Does MTB rider position affect bike cornering traction?

[willbikeformountains]

So there's this idea in mountain biking that keeping your weight over the tire contact patch equates to more cornering traction. What I mean is instead of leaning with the bike, you lean the bike under you while staying mostly over the tires (see the right "Bike more than body" image below).

So assuming the bike is leaning at the same angle and only the rider's angle relative to the bike is changing, does tire friction change?

I would think the normal force and side force would remain unchanged regardless of rider position so that F= μ N where N is simply N=mg.

I know rider location alters torque on the system (rider+bike) but I'm not sure that this alters friction in a cornering scenario.

 

[berkeman]

Welcome to PF.

That's a great question. Can you post a link or two from a reputable website (like Mountain Bike Action or similar) that discusses this?

On my MTB I usually keep a neutral position in corners (I think based on MTB Action and other YouTube tutorial videos), but I haven't tried the variations that you mention. In dirtbike riding, there is a preferred position for riding side-hills with low traction, but that's a different question.

 

[berkeman]

I know rider location alters torque on the system (rider+bike) but I'm not sure that this alters friction in a cornering scenario.

Can you provide a link for this, or else write out the torque equations?

The only thing I can see is that leaning the bike over more than the rider may lower the center of mass (CoM) of the rider+bike, which may change how the bike interacts with the dirt (assuming a fairly uniform tire tread pattern along the width of the tire).

 

[hutchphd]

Summary:: Does rider position (angle relative to bike) affect bike cornering traction?

I would think the normal force and side force would remain unchanged regardless of rider position so that F= μ N where N is simply N=mg.

That will be largely true but there are nuances. I can think of two: the position of the contact patch on the front wheel changes and the bike may track differently, and this is seldom a static situation. I too would love to see some research.
Welcome

 

[willbikeformountains]

Welcome to PF.

That's a great question. Can you post a link or two from a reputable website (like Mountain Bike Action or similar) that discusses this?

On my MTB I usually keep a neutral position in corners (I think based on MTB Action and other YouTube tutorial videos), but I haven't tried the variations that you mention. In dirtbike riding, there is a preferred position for riding side-hills with low traction, but that's a different question.

"simply dropping your outside foot you will DOUBLE your traction! Why? Because if your feet are level 50% of your weight has to be on the inside pedal! That means 50% of your weight is not above the tires! Which means you have half the amount of down force on your tires."
https://betterride.net/blog/2014/mountain-bike-cornering-foot-position-part-1/

This explanation sounds completely off the mark.

 

[willbikeformountains]

Can you provide a link for this, or else write out the torque equations?

The only thing I can see is that leaning the bike over more than the rider may lower the center of mass (CoM) of the rider+bike, which may change how the bike interacts with the dirt (assuming a fairly uniform tire tread pattern along the width of the tire).

The torque on the bike (that would change lean angle) is a function of angle and distance from where the tires contact the ground. I think it's T=mg*cos θ (d)...so at 90° Torque=0.

 

[berkeman]

so at 90° Torque=0

Not in a turn. But ultimately for your original question, what does torque have to do with traction? In a turn, the centripital force to force the MTB sideways around the arc of the turn depends on the speed and radius of the turn. The tread pattern on the ground matters, so if altering the angle of the bike wrt the ground changes the tread contact pattern, that would make a difference. Beyond that, I'm not seeing a big difference.

Even the lower CoM doesn't seem to make much difference so far to me. Can you post links to (even questionable) reading that you've been doing about this?

A more significant factor could be steering geometry and control with lean angle, instead of traction. That seems more likely at this point to me...

 

[cjl]

"simply dropping your outside foot you will DOUBLE your traction! Why? Because if your feet are level 50% of your weight has to be on the inside pedal! That means 50% of your weight is not above the tires! Which means you have half the amount of down force on your tires."
https://betterride.net/blog/2014/mountain-bike-cornering-foot-position-part-1/

This explanation sounds completely off the mark.

Yes, that explanation is complete nonsense. The actual forces on the tires just will depend on the acceleration you are undergoing and your weight, and you can't just double the downward force on the tires through this "one simple trick".

That having been said, tire interactions with surfaces like sand, dirt, clay, etc are far more complex than just the basic "physics 101" explanation of a normal force multiplied by a frictional coefficient. It could very well be the case that inclining the tires more to the surface changes this interaction in such a way as to give you more traction, due to a difference in how the dirt builds up and shifts under the tire or something like that. I think it's very likely that body position and bike inclination do impact the ultimate traction you can achieve, but it's certainly not for the reason espoused in that link.

 

[willbikeformountains]

Yeah, I'm assuming the the angle of the tires relative to the ground is constant for the purpose of this discussion.

 

[Lnewqban]

The suspension of the bike (springs, shocks and tire's carcase deformation) is less effective as the chassis of the bike leans more.
That negative effect may be compensated by the plowing effect of the tires sliding on soft dirt, sand and gravel, which improves traction, and steering precision where marginal, as explained in post #2.

 

[berkeman]

The suspension of the bike (springs, shocks and tire's carcase deformation) is less effective as the chassis of the bike leans more.

Excellent point. So far (given a constant tread pattern contact patch with lean angle), it sounds like the two biggest factors are suspension and steering angle. Let's all go ride and experiment!

(I have a singletrack BMX natural terrain track that I ride near my house, but will be on pavement tomorrow instead. Next dirt ride is Tuesday, and I'll try some experiments...)

 

[jack action]

So assuming the bike is leaning at the same angle and only the rider's angle relative to the bike is changing, does tire friction change?

I think your assumption is wrong. It is the rider that is at the same angle and the bike that leans more. It seems rather clear from the pictures.

I suspect the reason behind it has to do with camber thrust. When a tire leans, it creates a lateral force:

The effect of leaning the bike more means that you don't need to steer the wheel as much to create the needed lateral force. I think this is also clear from the pictures from your post. You could even lean the bike enough to create a need for negative steer.

I suspect the objective is to keep the wheel straight as much as possible. Not sure why, though. Maybe it has to do with the fact that there is a need for countersteering with a bike in a turn. You would want to keep your wheel straight such that you can correct rapidly either way. Just a guess.

 

[DaveE]

Theory aside, there is experimental evidence that, in the more ideal case with slick tires, and smooth pavement, you will want to lean into keep the bike more vertical. These guys can't win, or avoid killing themselves, when cornering at high speed without doing this. They all are leaning in as much as possible.

 

[willbikeformountains]

I think your assumption is wrong. It is the rider that is at the same angle and the bike that leans more. It seems rather clear from the pictures.

It is the rider at the same angle in the pictures. Are you saying it's not possible for the rider to move towards the inside and maintain the same bike lean angle?

 

[berkeman]

Theory aside, there is experimental evidence that, in the more ideal case with slick tires, and smooth pavement, you will want to lean into keep the bike more vertical. These guys can't win, or avoid killing themselves, when cornering at high speed without doing this. They all are leaning in as much as possible.

Great stuff, but the main reason for getting your body lower on sportbikes/superbikes/MotoGP on paved racetracks is to gain ground clearance for the lean angle. The bikes and tires are near their limits for lean angle, so with the rider doing all they can to lower the CoM, that increases cornering forces and speed.

The day that I can get a knee down on a dirt MTB track is the day that I retire to my couch...

 

[jack action]

Are you saying it's not possible for the rider to move towards the inside and maintain the same bike lean angle?

Assuming the bike has little effect on CG location (compared to the rider), yes. The angle depends only the lateral acceleration ($\frac{v^2}{r}$).

 

[willbikeformountains]

Assuming the bike has little effect on CG location (compared to the rider), yes. The angle depends only the lateral acceleration ($\frac{v^2}{r}$).

On a bicycle you can lean the bike more than necessary. I can lean the bike over an extreme and ride nearly straight by standing on the outside pedal and holding the outside (higher) grip. I would think unless we're considering minimum lean angles you can keep the bike lean angle the same and move the rider around a bit. I think the steering angle will differ to maintain balance and turning radius.

 

[Lnewqban]

Excellent point. So far (given a constant tread pattern contact patch with lean angle), it sounds like the two biggest factors are suspension and steering angle. Let's all go ride and experiment!

I believe that improving suspension is the only good reason behind hanging-off for motorcycle racing on pavement prior hard parts of the chassis start dragging.
Once the racer reaches the limit angle of the chassis, hanging off is limited by available clearance between bike and road/track surface.

To the OP: The lean angle of the center of mass of the bike-rider system must remain the same for similar speed and radius of turn: the total weight plus centripetal effect vector must go trough the line that joints both contact patches, just like only the weight vector does for staight trajectories in order to keep balance.

As the CG of the rider gets relocated, the CG of the bike must be relocated in the opposite direction, in such a way that the position of the total CG respect to a plane perpendicular to the contact patches line remains the same.
The respective relocation of the CG’s depends on the mass and spatial location of each.
That applies to any curve of constant radius/speed and to staight trajectory as well.

 

[rcgldr]

(racing motorcycles) These guys can't win, or avoid killing themselves, when cornering at high speed without doing this. They all are leaning in as much as possible.

Depends on the high speed, somewhere over 120 mph, no one hangs off because of aerodynamic drag issues. You see this on the high speed (in some cases over 200 mph) turns at Isle of Man TT, and also at Daytona where 2 of the banked straights are used, where the bikes are near horizontal on the banked turns leading onto the straights at 180+ mph.

About "bike's suspension working better when the bike is closer to vertical (less leaned over)". When encountering bumps, the tires track inwards over bumps based on the lean angle of the bike, not vertically. The tire experiences a lateral load due to the cornering, and an inward (relative to lean angle) load due to bumps. Body leaning inwards, means the bike is leaned inwards less, which could make it more stable, but another advantage is there's a bit more margin for error if the tires start to slip. Modern racing slicks are more forgiving than tires of the past, where not much grip is lost if they start to slide, allowing riders to drift through turns which wasn't possible with the tires of generations past.

 

[Claudio476479]

I wish to give an extra contribution. We may think that friction between tire and ground only depends on the force "pushing" the tire against the ground and the shape of the tire (tread pattern, size, etc...).

I think there something else to take into account. Think of a "cam cleat" (google it if you don't know what it is) the rope is subject to the same force and the pattern of the cam cleat does not change, yet the rope will slip in one direction and not the other, why? That's because in one case moving the rope will increase the clamping force and in the other case will decrease it.

When the bike is outside the curve (compared to the biker) as soon as it starts to slip it will get lighter on the ground and slip even more. When the bike is inside the curve and it starts to slip it acts as a wedge between you and the ground. I think that is this case the "get lighter" effect is less present

 

[berkeman]

Welcome to PF.

When the bike is outside the curve (compared to the biker) as soon as it starts to slip it will get lighter on the ground and slip even more. When the bike is inside the curve and it starts to slip it acts as a wedge between you and the ground. I think that is this case the "get lighter" effect is less present

So the difference between a low-side crash and a high-side crash?

 

[HilltopProf]

I think what it's about is putting the rider's weight through a larger arc so that the centripetal force required to move it through the corner is reduced. MTB coaches routinely demand the hips be swung outward, therefore putting that mass through an arc of greater radius than the bike.

 

[Lnewqban]

I think what it's about is putting the rider's weight through a larger arc so that the centripetal force required to move it through the corner is reduced. MTB coaches routinely demand the hips be swung outward, therefore putting that mass through an arc of greater radius than the bike.

Welcome!
As the weight of the rider moves in one direction (radially), the weight of the bike naturally moves in the opposite direction.

Therefore, the combined center of mass still continues on the same arc and aligned (lean angle) with the contact patches of both tires.
Otherwise, the bike would not be balanced respect to the summation of lateral vectors (centripetal and weight).

There are only two ways to reduce the required centrifugal force: reduced speed or/and wider arc (greater radius of turn).

 

[HilltopProf]

The rider's mass is far greater than the bike: so if the radius of the corner is given, the centripetal force required from the trail tread to move the rider through the arc will be reduced if you can move some of the (m)ass outwards so it can travel through a larger radius. This is unlike a motorcycle where the bike weighs more than the rider. Every MTB racer you'll see positions their hips outboard of the bike to wring a few extra milliseconds out of their race time. It looks odd but there's a science to it.

 

[berkeman]

Every MTB racer you'll see positions their hips outboard of the bike to wring a few extra milliseconds out of their race time. It looks odd but there's a science to it.

Can you share some pics? For me, getting my weight on the outside pedal (and low) is the biggest part of cornering well.

https://www.elasticinterface.com/magazine/downhill-mountainbiking-in-10-tips/

2 – The body position on the MTB​

A good body position on the bike is critical in all MTB disciplines.
As we pointed out in our guide to The perfect posture for downhill riding, when descending, a good rider’s position requires standing up in a centred position on the bike, looking ahead, with open elbows, low heels and knees slightly bent, ready to absorb the impact forces of the obstacles.

 

[hmmm27]

Shock absorption is articulated, longer.

The horizontaler the front tire is, the more it digs into the ground when it's turned into the turn ; momentarily higher friction than otherwise = higher centripetal traction = better recovery from a sideslip.

 

[mjw]

As pointed out already, the angle of the bike+body system has to be that which balances the acceleration through the corner, otherwise the rider would tip over.

Tipping the bike more than the rider helps for several reasons also brought up by other posters:

- camber thrust: a leaned tire produces lateral force without a lot of tire deformation whereas a steered tire produces lateral force through a lot of localized slippage in the contact patch

- tire construction: MTB tires are deliberately made with smaller knobs in the center for better rolling and larger knobs on the outside for cornering grip. Leaning the tire engages the larger knobs

- keeping the contact patch under the rider’s body: this has no advantage in a steady state turn but if the tire slips, the rider has a better chance of catching the slide than if the contact patch is already further from the rider when it slips.

A big part of mountain biking skill is varying the normal force to your advantage. Yes the average normal force is fixed by weight, but a rider can instantaneously increase normal force a lot when needed and recover the balance after.

One other thing about swinging hips out - Cornering involves two accelations, linear and angular. On a mtb, a rider can swing their hips out, increasing their yaw rate BEFORE the apex of the corner. This way the maximum angular acceleration can happen before the maximum linear acceleration. This spreads out the forces seen by the tires in time and reduces the peak force. When you see pros slashing turns and it looks like a skier carving powder, they are doing this.

 

[HilltopProf]

Mjw: they call that “schralping”

It’s done by pointing your toes downward as you go through the turn, unweighting your rear wheel so you can initiate a rear wheel skid without braking. Looks cool; erodes trails!

Back to cornering: by lowering your weight in the corner, you can keep the bike from overturning. So getting the body lower and hips outward lowers the rider’s CoM and puts it through a larger radius, reducing the centripetal force required to put rider+bike through the curve.

Other than the physics, Maxxis and other manufacturers have stated that the design of the tire and knobs is done to maximize lateral friction when the tire is leaned over. There’s a cool video Maxxis did regarding the Assegai tire and Greg Minaar’s input. They get into block design and sidewalls and all that good stuff: