Front brakes provide 70% of stopping power. But why?

[FreeForAll]

I'm not disputing that front brakes on a vehicle provide the majority of stopping power, but why is that?

I originally thought it was because of added traction at the front due to weight transfer, but even my bike moving at a walking pace has more powerful front brakes than the identical rear brakes (even with most of the weight on the rear wheel).

 

[Ranger Mike]

there is no such thing a weight transfer although this is a common term to help the uniformed about racing. The front brakes are bigger because the forward momentum of an object in motion requires this additional resistance when the brakes are applied. The front end ' thinks "there is more weight added but this is due to momentum or froward force that must be countered. Since most automobiles have engines in the front the front to rear percentage weight is usually 52/48 at static. Dynamic braking required additional brake area vs rear brakes to whoa up the car.

 

[berkeman]

For sportbikes, it's common for 100% of the stopping power to come from the front brakes and tire...

https://encrypted-tbn0.gstatic.com/...5zsYPrDwcnUvEu4w_7i5rzgBDwiaLL4713dU8AMzQMAVC

 

[FreeForAll]

For sportbikes, it's common for 100% of the stopping power to come from the front brakes and tire...

View attachment 234740

I had a sportbike, and I agree with that.
But on my bicycle, with identical front and rear brakes, and with most weight on the rear tire, the front brakes always stop the bike sooner than the rear with the same lever movement. Even when nowhere near the limit of traction.
This makes me think there's more to it than just weight transfer to the front (which allows for greater traction, and greater stopping force).

 

[FreeForAll]

The front brakes are bigger because the forward momentum of an object in motion requires this additional resistance when the brakes are applied.

Isn't the momentum the same for both the front and rear axle?

The front end ' thinks "there is more weight added but this is due to momentum or froward force that must be countered.

I think you're talking about weight transfer. If so, my experience is that front brakes are noticeably more powerful even when there is no noticeable weight transfer.

Since most automobiles have engines in the front the front to rear percentage weight is usually 52/48 at static. Dynamic braking required additional brake area vs rear brakes to whoa up the car.

Agreed, but my example is with slow speeds. To experiment, ride your bike at less than 10 km/h and see how long it takes to stop with just the rear brake. Do the same with just the front brake and it'll be less than 1/2 the distance with same brake pressure. Even when experimenting with many many different bikes the result is always the same. When my kids had their training bikes with training wheels one had only front brakes and the other only rear. The one with rear always had difficulty stopping, regardless who was riding.

 

[anorlunda]

Even when experimenting with many many different bikes the result is always the same.

Interesting stuff. But I have the impression that my retro bike with rear wheel coaster brakes can stop as fast as a caliper brake bicycle. Maybe I'm wrong about that. Could it be related to leaning my body back to exert pressure on the pedals for coaster brakes?

 

[russ_watters]

I had a sportbike, and I agree with that.
But on my bicycle, with identical front and rear brakes, and with most weight on the rear tire, the front brakes always stop the bike sooner than the rear with the same lever movement. Even when nowhere near the limit of traction.

How sure are you that the brakes are identical? Because if the only stipulation is that you are below both traction limits, the relationship between front and rear brake force could be literally anything.

What kind of brakes are they?

I'll have a look at mine when I got home, but I'd consider any engineer who designed the front and rear brakes of a bike to be identical, lazy.

 

[Randy Beikmann]

While people do call it "weight transfer", it is actually "load transfer." When you are braking, the traction forces act on the car at ground level straight rearward(lower than the car's center of gravity). That will rotate the car in pitch (nose down, tail up) until the vertical tire forces (loads) act to counter the rotation. To stop the rotation, the front tires must push up on the car more than before, and the rear tires less.

So let's say that at rest, a car has 55/45 weight distribution (front tires carry 55% of the load, rear tires 45%). When you brake at 1 G, the front tires might support a load equal to 75% of the car's weight, and the rear 25%. The change in load distribution depends on the wheelbase and CG height.

So yes, with more load on the front tires, they have more traction, and you need to put more braking pressure on the front brakes to take advantage of that.

 

[AZFIREBALL]

Its all a matter of center of gravity height above ground level.

 

[FreeForAll]

How sure are you that the brakes are identical?

Identical part numbers in the breakdown.
Except for the very high end, all the bikes I see for sale have identical brakes.
Some of my bikes have hydraulic disc brakes, others have traditional rim brakes.

Remember I'm talking bicycle, not motorcycle.

 

[FreeForAll]

For people talking about weight transfer or load transfer, I believe I understand how that works, and that it becomes more prevalent the higher the braking force involved.

Just to get some numbers from the first 2 websites I on the search list:

"With a rider aboard, the center of mass moves rearward, with nearly 60% of the total load on the rear wheel."
https://www.vernier.com/innovate/investigating-weight-distribution-on-a-bicycle/

Someone measured a "39.9%/60.1% front/rear weight distribution." https://forums.roadbikereview.com/general-cycling-discussion/front-rear-weight-distribution-55062.html

These numbers are for road bikes. My mountain bike is probably closer to 65-70% on the rear as I sit more upright.So I guess my question breaks down to this:
At slightly over a walking speed with moderate brake pressure, does load transfer and traction even become a measurable factor? I don't think so but I could be underestimating it.

If it is not a factor, then why do the rear brakes work so poorly compared to the front, even with most of the weight on the back?

Please don't take my word for it - next time you're on a bike try the experiment. Give it 1 to 2 pedals and stop with only the front brake. Do the same with the rear and you'll notice a difference.

 

[russ_watters]

Identical part numbers in the breakdown.
Except for the very high end, all the bikes I see for sale have identical brakes.
Some of my bikes have hydraulic disc brakes, others have traditional rim brakes.

Hmm. Well, my curiosity is piqued.

I have disc brakes. Last time I borrowed a bike with rim brakes, the first time I squeezed the brake handles, I vaguely noticed a slight sensation of deceleration...

Please don't take my word for it - next time you're on a bike try the experiment. Give it 1 to 2 pedals and stop with only the front brake. Do the same with the rear and you'll notice a difference.

It's Novemenber and I live in Pennsylvania...I'll get back to you in 5 months on that.

 

[FreeForAll]

I have disc brakes. Last time I borrowed a bike with rim brakes, the first time I squeezed the brake handles, I vaguely noticed a slight sensation of deceleration...

Agree, discs are superior in almost all aspects. But I'm only comparing front vs rear on the same bike.

It's Novemenber and I live in Pennsylvania...I'll get back to you in 5 months on that.

Get a mountain or fat bike and double your riding season :)

But your comment got me thinking - a good experiment would be to test the brakes on bike rollers - completely eliminating load transfer. If the front and rear performance is identical then maybe load transfer does play a larger part in this, even at such slow speeds.

Too bad I don't have rollers...

 

[Randy Beikmann]

At slightly over a walking speed with moderate brake pressure, does load transfer and traction even become a measurable factor? I don't think so but I could be underestimating it.

Load transfer is not dependent on vehicle speed, only its deceleration rate. Having said that, since brake material (probably) grabs better at low speeds because there is negligible heating, the load transfer could be even more pronounced than at high speeds.

 

[FreeForAll]

Load transfer is not dependent on vehicle speed, only its deceleration rate. Having said that, since brake material (probably) grabs better at low speeds because there is negligible heating, the load transfer could be even more pronounced than at high speeds.

Sorry but I don't understand why load transfer is even a factor if the tire is nowhere near its traction limit.
For example, if a tire is at 10% of its limit before it begins to slide, and load transfer raises that to 20%, or even 50%, how does that make a difference to braking distance if the limit is never reached? I thought if the brake force is the same (applying only front vs only rear), and traction is the same, then braking distance should be the same.I understand that load transfer is beneficial in the sense that it increases road grip on the front tire and allows it to brake harder before losing traction. Is there some other benefit I'm not considering?

 

[cjl]

If the tire starts to slide, you reached it's traction limit (by definition).

Also, you cannot apply the same braking force when applying only rear vs only front brake. If you apply only the front brake, and you keep gradually increasing intensity, you keep increasing front load as well, increasing the front tire's resistance to sliding. On a high traction surface like a road or sidewalk, I'd expect most bikes to be able to put 100% of their load on the front tire while braking (picking up the rear tire), which then allows your deceleration rate to only be limited by the bike's geometry and resistance to flipping over the front wheel. On the other hand, it's not physically possible for the rear to provide this much brake force, since as you increase deceleration, the rear end has less and less load. You'll always hit the tire's traction limit and therefore induce a slide at some point which is lower than the front limit.

Note that on a very slick surface, the opposite could be the case. If your grip level is low enough, the peak deceleration rate still won't cause very much load transfer. In this case, your rear brake will be more effective because it has more static load on it due to the geometry of the bicycle. On any surface where you can generate sufficient braking force to cause the front tire load to exceed the rear though (which I'd expect to be on most surfaces except snow/ice), the front will be the more important brake for stopping.

 

[FreeForAll]

I think I'm doing a bad job at explaining this. Everyone is talking about traction limits and load transfer, when I don't think they have any impact on my scenario.

Let me try to simplify. Imagine this:
- A bike with identical brakes front and rear. New tires, on a high friction surface. 40/60 front/rear weight bias.
- Brake levers are labeled 0-10 (0 being off, 10 being maximum power).
- Riding at 10 km/h, apply front brakes at "power level" 2. Measure stopping distance.
- Do the same with rear brakes, at same "power level".

Whenever I try this the front always stop the bike in a shorter distance. Why is that?

When using the rear brakes I'm nowhere near the traction limit, and if there is any load transfer I'm now at 50/50, nowhere near 90/10 or whatever will cause the rear to slide. I'd imagine if rear tires slide at "power level 2" at under 10 km/h we'd have to replace rear tires way more often than front tires. This is simply not the case with a normal city bike.

If I'm trying to brake at "power level 8" or so, then absolutely I understand how reduced rear traction will lengthen stopping distance.

 

[cjl]

If you're applying identical brakes with an identical amount of force, and you're well below the traction limit, which brake you use should be irrelevant. Are you sure you're applying them with the same amount of force? Perhaps the front one is rigged with a larger mechanical advantage (such that the same brake lever force on both levers results in more front brake force), in order to allow you to more easily take advantage of the larger available front grip?

 

[Dullard]

I understand your question. a couple of points:

I'm having trouble with how I would accurately evaluate the 'power level' of brakes.
I'm wondering how the length of the cables (front vs rear) might impact the perceived (or actual) effort.
I know that pad clearance/travel will impact the mechanical advantage (for many styles of caliper brakes) - are your front/rear 'hands off' pad clearances the same?

 

[CWatters]

Given the conditions described in #17 it shouldn't matter which brake you use. For example if it takes the same force to rotate the wheel against the brake the deceleration should be the same.

My bet is you are applying different amounts of braking forces for some reason. Perhaps there is more friction in the longer cable to the rear brake? Who knows.

I think I'd want to see more scientific evidence that the same braking force is actually being applied.

Could it be the rear brake is operating in the spray or dust thrown up by the front wheel?

 

[DaveC426913]

You've raised the factor I first thought of: how can you be sure that a 2 on the rear brakes is actually applying the same braking power as a 2 on the front brakes? Without factoring this out, you're taking it on faith.

 

[FreeForAll]

Thanks to everyone for confirming that:

a) The rate of deceleration should be the same for both the front and rear brakes if both wheels experience the same traction and braking force.
b) Any perceived difference is most likely from operator error or some mechanical difference between the front and rear setups.

I really wasn't sure if there was something else at play here that I hadn't thought of.
Thanks again to everyone for their input - I learned a lot more than intended, which is always great!

 

[Stephen Tashi]

What about the timing of which brakes are applied first. When a vehicle tows a trailer, isn't it safer if the trailer brakes act first - to prevent "jacknifing"?