front wheel, and therefore it provides most of the stopping power. Essentially you need to know when the rear wheel starts to skid, and lower then pressure on the front brake slightly, as if you don't there is a possibility that you will lose complete rear wheel contact with the ground, this is not a good thing. The same physical laws apply to all wheeled vehicles, and is why for a long time cars had disc brakes only on the front. Drum brakes were good enough for the rear wheels.

I've only had one instance where brakes almost caused an endo, when a rear brake cable snapped, had a full hand of front brake (OH $%#@ ), but managed to keep control. I did an endo once, front wheel hit a 20cm wide and deep ditch cut in some grass, that one hurt, 4 years later that shoulder still bothers me sometimes.

Top tube, nope, never managed to do that one, been riding on and off for 40 years.
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Old 12-22-09, 06:51 PM
  #34  
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Originally Posted by MrCjolsen View Post
Two common fears of cyclists - crashing down on a too-high top tube and going over the bars as a result of applying the front brake - have never even come close to reality for me in over 40 years of cycling. I jam on my front brake all the time, and my rear wheel stays firmly attached to the ground, and my body stays on the bike without any problems. And I've never worried about somehow slipping off my seat and injuring myself by hitting the top tube. And all of my bikes are on the large side.

I find this interesting considering the most common determiner of whether a bike is "too big" is the inability to lift it 3" while straddling the top tube. If I can do that, the bike is too small. Yet hundreds of thousands of bikes are purchased at department stores with this as the main sizing factor.

And worse that that, I actually saw a kid riding a bike at the school where I teach and his front brake had been disconnected. He said his dad did it because he didn't want him to go over the bars.

Are these two types of accidents myths or do they actually happen to people in real life?
They do happen... but like you, with over 40 years on a bike and the the last 30+ using hand brakes... I don't recall ever being close to going over the top.

I do know a couple of people who have flipped... and it was bad in both cases.
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Old 12-22-09, 07:06 PM
  #35  
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I couldn't tell you the exact ratio of front to rear brake I use on a regular basis. All this theorizing is just idle chatter, IMO. Ride yer bike and get a feel for sudden stops. Experience is the best teacher, and just brake by feel. If you're calculating how much front brake to use vs. how much rear brake, you're probably thinking too hard.
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Old 12-22-09, 07:54 PM
  #36  
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Originally Posted by chipcom View Post
Careful, you are peddling a myth of your own here. Just because a bike has lots of stand over doesn't necessarily mean that it is too small either.
Giant's relaxed geometry took care of that.

And also took care of the OP's concern.

Ya, they're ugly, but I own two of them - because they're easy to get and fit to my long-armed galoot of a body frame.
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Old 12-23-09, 05:17 AM
  #37  
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Originally Posted by Pscyclepath View Post
I rarely cease to be amused by the ignorance that gets perpetrated in these forums.

The braking power of a wheel is affected by the amount of weight it bears. When you're JRA, the weight is fairly evenly distributed, and the braking power of each wheel is about the same. When you apply brakes and the bike slows, momentum carries the rider's weight forward, placing more weight on the front wheel and greatly increasing its braking force. In a hard braking maneuver such as a panic or emergency stop, enough weight can be transferred forward that the rear wheel loses traction (skids) and can actually come up off the ground. This is the phenomenon that throws a rider over the handlebars.
For someone who is amused at others' ignorance, you're awfully ignorant yourself.

"momentum carries the rider's weight forward" No! As long as you keep your arms stiff your weight remains wear it was.

" In a hard braking maneuver such as a panic or emergency stop, enough weight can be transferred forward that the rear wheel loses traction (skids) and can actually come up off the ground. This is the phenomenon that throws a rider over the handlebars" No! The force that reduces grip at the back wheel is the torque exerted by deceleration. Which isn't enough to cause an endo on the level if you keep your weight planted.

..You're not only braking with the worst possible technique, you're sneering at other people for not being silly enough to accept the rationale you've come up with to excuse your failure!

Go and read https://sheldonbrown.com/brakturn.html.

Oh - and the reason to go easy on the rear in hard braking is that if the rear wheel skids then the bike can slide, which itself can throw off the rider. The less grip at a wheel then the less brake is needed to make it skid.
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Old 12-23-09, 08:51 AM
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After years of bicycle and motorcycle riding, I've found it's better to apply the rear first then the front. I can control a rear wheel skid/slide far better than a front wheel version, and on the few times that I did have a front wheel slide, be it bicycle or motorcycle, I generally ended up on the ground so fast that I had very little time to react or prepare for the fall.
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Old 12-23-09, 09:01 AM
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This morning I hit a patch of ice on the side of the road and lost traction in both wheels. I put my foot down and waddled with my bike for a few steps (at low speed) and was glad that I could stand over my top tube.
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Old 12-23-09, 09:25 AM
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Originally Posted by Dan The Man View Post
This morning I hit a patch of ice on the side of the road and lost traction in both wheels. I put my foot down and waddled with my bike for a few steps (at low speed) and was glad that I could stand over my top tube.
How much clearance did you have between crotch and top tube?

The one time I crashed on ice, I landed on my side. Was on the ground before I even knew what happened.
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Old 12-23-09, 09:43 AM
  #41  
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Originally Posted by meanwhile View Post
For someone who is amused at others' ignorance, you're awfully ignorant yourself.

"momentum carries the rider's weight forward" No! As long as you keep your arms stiff your weight remains wear it was.
Mr. Newton, meanwhile. meanwhile, Mr. Newton.

Every time you brake in a car you feel the front end dive downwards. If you ride a bike with suspension, you'll feel the front shock dive and, if you have rear suspension, the rear suspension elongate. On an unsuspended bike, you won't feel the suspension compress because there isn't any but you will feel the center of gravity pitch forward. All of these are the momentum of the vehicle being transferred to the front wheel(s). All are examples of Newton's Third Law.

Keeping "your arms stiff" does not negate Newton's Third Law...and is an improper technique for good braking practices. You should push your body back (by straightening your arms and lowering your chest) to move the center of gravity rearwards and down. Movement of the center of gravity further back and down mitigates the weight transfer somewhat but doesn't make it go away.

Just stiffening your arms does nothing to mitigate the weight transfer and leaves you with very little ability to respond with steering input if you need it.

Originally Posted by meanwhile View Post
" In a hard braking maneuver such as a panic or emergency stop, enough weight can be transferred forward that the rear wheel loses traction (skids) and can actually come up off the ground. This is the phenomenon that throws a rider over the handlebars" No! The force that reduces grip at the back wheel is the torque exerted by deceleration. Which isn't enough to cause an endo on the level if you keep your weight planted.
Brake torque is only part of the equation. Brake torque is proportional to the sum of the braking force and the height of the center of gravity. Change the center of gravity, i.e. push back and down with your body, and you'll change that proportionality. Keeping "your weight planted" does nothing to change the CG and doesn't help with braking. It's dynamic...not static.

Originally Posted by meanwhile View Post
..You're not only braking with the worst possible technique, you're sneering at other people for not being silly enough to accept the rationale you've come up with to excuse your failure!

Go and read https://sheldonbrown.com/brakturn.html.

Oh - and the reason to go easy on the rear in hard braking is that if the rear wheel skids then the bike can slide, which itself can throw off the rider. The less grip at a wheel then the less brake is needed to make it skid.
Yes, the rear wheel can slide if you use too much front brake but a sliding rear wheel doesn't mean that the bike will automatically fall over. Any semi-competent rider should be able to deal with a sliding rear wheel. A little body english or a reduction of force on the brake levers (both but more on the front to reduce the weight transfer) can be used to deal with a sliding rear wheel. Knowing how to slide the rear wheel and knowing how to deal with it goes a long way towards avoiding crashes due to wheel slip. You can even get more sliding out of the rear wheel by moving your CG forward and enhancing the weight transfer. Ask a 10 year old kid if you need lessons on how to skid a rear tire
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Old 12-23-09, 09:51 AM
  #42  
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Old 12-23-09, 09:52 AM
  #43  
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Originally Posted by meanwhile View Post
"momentum carries the rider's weight forward" No! As long as you keep your arms stiff your weight remains wear it was.
It's the forward momentum (most of which is associated with the rider's mass) that braking is reducing. Ideally, the rider and the bicycle should have the same speed when braking.

Originally Posted by meanwhile View Post
It took 37 posts to link to Sheldon Brown. It took 37 posts to link to anything!


Here's a direct link to John Forester's contribution to the FAQ that Sheldon Brown's page mentions:

https://draco.acs.uci.edu/rbfaq/FAQ/9.17.html

Originally Posted by cyccommute View Post
Yes, the rear wheel can slide if you use too much front brake but a sliding rear wheel doesn't mean that the bike will automatically fall over.
He didn't say it would "automatically" fall over. He said it could ("can").

Last edited by njkayaker; 12-23-09 at 10:24 AM.
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Old 12-23-09, 10:42 AM
  #44  
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Originally Posted by cyccommute View Post
For someone who is amused at others' ignorance, you're awfully ignorant yourself.

"momentum carries the rider's weight forward" No! As long as you keep your arms stiff your weight remains wear it was.
Mr. Newton, meanwhile. meanwhile, Mr. Newton.

Every time you brake in a car you feel the front end dive downwards. If you ride a bike with suspension, you'll feel the front shock dive and, if you have rear suspension, the rear suspension elongate. On an unsuspended bike, you won't feel the suspension compress because there isn't any but you will feel the center of gravity pitch forward. All of these are the momentum of the vehicle being transferred to the front wheel(s). All are examples of Newton's Third Law.
The centre of gravity may move forwards, but that's because of deceleration torque compressing the suspension/tyre. It wouldn't happen if the tyres were completely solid - you're confusing an effect with a cause.

Keeping "your arms stiff" does not negate Newton's Third Law..
I didn't say that it would. Unfortunately you'd don't understand what Newton's 3rd Law is or how it applies to braking.

As you're not smart enough to understand the physics, you'll just have to

1. Take my word for it - and that I have a real physics degree rather than a joke SIG that uses the word scientist

2. Take the word of the famous Jobst Brandt - an engineer on brake and suspension systems for Porsche, an expert on bicycle physics who designed tires for Avocet and wrote the standard book on wheel building. Here he is talking to St Sheldon Brown:

https://www.sheldonbrown.com/brakturn.html

Jobst Brandt has a quite plausible theory that the typical "over-the-bars" crash is caused, not so much by braking too hard, but by braking hard without using the rider's arms to brace against the deceleration: The bike stops, the rider keeps going until the rider's thighs bump into the handlebars, and the bike, which is no longer supporting the weight of the rider, flips.
Once again:

- Braking does NOT magically make your weight move forwards even if you stay in the same position! Your weight will only move forwards if your body moves forwards! Anyone who tells you otherwise is a dangerously ignorant. To point out the very obvious, your body won't move relative to the bike if your arms stay locked and so, despite cyccommute's mangling of high school physics, no, your weight won't move.

- Keeping your weight back while braking is what keeps your rear wheel down - the further back your weight is, the more leverage it exerts. (Welcome to MTBing 101!)

As for Newton 3 and braking - it's application to braking is actually damn simple:

1. The road exerts a force on the tyre, and the tyre exerts an equal force on the bike. If you road over a plywood sheet and hit the brakes, then the sheet would skitter away. The road doesn't move because it is fixed to the ground.

2. F=ma says that there will be force approximate to your body's mass acting forwards, it's magnitude being the product of your mass and the bike's deceleration. This is the force that exerts that rear wheel- lightening torque.

..Really, this isn't rocket science...

If my reference to Brandt won't convince you, we can take this to a forum where a professional physicist or engineer may be willing to explain to you what you should have grasped in high school.

Last edited by meanwhile; 12-23-09 at 10:53 AM.
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Old 12-23-09, 10:51 AM
  #45  
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However, while we are on the subject of St Sheldon and his page on braking, I don't think that rear brakes are as ineffective as he claims on all bikes in all conditions. My hardtail MTB can stop nearly as quickly on the road with the rear as the front - and that's very quickly indeed with performance slicks on. Otoh my crosser's rear brake is almost useless on slippery off road. I'll do some googling later to see what hard data I can find.
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Old 12-23-09, 11:10 AM
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Just how much weight transfer do you get with a road bike with no suspension and 100 PSI in the tires?? The front tire probably doesn't compress more than 5-10mm?

Does anyone have any numbers to use as examples??? For example with a 170 lb rider 30 lb bike even weight distribution when not braking -either stopped or at a steady velocity)you would measure 100 lbs on either wheel.Once you start to brake, you would get forces over 200 lbs(both wheels added) for a very short period until whatever compression was going to happen, happened. Once compressed to maximum I think the total weights should revert to 200 lbs with the front wheels having over 100 lbs since the CG has changed slightly because of the front tire compressing.?

Anyone out there have any convincing numbers they could walk us thru?
Thanks
Charlie
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Old 12-23-09, 11:13 AM
  #47  
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Originally Posted by phoebeisis View Post
Just how much weight transfer do you get with a road bike with no suspension and 100 PSI in the tires?? The front tire probably doesn't compress more than 5-10mm?

Does anyone have any numbers to use as examples??? For example with a 170 lb rider 30 lb bike even weight distribution when not braking -either stopped or at a steady velocity)you would measure 100 lbs on either wheel.Once you start to brake, you would get forces over 200 lbs(both wheels added) for a very short period until whatever compression was going to happen, happened. Once compressed to maximum I think the total weights should revert to 200 lbs with the front wheels having over 100 lbs since the CG has changed slightly because of the front tire compressing.?

Anyone out there have any convincing numbers they could walk us thru?
Thanks
Charlie
I don't have numbers, but a body can be perfectly rigid (no compression at all) and there will still be weight transfer. The suspension/compression thing is not the issue.
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Old 12-23-09, 11:13 AM
  #48  
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After reading about Sheldon's braking techniques, I'd still like my way of using the rear brake first, somehow , I like the idea of sliding on my butt if the rear tire gets away from me than that hard slamming face plant if my front tire traction decides to take momentary leave.
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Old 12-23-09, 11:18 AM
  #49  
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Originally Posted by meanwhile View Post
However, while we are on the subject of St Sheldon and his page on braking, I don't think that rear brakes are as ineffective as he claims on all bikes in all conditions. My hardtail MTB can stop nearly as quickly on the road with the rear as the front - and that's very quickly indeed with performance slicks on. Otoh my crosser's rear brake is almost useless on slippery off road. I'll do some googling later to see what hard data I can find.
I'm trying to understand this. Do you mean that it's easy to skid the rear wheel on the cross bike?

I think what you're describing has more to do with your particular brake set up on your bikes than the difference between front and rear brakes in general.
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Old 12-23-09, 11:23 AM
  #50  
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"Jobst Brandt has a quite plausible theory that the typical "over-the-bars" crash is caused, not so much by braking too hard, but by braking hard without using the rider's arms to brace against the deceleration: The bike stops, the rider keeps going until the rider's thighs bump into the handlebars, and the bike, which is no longer supporting the weight of the rider, flips."

------------------

Nope. When I flipped over the bars, my ass stayed on the seat until my shoulder(s) hit the ground. Accounts of many other "over the bars" flips shows the same thing. Bracing your arms does not stop the forward momentum of the body.
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