Mountain Biking Physics Questions

A couple years ago I bought a new bike and got back into mountain biking. I missed a bit during my time away from the mid '90's to today. Bikes are vastly different, and I've had fun relearning. But every so often I come across something that is just accepted knowledge that goes against my understandings of physics and my experience with cars. I just saw a biking how to that really left me scratching my head. 
 

It was a demonstration on how to do a Scandanavian flick on a mountain bike. 
 

https://youtu.be/jn5fYdhc-Qc?si=R2Iq3tTY25Nfvrn5
 

This is a well respected mountain biking channel, and a quick search showed that there are many more like it. I'm very familiar with using it in a car, l've used it rally crossing. I never thought to apply it to a bike, and I can't see how it makes any sense. With a car, you have limited options on how to control weight transfer and brake proportioning. You also have weight transfer front to rear and side to side. On a bike, your body is the majority of the weight, and you can drastically alter the balance over just two tires. You also have infinite brake balance. In cars, I don't think the flick is about maximizing cornering speed, but about getting your car in position for the best corner exit. On a bike, cornering speed and momentum are important because you can only pedal with so much power. Not like a car accelerating all the way down a straight. Ignoring that you aren't supposed to be skidding around and tearing up trails, I don't think this is really a scandi flick. More like just pulling the e brake, but in the wrong direction for no apparent benefit. What an I missing here? 

Another frequent bike "truth" I hear is how 4 piston calipers are more powerful than 2 piston calipers. This also doesn't make sense. My understanding is that the number of pistons don't affect the power, it's the ratio of the master cylinder to caliper and the length of the brake lever movement. To be more powerful, the lever would need to travel farther. More pistons can provide better feel and maybe a slightly larger pad, but should not have much effect on total power. Larger rotors and better pads should have much more effect than more pistons. My bike has 2 piston caliper, I upgraded the rotors and pads (203mm front, 180mm rear.) They have plenty of power, but they are not quite "1 finger brakes" like I'm told they should be. That makes me think that the 4 pistons do indeed have a longer lever throw. Or am I missing something? 

I think you're right that it's more akin to a handbrake turn, one possible good reason to start with a small slide in the other direction might be to get a feel for how the rear tire slides on that particular surface before you initiate the real turn and potentially give yourself a balance problem on top of a messed-up line. I'm not a great MTB rider though and I just avoid locking up, it's not only bad for the trails but also kills tires fast.

More pistons do seem to be more powerful in MTB brakes from my experience, I think that's from spreading pressure more evenly across the pad. In cars there are even race calipers with multiple different-sized pistons to fine-tune the pressure across different parts of the pad.

Racers do it into tight turns, especially in Enduro events, because of hairpin turns. Folks will also show off by doing stoppy turns or using French Lines. By keeping the back end loose, you can decrease the radius of turn-in. 

Outside of racing or by accident, I wouldn't suggest intentionally doing it very often. 

More pistons definitely stop you better to a point.  Just like in cars, you are traction limited, there are limits to what fits, and at some point there are diminishing returns.  

• My road bike has 140mm rotors on it and single piston calipers.  On big downhills you get brake fade.  It could probably use bigger rotors for more heat dissipation if I was always bombing downhills.
• My e-Bike is 50 lbs, long travel Trek Rail, made for hitting big stuff and going real fast.  It has 220mm Hope rotors front and rear with 4 piston Hayes Dominion brakes on it.  With 29x2.6 tires front and rear, there is a ton of traction on most surfaces.  Since you can session the E36 M3 out of it, its nice to have something with some fade resistance.  There was a yuuuuuuge upgrade going from 200mm Ice Tech rotors and XT 4 piston brakes (generally considered to be pretty good) to the 220s and Hayes Dominions.  It now has the 1 finger braking that everyone talks about, even on a big bike rode hard by a big dude.
• My fat tire bike has 2 piston Hayes Dominions and 180mm front/160mm rear Sram Centerline rotors.  Since it is not a bomber bike, mainly ridden slowly in cold temperatures, and is traction limited because of riding in snow, there is no value in going to some big ol honking rotors and calipers on it.  

There is also some real value in having something better than the cheapest Sram/Shimano/Tektro brakes and going to a Hope or a Hayes caliper and lever.  Of course the Hayes Dominions are around $600 to outfit a bike, so now we are back to the laws of diminshing returns.  

Piston area is key here, not pad area. With more pistons, the brake fluid pressure (P) is acting over a great fluid area (A=n * pi()/4 * D^2, where n is number of pistons and D is their diameter). The brake force is then F=P*A. You may notice a longer stroke because the swept volume of the pistons may also increase, but in a perfectly rigid system with no pad gap, there won't be much stroke to notice. 

With my 29er hardtail, I've need a scandi flick / trailbraking to rotate tight turns where the long bike just has too large a of a turn radius. Rather than slow to a crawl, the extra yaw just help clear the turn. 

I am here to just echo the others, I think the scandinavian flick is just a poor name for what actually happens, which is more trailbrake induced oversteer.  But, yes, it is a very nice tool to have in the arsenal, with bikes just getting stupid long and low now it's difficult to get them to rotate tight hairpins without using some sort of rear lockup and slide-out.   It's either this, or a manual, or a series of manuals. 

In reply to pheller :

Racers do it into tight turns, especially in Enduro events, because of hairpin turns. Folks will also show off by doing stoppy turns or using French Lines. By keeping the back end loose, you can decrease the radius of turn-in. 

Outside of racing or by accident, I wouldn't suggest intentionally doing it very often. 
 

I completely get rotating the bike to make tight turns. I question the reasoning for the extra step that the flick requires on a bike. You do it in a car to accentuate weight transfer and break rear traction to get the rear to come around, because you can't finesse a car around with your body weight. A skilled ride can literally lift their bike off the ground and flip it 180 degrees if they choose to. They would have no problem just bringing the the tail around without the extra flick. It doesn't appear to have anything to do with weight transfer like the car version. My best guess is that it possibly gives you more time to "wind up" and store energy to whip the tail out, like pitching a baseball. But actually whipping the the tail the opposite direction first appears to be counter productive, wasting energy, and now it takes more energy to flick it back farther in the other direction. It would seem that just flicking the tail out in one direction would be just as effective. 
 

In a car, it makes sense. To make a right turn, you steer left. The whole car, including the rear, moves left of center. The balance shifts to the right. You turn back to the right. Now the car wants to go right, and the weight distribution suddenly shifts left, giving it an extra kick to break traction. Since more weight of car is shifted forward due to braking, the front has more traction, resisting the sideways force more than the rear, which now more readily steps out to help rotate the car. You don't get that same kind of weight transfer on a bike. But you have much greater control of overall weight transfer with your body. When you break it down moment by moment, it appears that the flick on a bike just makes a tight turn even tighter. 

In reply to cyow5 :

Piston area is key here, not pad area. With more pistons, the brake fluid pressure (P) is acting over a great fluid area (A=n * pi()/4 * D^2, where n is number of pistons and D is their diameter). The brake force is then F=P*A. You may notice a longer stroke because the swept volume of the pistons may also increase, but in a perfectly rigid system with no pad gap, there won't be much stroke to notice. 

With my 29er hardtail, I've need a scandi flick / trailbraking to rotate tight turns where the long bike just has too large a of a turn radius. Rather than slow to a crawl, the extra yaw just help clear the turn. 

I think I figured out what I was missing. According to the math, the number of pistons has zero effect on the clamping force of the brakes. You can change the ratios between the caliper pistons and master cylinder piston, but it doesn't change the amount of power available from your hand squeezing the brake lever. If the combined area of caliper pistons in 2 piston and 4 piston brakes were the same, they would have the same clamping force for the same lever effort (and travel.) 4 piston bike brakes don't have more power because of the piston count. They have more power because the skinny rotor surface limits the size of pistons a two piston caliper can have, which is below optimal. Resulting in shorter handle throw and more effort. You would think that they could balance that with master cylinder sizing, but they don't for whatever reason. 

The thing with bikes is, you greatly outmass the bike (even the modern ultra heavy bikes) so body English is key.

One of my favorite techniques for a tight corner was to lean way back past the saddle, set my outside knee on the seat post clamp, and just lay the bike over.  i'd turn practically inside the bike's wheelbase and the act of turning while falling set me upright again and away I went in a completely different direction.  Setting all of your mass as far back as possible kept the front tire from digging in and highsiding you, I think.

Real fun was to try it on road slicks, on asphalt.

I got the idea from hearing stories about riding a locally made recumbent that had a ridiculously short wheelbase, you could just flop it over and change direction like a ball bouncing off of a wall.  Madness to think about, amazing to feel happen.