Best braking technique?

[trailgumby]

@human909, here's another piece of information for you from https://en.wikipedia.org/wiki/Tire_load_sensitivity:

Tire load sensitivity describes the behaviour of tires under load. Conventional pneumatic tires do not behave as classical friction theory would suggest. The load sensitivity of most real tires in their typical operating range is such that the coefficient of friction decreases as the vertical load, Fz, increases. The maximum lateral force that can be developed does increase as the vertical load increases, but at a diminishing rate.

Coulomb friction theory says that the maximum horizontal force developed should be proportional to the vertical load on the tire. In practice, the maximum horizontal force Fy that can be generated is proportional, roughly, to the vertical load Fz raised to the power of somewhere between 0.7 and 0.9, typically.

[human909]

@human909, here's another piece of information for you from https://en.wikipedia.org/wiki/Tire_load_sensitivity:

Tire load sensitivity describes the behaviour of tires under load. Conventional pneumatic tires do not behave as classical friction theory would suggest. The load sensitivity of most real tires in their typical operating range is such that the coefficient of friction decreases as the vertical load, Fz, increases. The maximum lateral force that can be developed does increase as the vertical load increases, but at a diminishing rate.

Coulomb friction theory says that the maximum horizontal force developed should be proportional to the vertical load on the tire. In practice, the maximum horizontal force Fy that can be generated is proportional, roughly, to the vertical load Fz raised to the power of somewhere between 0.7 and 0.9, typically.

Thanks trailgumby. That snippet is about corning force in race cars. It really isn't the same as that is lateral slip in a rolling tyre which is significantly different to slip the longitudinal direction.

For example, earlier I said this.
(Now friction can be more complicated by for these purposes it is a very close model. Otherwise a we'd have all sorts of oddness occuring in our world.)

Do you wonder what sort of oddness would occur? Well now that you gave me some numbers to play with I'll explain the oddness. Sorry for the mass text...

~~~~~~~~~~~~~~~~~~~~~~

Lay a large thin rubber mat on the asphalt lets say it has a coefficient of friction of 0.7. So it will happily not slide on a slope until it reaches 35 degrees. Seems sensible. Lets be safe and put it on a 15bdegree slope can't be too careful can we. Now this rubber weighs 1kg and we can readily do the mathematics and confirm that it won't slip. Also we can just glance over at the rubber mat and yep it is still there...

In fact lets walk over and stand on it yep not issues. I am now standing on a 1kg mat on a 15degree slope. All good, I was a bit worried there that my 130kg of tubbyness would cause the mat that I'm now standing on to slip. But thankfully after reading about classical simple friction I'm relieved and I'm still stable.

But I'm curious so I now start calculating the slip angle based a link trailgumby gave me. With an exponent of 0.8 and an increase in weight of 130x this slip angle drops drastically to below 15degrees. With shock I see the rubber mat now sliding down the 15degree slope with me on top of it. Frantically I recalculate with classical friction and the mat stops.

I could describe a hundred other examples involving sand grains, pebbles or boulders. If you start heading away from DIRECT proportionality then really weird stuff would result in our general physical world. Don't get me started on mountains falling over and landslides everywhere... Thankfully it doesn't occur in 'normal circumstances'.

(Of course there are also exceptions that can be readily or not so readily explained. For example the behavior of fat fast skis on snow, or thin bladed skates on ice. Both those definitely don't display the normal behaviour and in completely the opposite ways.)

EDIT: And one last thing. I'm not saying that tyres precisely follow classic friction. There is plenty of research on this issue. But it follows it very closely. There has been a hell of a lot of research in racing tyres to get that 1% extra grip....
~~~~~~~~~~~~~~~~~~~

But also none of that changes that under normal conditions your straight line braking is limitted by OTB failure. Given that is the failure point you want to push you braking up to but not exceed it. At this point there is zero or extremely close to zero weight on your rear tyre. Your rear tyre is simply incapable of providing any extra braking under these conditions.

Now if we head away from conditions where OTB isn't the failure mode then absolutely you want your rear brake.

[trailgumby]

Thanks trailgumby. That snippet is about corning force in race cars. It really isn't the same as that is lateral slip in a rolling tyre which is significantly different to slip the longitudinal direction.

Not really. That's a common misconception. The limits of adhesion for any combination of braking, acceleration and cornering force vectors is generally described by a circle for slick tyres (perhaps mildly elliptical for treaded tyres depending on tread design and intended surface). To get maximum braking force you need to be in a straight line, to get maximum cornering force you need to be off the throttle or brakes.



(Limited exception: you can blend brake with cornering as you start your turn in, and throttle as you unwind the steering on exit, since for those fractional seconds you are not quite at the cornering force limit.)

The shape of the contact patch has little to do with total grip levels on most tarmac surfaces, it is more to do with total contact patch area, although loose uneven terrain will differ case by case (eg mountain bike tyres climbing loose steep terrain).

There is slip while braking as well. There is in fact slip at any point where the tyre is exerting any cornering, braking or accelerative force on the road, or combination thereof. The principle of load transfer and non-linearity still applies. You lose more from the unloaded tyre than you gain from loaded tyre.

Bringing this back to applying to bikes, you're right about the limit being an OTB, but I have found if you move your centre of gravity appropriately you can get to the point of breaking traction at the front before you go over the bars. In that case, there are definitely gains to be had from using both brakes. The closer you can get to 50/50 weighting the shorter the stopping distance.

[human909]

Thanks trailgumby. That snippet is about corning force in race cars. It really isn't the same as that is lateral slip in a rolling tyre which is significantly different to slip the longitudinal direction.

Not really. That's a common misconception.

The limits of adhesion for any combination of braking, acceleration and cornering force vectors is generally described by a circle for slick tyres (perhaps mildly elliptical for treaded tyres depending on tread design and intended surface). To get maximum braking force you need to be in a straight line, to get maximum cornering force you need to be off the throttle or brakes.

Complete agreement there and certainly not my misconception.

I was just stating why you can't go grabbing figures and relationships off bleeding edge racing and applying it to dispute the basics of physics. I also explained why if you get away from anything but a directly proportional relationship life starts getting really weird really quickly.

Bringing this back to applying to bikes, you're right about the limit being an OTB, but I have found if you move your centre of gravity appropriately you can get to the point of breaking traction at the front before you go over the bars.

I'd find it impressive if you could break front wheel traction in a straight line in normal conditions. You'd need you CoG quite low and far back.

I've certainly never broken traction on my front wheel even during extremely hard braking.

[trailgumby]

I'd find it impressive if you could break front wheel traction in a straight line in normal conditions. You'd need you CoG quite low and far back.

I've certainly never broken traction on my front wheel even during extremely hard braking.

LOL, yeah. I wasn't real impressed at the time.

[human909]

LOL, yeah. I wasn't real impressed at the time.

Well my equivalent was probably over reacting and braking too firmly (with my heavy front bias) in a steep rock garden. The braking combined with the front wheel wedging itself a little between two rock was enough for the front wheel to decide to stop moving. The rest of the bike and myself kept going.

Maybe I've been lucky, but in my half dozen OTB incidents I've largely been unhurt often landing on my feet except for the time which I had something get caught in my front brake at 35kph.... (My worst biking injury happened at 5kph!, dislocated and broken toe! )


Anyway we probably both have had enough of the technical discussion.... To summerise on the original question:
-practice (IMO practice with front brake only in grippy conditions)
-weight back for the best braking performance
-in slippery/loose conditions it comes down to experience and skill which can be trained and learnt by time and the above tips

[trailgumby]

To summerise on the original question:
-practice (IMO practice with front brake only in grippy conditions)
-weight back for the best braking performance
-in slippery/loose conditions it comes down to experience and skill which can be trained and learnt by time and the above tips

I can agree with that.

[CKinnard]

Braking questions and responses are as entertaining now as they were 20 years ago.

- the OP's bike is a flat bar. sitting more upright (cf roadie) with CoG further rearwards means rear wheel lateral slide is more difficult to correct with bodyweight adjustment. i.e.
higher and more rearward CoG will more quickly deviate away from the line between front and rear contact patches, and be harder to bring back onto line.

- static coefficient of friction for generic bicycle tire rubber on clean dry surfaces is around 0.8.
on wet surfaces 0.3 is typical. that's a massive difference.

- tire coefficient of friction changes with tire pressure and temperature.
traction is different between wider lower pressure tires and narrower higher pressure tires.

- rubber coefficient of friction is not linear for contact area or normal force.
review the molecular elements of rubber traction (adhesion forces including Van der Waals, hysteresis, wear)

- the primary consideration when braking is to maintain balance i.e. keep CoG between contact patches. this is not difficult, but requires practise and staying more agile on the bike so you can shift bodyweight easier, and front wheel direction (steering) easier. This is harder sitting more upright in the saddle such as with the OP's bike. It is near impossible to stop rear wheel lateral slide during high speed heavy braking, without dynamic CoG countershifting. As soon as CoG moves laterally away from the line between contact patches, an opposing lateral force is applied to both contact patches, and the lower loaded rear tire is usually the one to wash out first.

- personally, I've lost traction with both tires half a dozen times on wet descents. Fortunately I was able to stay balanced all but for one time, by balance and steer adjustments.

- as for ratio of front to rear braking, it is not as important an issue as maintaining CoG between contact patches (staying balanced).

An interesting consideration is - does rear braking help one keep CoG between contact patches when braking hard?

As always, safer higher performance braking is a learned skill.
So it really is worth getting out and practicing/learning.

[ball bearing]

Yesterday was interesting - wet roads riding my Lynskey CX with discs and Continental Cyclocross Speed 32 tyres. I was slowing for the final downhill turn into my home street and my rear wheel locked and skidded twice. I was not braking hard at all on the rear, so the skid was a total surprise.

Is there a known problem with the Continental Cyclocross Speed 35 tires? The pressures were 50psi on both tyres. I am sure there was no oil on the road and my braking technique is pretty good.

My instinct is to mount a pair of 28mm Continental Grand Prix 4000S II. I'm just not sure about using them on gravel.

[human909]

Down hill and braking means very little weight on the rear wheels. It certainly sounds like you were too heavily rear biased.

[ball bearing]

Down hill and braking means very little weight on the rear wheels. It certainly sounds like you were too heavily rear biased.

You are probably right. It pouring today so I'll head out and try a few practice stops. These are my first set of Continental Cyclocross Speed tyres, so maybe I'll try lower pressures.

[human909]

You are probably right. It pouring today so I'll head out and try a few practice stops. These are my first set of Continental Cyclocross Speed tyres, so maybe I'll try lower pressures.

Lower pressures does not necessarily mean more grip on tarmac.** And if it does it is normally quite minor. 50PSI seems fine to me, IMO.

**Let's try not to get back into the previous discussion about how much difference the contact patch makes. Simple physics says no difference. More complex physics and experimental data says only slightly but depends on the circumstances and gets complicated quickly.

[NASHIE]

Cant really compare cars to bikes but for wet weather i increase tyre pressure on the track car. You want to be cutting through standing water and not ballooning out softening the tyre so its aquaplanes over the water.

[human909]

Cant really compare cars to bikes but for wet weather i increase tyre pressure on the track car. You want to be cutting through standing water and not ballooning out softening the tyre so its aquaplanes over the water.

Makes sense. That is something I didn't know was done.

However it isn't applicable to bicycle tyres as their pressure and contact patch shape means that aquaplaning is not an issue.

(I presume NASHIE knows, but for those that don't know. Aquaplaning is very much different to traction with water acting as a thin lubricate between contact surfaces. It can occur in a straight line without any braking action.)
https://en.wikipedia.org/wiki/Aquaplaning

[NASHIE]

Yes agree, little that applies to cars converts to bikes. Massive amount of variable's in braking. As has been well covered its practice at varied brake modulation over varying surfaces thats get you to the best technique. Some people have good feel at braking either with the foot or hands and others not as good. For those that don't race or push the limits is good practice to get out in a controlled or quite location and practice emergency stops and hard braking (wet grass dosent hurt to much). One thing is for sure everyone even the best in cars or on bikes get it wrong from time to time due to human error or the environment just throws you a curly one.

[CKinnard]

Cant really compare cars to bikes but for wet weather i increase tyre pressure on the track car. You want to be cutting through standing water and not ballooning out softening the tyre so its aquaplanes over the water.

preventing aquaplaning is a reason to run car tire pressure higher in the wet.....but it has a lot to do with target tire temperature.
in the wet, the tire won't heat up as much as on dry road, resulting in lower heat related inflation. therefore you start with higher cold psi.

[trailgumby]

Cant really compare cars to bikes but for wet weather i increase tyre pressure on the track car. You want to be cutting through standing water and not ballooning out softening the tyre so its aquaplanes over the water.

preventing aquaplaning is a reason to run car tire pressure higher in the wet.....but it has a lot to do with target tire temperature.
in the wet, the tire won't heat up as much as on dry road, resulting in lower heat related inflation. therefore you start with higher cold psi.

Absolutely. Operating temperature is going to be lower in the wet, so you have to compensate.

[ironhanglider]

I have been enjoying the technical discussion of traction, and it seems that ground pressure is a significant factor.

I have some experience on tandems and a particularly heavy one at that with north of 200kg since neither rider is a lightweight. I know that we never lost traction on a normal surface, and that the braking limit was down to the brakes. I could haul on the levers as hard as I could and the bike could stop at a comparable rate to a single bike and I only managed a back wheel skid once when we went through a puddle. I suspect with the best available brakes that the tandem may even outperform a single bike, largely because there are two loaded wheels. However that does require at least with some warning, because having the stoker collapse into your back does throw the concentration off a little.

It does make me a little sad because one of my favourite memories of racing hard was doing a criterium in the rain with one corner in particular that was fast and tight. Throughout the race I kept going through that corner faster and faster testing the limits of traction until on the ultimate lap I led through that corner at full speed and heard the sounds of a crash as the rider behind me failed to keep his bike upright. For years I have smugly thought that it was down to my clearly superior skills , but might it actually be that I was just fatter which gave me more traction?

Cheers,

Cameron

[human909]

I have been enjoying the technical discussion of traction, and it seems that ground pressure is a significant factor.

In normal circumstances ground pressure is 99% of the story. In normal circumstances friction is DIRECTLY proportional to the load. Contact area of tyres makes no difference in the circumstances described particularly at higher pressures. (In reality the latter is less true particularly for 'sticky' substances, but for our purposes it really doesn't aid the discussion.)

I suspect with the best available brakes that the tandem may even outperform a single bike, largely because there are two loaded wheels.

Your suspicious are correct. With more weight over the rear wheel your limiting factors become brakes and tyre traction which can give you ~50-100% faster stopping distance than a normal bike! (Maintaining balance during such rapid deceleration with two people migh introduce additional challengers.)

For years I have smugly thought that it was down to my clearly superior skills , but might it actually be that I was just fatter which gave me more traction?

Keep being smug. Your weight also means that the needs proportionally more traction to corner or to brake. (50% more weight means 50% more traction available, but also 50% more traction needed)

In theory it all balances out exactly. (Like I keep hedging my comments, in practices some subtleties may arise.)

[CKinnard]

Here's a question!

Losing grip in a corner occurs when side force on a tire contact patch rises just above friction force.
Mathematically
friction force = static coefficient of friction * normal force
side force = mass * radial acceleration
In the above, for a flat surface, normal force and mass (of rider and bicycle) are the same.

To find what side force is required to lose grip, the equation is
side force = friction force
or
mv^2/ r = umg*cos(surface slope)

For this equation, mass cancels out because it effects friction and side forces equally.
Therefore, the mass of an object should not effect how fast it can go around the corner.
i.e.
every time you increase rider weight, you increase friction force to the same degree you increase side force!

___________________


On another note, the Holy Grail of motor racing is to reduce car weight to improve performance with cornering.

If traction at speed in corners was purely a matter of greater mass, then lighter cars would have less traction through corners.

But there's a way to increase friction force without increasing side force - the use of aerodynamic downforce which adds no mass to the car, but increases normal force.

Aerodynamic downforce is a lot harder for a cyclist to accomplish, but it is worth keeping in mind.
I don't think I've ever seen an analysis of the down force a cyclist achieves in various positions on the bike.

[trailgumby]

Therefore, the mass of an object should not effect how fast it can go around the corner.
i.e.
every time you increase rider weight, you increase friction force to the same degree you increase side force!

You're also increasing the work the tyres have to do, which results in higher operating temperatures and higher wear. Lighter cars are easier on their tyres and stay in the "sweet spot" for longer during a race which, in addition to being faster to accelerate and brake, is another reason why car constructors pursue light weight. Heavier cars can struggle with their tyres going off earlier, especially on tracks with lots of bends, affecting performance and therefore race strategy.

Not really a factor for bicycles with current tyre tech though.

Edit: you're also assuming that tyre grip improves in a linear relationship with load, and we've already established that pneumatic tyres do *not* follow the standard friction model.

[CKinnard]

Edit: you're also assuming that tyre grip improves in a linear relationship with load, and we've already established that pneumatic tyres do *not* follow the standard friction model.

yes, the non linear load:traction relationship discounts mass canceling out in the above friction force = side force equation.
BUT to what extent.

I read a few tech papers about non linearity years ago, but vaguely remember it not being that large.

[human909]

Therefore, the mass of an object should not effect how fast it can go around the corner.
every time you increase rider weight, you increase friction force to the same degree you increase side force!

Yep there shouldn't be too much surprises there otherwise. Shrink a car to 1/100 of the size to increase the speed! Nup all this pretty much scales with mass. **(normal fine print)

On another note, the Holy Grail of motor racing is to reduce car weight to improve performance with cornering.

Reducing the car weight is primarily about acceleration. When you get down to the weight, the soft STICKY tyres of a F1 car and the low pressures, the other cohesive forces come more into play.

Given that bicycle tyres are 3-5x higher pressure that F1 tyres and made of material that actually lasts for years rather than just one race we are really comparing apples with oranges.

If traction at speed in corners was purely a matter of greater mass, then lighter cars would have less traction through corners.

Nobody has said that, and your own statements earlier disagree with that.

[Zippy7]

Wow, so scientific about braking.

Just adding my experiences. On a flat bar, commuting to work, all too easy to go over the bars.
Have done so in the dry when a car pulled out suddenly (and drove off whilst I stacked it).
No contact with the car, so not their fault and cops not interested in it.

I'd suggest moving your butt off the seat and shifting it back just as you want to brake hard (think of it as pushing yourself back with your arms and getting lower). That should help with the rear traction.

Also, as a kid, rear skids were fun, but I remember doing them on a BMX with a pedal brake for the rear.

In terms of rear skids now, maybe practise rear skids on a bit of dirt, with the goal to be flicking dirt up with the rear wheel?

Anyway, from my perspective, I think I'm too big and heavy compared to being a kid, and its way too easy to fly over the bars now. Perhaps slow down for the commute and save the speed for the bunch rides?

[human909]

Wow, so scientific about braking.

Yep. Science/Engeneery/Bikie Geeks!

Anyway, from my perspective, I think I'm too big and heavy compared to being a kid, and its way too easy to fly over the bars now. Perhaps slow down for the commute and save the speed for the bunch rides?

You are neither too heavy or too big. You just need better front brake modulation. Best way to get that is practice.
{Seriously, practice}