Why does KTM work?

[tobyzusa]

In the past there have been a number of interesting threads on the mechanics of turning a motorcycle. I’ve got a bit of a different question on that old theme.

 

I ride using the KTM (kiss the mirrors) riding method. In fact, while I don’t truly hang off, the more I move off the seat and forward, the more stable the bike is in aggressive cornering scenarios. KTM also seems to make the bike turn-in effortlessly. Many have stated (correctly I believe) that the main benefit of KTM is to provide a reduced lean angle and therefore greater ground clearance at a given speed. So here’s the question, if the lean angle is truly reduced by KTM, what is the mechanism that actually turns the bike? I thought it was by moving the contact patch off center and toward the inside of the tire, causing the bike to follow the leaned/reduced radius of the tire, but how can this be if I’m reducing the lean angle by using KTM? I know that KTM works – I just don’t know why anymore.

 

 

Inquiring minds want to know.

 

[Lawman]

Brad,

It's 11:49 p.m.

Go to bed..

[Quinn]

My guess is that it's because your weight is not centered over the contact patch. The additional pressure smushes (technical term) the contact patch on the outside and decreases its radius in that area.

 

Now learned people will correct me.

 

 

 

 

[T__]

In the past there have been a number of interesting threads on the mechanics of turning a motorcycle. I’ve got a bit of a different question on that old theme.

 

I ride using the KTM (kiss the mirrors) riding method. In fact, while I don’t truly hang off, the more I move off the seat and forward, the more stable the bike is in aggressive cornering scenarios. KTM also seems to make the bike turn-in effortlessly. Many have stated (correctly I believe) that the main benefit of KTM is to provide a reduced lean angle and therefore greater ground clearance at a given speed. So here’s the question, if the lean angle is truly reduced by KTM, what is the mechanism that actually turns the bike? I thought it was by moving the contact patch off center and toward the inside of the tire, causing the bike to follow the leaned/reduced radius of the tire, but how can this be if I’m reducing the lean angle by using KTM? I know that KTM works – I just don’t know why anymore.

 

 

Inquiring minds want to know.

 

Brad, it might just be as simple as you are pulling more with your outside arm or pushing more with your inside arm by leaning over & kissing the mirror..

 

Try leaning it (the bike) over with only your inside arm & see if feels different..

 

I usually use more of my outside arm to get the bike positioned for the curve then once ready to corner hard release pressure on the outside arm & allow the inside arm to take over (that way both arms are not fighting each other).. It seems as I release the pressure from the outside arm the bike will just fall over into the curve naturally.. I do most of my actual cornering with the inside arm..

 

Twisty

 

[Twisties]

I've wondered the same thing. What I've heard is that it is the location of your center-of-gravity (cog) that matters, but like you I can't see how this turns the bike.

 

I did learn from dirt riding that forward weighting makes the front tire bite more, but I'm not sure ktm results in forward weighting, as you should have your butt back and your head forward, implying more low weighting than forward weighting.

 

Ok, I'll sit back and weight for some experts to chime in.

 

Jan

[Joe Frickin' Friday]

A typical motorcycle has steering geometry which is self-correcting. Cruisers have it to a greater degree, sport bikes have it to a lesser degree, but they all have it. This is due to the angle of the steering axis, and the position of the tire's contact patch relative to that axis.

 

By "self-correcting," I mean that the bike wants to go straight down the road: if you apply handlebar inputs to make the bike lean over, then let go of the bars, the bike will try to straighten itself up again. It follows, of course, that if you want to keep the bike in the turn, you have to keep pushing on those bars until the turn is done.

 

One way to cancel out this self-correction is to move your body weight off to the inside of the turn. You'll still need some initial steering input to snap the bike into the turn (the famous countersteer!!!), but once that's done, a relatively small shift of your body is enough to cancel the self-correcting tendency and keep the bike in the turn without any significant handlebar inputs.

 

In fact, you can hang your body off even further to the inside of the turn, in which case you will have to apply handlebar inputs to prevent the bike from spiraling in even tighter. Alternatively, you could still skip those handlebar inputs and instead roll on the throttle to keep from spiraling inward. Whaddyaknow - moderate throttle application also helps to get the suspension where it is supposed to be, further stabilizing things. The front end picks up a smidge, increasing the steering rake and making the handling less twitchy. Moreover, the traction demand is also distributed better: you're asking for less grip from the small front wheel, and letting the big rear wheel do more. The turn is starting to feel pretty good.

 

It's difficult to isolate yourself from the handlebars, but if your butt, feet, and knees are properly braced, it can be done - and then you'll be able to observe these effects directly. It also helps if you're on longer sweepers instead of super-tight stuff, so you have some time during each turn to feel what's happening. Let your left hand kind of hover around the grip, and try to make sure your right hand is just holding the throttle, and not pushing the bar in any particular direction. Watch what happens as you go through the turns with various degrees of KTM, without wrestling the handlebars in the middle of the turn.

 

So here’s the question, if the lean angle is truly reduced by KTM, what is the mechanism that actually turns the bike?

 

People seem to get confused by lean angle, and lean angle reduction, especially when we get to talking about a rider who is moving around on the bike. So let's clear some of that up.

 

First, we have three centers-of-gravity (more accurately called "centers of mass", or COM) to consider:

 

1. the COM of the rider;

2. the COM of the bike;

3. the combined COM of the rider and bike, which will exist somewhere on a straight line between the first two.

 

Second, the lean angle of a turn. When we talk about the lean angle of a turn, we're talking about the lean angle of the combined COM with respect to the contact patches. And this is dictated entirely by the speed and the radius of the turn, as in the plot below. Nothing you do - other than changing speed or radius - will change this lean angle.

 

When you move your body to the inside of the turn, you are changing the lean angle of your own body's COM. Since the combined COM does not change (see previous paragraph!), the bike's COM must move to a lesser lean angle. In the extreme case, if you could move your body far enough to the inside of the turn, the bike itself would end up perfectly vertical. So how is the bike turning in this case? Well, the combined COM is still at its original lean angle relative to the contact patches. The now-vertical bike is analogous to an SUV on the verge of rolling over, with all of its weight on the two outside wheels. The front wheel is turned a bit, and so the bike is following a curved track, even though it's standing straight up.

[Silver Surfer/AKAButters]

Amazing!

[gottabmw]

Amazing!

What's even more amazing is that I actually understood all that! Best explanation I've ever read. Thanks Mitch!

[tobyzusa]

Mitch,

 

Well written. Thank you.

 

I'm sure I'm just dense, but I still don't get it. The graph implies that that the only thing that affects the turning radius at a given speed is the lean angle. If that's true, then KTM reduces the lean angle at a given speed and therefore must increase the turning radius.

 

What am I missing?

[David]

I'm sure I'm just dense, but I still don't get it. The graph implies that that the only thing that affects the turning radius at a given speed is the lean angle. If that's true, then KTM reduces the lean angle at a given speed and therefore must increase the turning radius.

 

It's all about the COMBINED lean angle. Your bike is leaned less, but your body is leaned more.

[Tony_K]

It's all about the COMBINED lean angle. Your bike is leaned less, but your body is leaned more.

 

and to regurgitate info...

 

Center of mass is LOWER.

It takes less overall lean to accomplish the turn with your body down a bit more.

 

I know you got it now

[Joe Frickin' Friday]

What am I missing?

 

David summarized it; you're thinking about the lean angle of the bike all by itself, but there are two other lean angles you need to think about.

 

For the longer version, go back to my previous post and re-read everything after where I quoted you.

 

Maybe a slightly different example can help clear things up (or make them even muddier?):

 

Suppose you're cruising on a straight road. While maintaining a straight track down the road, you lean your body out toward the left mirror, and here's what you end up with:

 

1. the COM of the rider is to the left of the contact patches; that is, the rider is now leaned to the left.

2. the COM of the bike (which is what you're thinking about) is to the right of the contact patches; that is, the bike is now leaned to the right.

3. the combined COM of rider and bike (which is what the plot in my earlier post is about) is still positioned directly above the contact patches. This lean angle doesn't change, since the bike and rider are still cruising straight down the road.

 

KTM does indeed adjust the lean angle of the bike, but regarding the bike and rider as a single entity, that entity's lean angle is unchanged (unless you tweak speed or turn radius).

[tobyzusa]

Thanks Mitch and David. That's pretty clear and convincing. Appreciate the tutorials.

[Martyn]

I saw the title & thought this was going to about the rise of a European company making bright orange, mostly dirt orientated motorcycles.............

[Kerry in Mpls]

I saw the title & thought this was going to about the rise of a European company making bright orange, mostly dirt orientated motorcycles.............

Yep, me too.

[knight88]

Great question, and even greater answers... I was also wondering about how I was able to make the same turns as before without having to lean my bike over and scraping my center stand when I started to move more of my weight into the inside of my turns and having the bike lean less. Great explanations, now can you all answer this next puzzling question...."which came first the chicken or the egg?"

[leikam]

My experience is that the KTM body position also encourages looser arms so that I'm less inclined to fight my own steering inputs. That makes turning feel easier and the corner more stable.