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Sportbike Steering Vs Cruiser


Hotfoot
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I took my little cruiser bike out for the first time in a long time, after doing a lot of sportbike riding. It's a Suzuki Savage 650. Obviously it steers differently than my Ninja. But one thing in particular jumped out at me. One of the things my coach told me at school is that once the bike is leaned over in a corner, getting on the throttle does not make the bike stand up, that it maintains the same lean angle. OK, fine.

 

However, when I lean over that cruiser, and get on the throttle, it stands up noticeably. It is most pronounced at low to medium speeds, like 20-40 mph, but it took me by surprise a couple of times until I got used to it. I never really noticed it before - probably because I thought ALL bikes would stand up when you roll on the gas, so I used to maintain some pressure on the bars to "hold" the bike in the turn. I don't do that anymore. :)

 

So... what's the deal? Is it because it has a much shallower steering angle and/or greater trail? Or is it because the suspension is soft and the rear end squats under acceleration, changing the trail when I get on the gas? I'd really like to have a better technical understanding of how this works, anyone want to help me out? :rolleyes:

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My Intruder 1400 had to be fought to be kept on line under acceleration while cornering, or it would just head straight. But in my unskilled opinion, accelerating hard while leaned over will make any bike want to stand up. This isn't equally pronounced between bikes, but I've always used hard acceleration on corner exits to let the bike right itself. Maybe it's just something in my head.

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You said you'd like to have a more technical understanding of how it works, well I could write pages and pages about this stuff, but basically I think you are right that it is because your cruiser has "much shallower steering angle and/or greater trail?" But search a little on rake and trail and how they are used to make the bike balance itself, and motorcycle geometry and motorcycle physics, there's no shortage of fascinating stuff out there! For starters, try tonyfoale.com, then go to Articles and read the one about "Experiments with steering geometry." I thought that one was quite interesting. There's also a very well done article on wikipedia about countersteering.

 

I've owned several very different kinds of bikes. My observation is that the sportbikes usually hold their lean in the turns, but some bikes I have owned in the past wanted to stand up with no pressure on the bars, and my current KLR650 with knobby tires wants to fall in (I theorize that it would be more neutral with street tires). Like you, I also became quite aware of these differences after taking the superbike school.

 

My understanding is that the bike designers set the rake and trail to get the desired outcome, and I bet it depends a lot on the front tire profile. Sportbikes are most neutral because optimal handling is the primary goal of their design. I've never owned a cruiser but I'd bet they are set up for "stability" and wouldn't be surprised at all if some of them stand up in turns if no pressure on the bars, just like Eirik's comment about his Intruder 1400. In bike reviews in magazines I've often seen bikes praised for being "neutral in the turns." I figure this is what they are talking about.

 

Every sportbike I've owned would acquire a tendency to stand up with a worn front tire, which is why I often look forward to fresh tires. This issue only arises in street riding, not track riding, due to milder lean angles usually used. Increasing front tire pressure alleviates the problem somewhat in the interim. Hanging off seems to counteract it somewhat too. Try hanging off on your cruiser. :lol:

 

This comment that some riders make that they accelerate out of turns to help the bike stand up, it implies that we can not simultaneously slow down and stand up, or speed up and lean more, etc, and we all know we can do all combinations of steering/speed changes and it would be impossibly dangerous to ride a motorcycle if we couldn't. And countersteering gives us such direct and powerful control over the lean angle of the bike, so it seems kinda pointless to get fixated on the idea of using the throttle to change lean angle. But I do think that we may have to use some mild pressure on the bars sometimes in order to deal with the reality that some bikes in some circumstances are not all that neutral.

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Good stuff harnois. I think it is interesting that a bike will stand up when you get on the front brake which should effect the suspension in just the oposite way. I think that mostly has to do with the way the contact patch kinda pulls on the front wheel when the brake is applied though, not really what the suspension is doing.

 

Anyway, sorry I didn't mean to side track the thread.

 

Anyone able to provide a good, technical explanation as to why some bikes stand up when you roll on the throttle?

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accelerating hard while leaned over will make any bike want to stand up.

 

Common misconception. Most sport bikes will not stand up. It might run wider, but shouldn't stand up.

 

I can buy that. Maybe even with my cruiser, come to think of it. I can see myself getting confused between the tendency to running wide and standing up - because if you do not want to go wide, you have to use more force to increase the lean. Hence it will feel like the bike is standing up a bit because the bike will sort of finish off the turning and you will just naturally let it come back up. Food for thought - and testing. And now that the snow is going, I may even be able to go for a ride and test this soon :)

 

Also, as has been mentioned, different bikes act differently. For instance, the Intruder had to be "held down" by constant counter steering or it would just stand up at any time. Other bikes will want to fall in and will have to be steered into the corner to prevent it from toppling over. Some bikes will stand up dramatically under the brakes, others are almost oblivious to this. Tyres matters greatly - I've read tests that claim the new Honda VFR1200 (which comes with two brands of tyres, depending on market or luck) are fine with one brand and evil with the other.

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I searched Tony Foles book for a better explanation of this last night and read some interesting stuff about how rake, trail and front tire width would have an effect on this.

 

Then my head exploded before I was able to find what I was looking for :)

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You're doing good for a man with an exploded head, old chap :)

 

The CX500 had fairly little rake (26.5 degrees) and trail (100 mm or 3.9 in) and a narrow front tyre (3.25 in or 88 mm), yet it would stand up like the most eager A person on the most exiting day of his life if you touched the front brakes mid-corner. My Daytona-based Triumph has 27 degrees of rake, 105 mm (4.2 in) of trail and a 120 mm (4.7 in) front tyre and has just a gentle tendency to straighten itself under similar conditions.

 

How does that fit with the theories? Oh, sorry, forgot you're head's gone :P

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Check out the article on "camber thrust" on tonyfoale.com. Note the 3rd image and the explanation that follows it. So this camber thrust thing makes the front tire really want to turn in a very tight circle, toward the inside of the turn, like a coffee cup or cone rolling on a flat surface. This drives the tires up under the bike and thus the bike wants to stand up.

 

But the trail, just like the front wheels of a grocery cart, is trying to pull the front wheel in whatever direction the inertia of the bike wants to go (straight). This drives the tires out from under the bike and thus the bike will fall in until it falls over.

 

If the camber thrust and the pull of the trail are properly balanced against each other, the front wheel will naturally turn in just the right amount to allow the bike to continue around the turn and hold it's lean angle with no pressure on the handlebars, and thus the bike will be neutral. If the camber thrust gains some leverage over the trail, the bike will stand up. If the trail gains some leverage over the camber thrust, the bike will fall in. Of course we can counteract any of this by pushing on the bars ourselves.

 

There's no doubt a lot more going on than just this little battle between camber thrust and trail, but the concept in my opinion is quite practical in understanding a lot of bike behaviors and bike handling problems and how to fix them or adjust to them. For example:

 

Stuman mentioned that "a bike will stand up when you get on the front brake which should effect the suspension in just the oposite way."

Getting on the front brake causes the bike to dip forward, which decreases trail, thus the trail loses leverage. The braking also causes the front tire to squish down more increasing the width of the contact patch, which increases the leverage of the camber thrust. Trail loses, camber thrust wins, the bike stands up.

 

Now lets take my example of the worn front tire making my sportbikes want to stand up. The reason is that I have a certain lean angle that I'm comfortable with on public roads and I use that angle a lot. So the part of the front tire that's on the ground at that lean angle wears more and thus the tire profile develops a flat spot. That flat spot means that when I reach that lean angle, the contact patch gets wider (and shorter), increasing the leverage of the camber thrust, and thus the bike wants to stand up. Adding throttle helps to make the bike more neutral because it decreases the size of the front tire contact patch. Increasing front tire pressure helps to for the same reason. But overall the bike is generally not enjoyable to ride because it's neutrality changes with lean angle which is annoying and prevents me from completely relaxing in the turns.

 

Let's consider the KLR650 I mentioned, with knobby tires, that wants to fall in. I theorize that the knobby tires reduce the leverage of the camber thrust and thus installing street tires would make it more neutral. But I'm not going to find out any time soon because I have no interest in being restricted to pavement with this thing.

 

Hotfoot, I bet, if you experimented some more, you would find that your cruiser wants to stand up all the time, not just on the throttle, and that getting on the throttle actually makes it less severe. Consider that your throttle control habits also might have changed as a result of the superbike school. You'd have to go into a turn off throttle with no pressure on the bars, observe the behavior of the bike, then get on throttle and remain no pressure on bars, and see what changes. Of course there are lots of things you could do to make it more neutral if you wanted.

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I didn't want to side track us with the standing up on the brakes thing... But since it has been brought up, one thing that Tony talked about in a seminar a couple years ago I found very interesting. Basically he said one of the main reasons why a bike will tend to stand up when the front brake is applied is because of the way the front tire will steer the front wheel into the corner.

 

 

In terms that I can understand...

 

If you are in a left hand corner, leaned over a bit and apply the front brake, there will be a force applied to the left side of the leading edge of the contact patch of the front tire. This in effect pulls the front of the front wheel to the left, counter steering the bike upright.

 

there it is clear as mud.

 

Anyway in my experience, every bike will want to stand up if you apply the front brake while leaned over in a corner. Some to a lesser degree, but all do want to stand up.

 

On the gas it is obviously a different story. Most sport bikes will hold their line and application of the throttle shouldn't effect lean angle. But other bikes with different geometry will react differently when you roll on the throttle.

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I see what you mean... when leaned left the contact patch is on the left edge of the tire and thus to the to the left of the steering pivot, so it would get pulled back towards the center under braking, turning the front wheel to the left. I hadn't thought of that one. Yet another interesting tidbit!

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It does seem freak'n hard to figure out how to explain some of this stuff without it sounding a lot more complicated than it really is.

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Harnois, nice explanation about rake and trail, thank you for that. To answer your later question, about whether the cruiser always wants to stand up: I experimented with that a lot on the day that I noticed it happening. I tried turns off-gas, maintenance throttle, light acceleration, and hard (kinda) acceleration. Off throttle it was nuetral, maintenance it stood up slightly, light throttle it stood up a LOT. Hard acceleration didn't seem to affect any more than light roll-ons. I'm still a little suspicious that the thing is squatting a lot, the suspension on it is very soft and not adjustable.

 

Stu, I totally agree with your explanation about the bike standing up under braking. There is a GREAT computer animation of this in the Twist of the Wrist II DVD, that is what finally made it click for me. I also agree with your head-exploding feeling, I read the Tony Foale stuff too. :)

 

Harnois, you said:

"Getting on the front brake causes the bike to dip forward, which decreases trail"

 

I was thinking about that, and I don't really see how the front of the bike dipping forward would change the trail, are you sure that it does? I think trail is dependent on rake, not fork length, and I think rake is fixed mechanically by the bike. Am I confused?

 

I was thinking this over because since we started this whole discussion, I also started wondering WHAT actually causes the front wheel to turn when you hit a bump while leaned over. We know it does, but why? I think it must be a similar thing to why it stands up under braking...? :blink:

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I was thinking about that, and I don't really see how the front of the bike dipping forward would change the trail, are you sure that it does? I think trail is dependent on rake, not fork length, and I think rake is fixed mechanically by the bike. Am I confused?"

Under braking the back end of the bike lifts and the front compresses, so the whole bike is rotated forwards which makes the head angle more vertical, which reduces trail. Here is a good picture of it: http://en.wikipedia.org/wiki/File:TrailDIAG2.jpg

It's from this article: http://en.wikipedia.org/wiki/Rake_and_trail

 

Reducing the trail means that the trail has less leverage against the "camber thrust," but it also makes it have less leverage against the rider's steering inputs, which is why racers often try to adjust their suspension lower at the front and taller at the back to make the countersteering easier or faster. But if taken too far it can create stability problems.

 

It's possible for the back of the bike to either squat, jack up, or do nothing as a result of the acceleration. Racers sometimes adjust swingarm angle to make it do different things. I've noticed some cruisers jack up at the back with acceleration, especially the shaft drive ones (except BMW).

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A chain driven bike will raise its rear under acceleration if the rear axle sits below that of the output shaft. OTOH, if the rear axle sits above the output shaft, it will squat under acceleration. Shaft driven bikes will always raise the rear end under acceleration unless you build a system (like BMW and others) that place all of those forces elsewhere. BMW haven't eliminated the climbing tendency (but they could - they could also have reversed them if they so desired), but very little remains.

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Under braking the back end of the bike lifts and the front compresses, so the whole bike is rotated forwards which makes the head angle more vertical, which reduces trail.

 

I get the idea, and it makes sense intuitively, BUT... if trail is dependent on the steering head angle, and the steering head angle is mechanically FIXED on the bike, how does the trail actually change? Also, if you draw out a picture of a bike, draw the vertical line to the ground and the line extending the angle of the forks to see the trail, then SHORTEN the forks (compressed shocks) and redraw the trail, it doesn't change... because the wheel radius doesn't change, and the rake doesn't change. Not unless you "allow" the forks to rotate at the steering head, changing the rake angle.

 

Trail=RSinø-0/cosø

R=tire radius

0=triple-clamp offset

ø=rake

 

I do see that rotating the whole bike forwards would shorten the wheelbase, but don't see that it would change trail.

 

Sorry if I sound a little argumentative, I am not trying to be, I am just frustrated because I don't quite understand this and it's bugging me! :huh:

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You are right that shortening the forks in itself does not change the trail. It changes because the whole bike rotates forward when braking, so the head angle rotates forward along with it. Yeah the frame angles are mechanically fixed, but there's 4 or 5 inches of suspension travel front and back, so the whole frame can rotate relative to the ground due to braking or acceleration. If you hit a dip and both front and rear compress equally, no change in trail, but braking and throttle can cause the front and rear to do the opposite of each other, and there will be a change in trail.

 

Perhaps considering an extreme example will make it more clear. Consider a guy doing a "stoppie" draw that on paper, draw your lines, now how much trail is there? At some point it will even become negative (the tire contact point will be in front of the steering pivot).

 

Back to the original subject, I have an SV650 that has a slight tendency to want to stand up in turns. So now that you got me thinking about this "camber thrust vs. trail" theory, I'm thinking of doing a bit of testing to see if I can use that to make it better or worse.

 

You said your bike does not have adjustable suspension, but I don't think I've ever seen a bike that didn't at least have preload adjustment on the back (to deal with the possibility of riding with a passenger). So if you have that, try cranking that all the way in one direction, then ride, then all the way in the other direction, then ride. I wonder if there'll be a noticeable difference in the bikes tendency to want to stand up, for better or worse. Also there's the possibility of sliding the forks up in the tripple clamps a little bit. Both of these changes would pitch the bike slightly forward or backward which would change the trail a little.

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You are right that shortening the forks in itself does not change the trail. It changes because the whole bike rotates forward when braking, so the head angle rotates forward along with it. Yeah the frame angles are mechanically fixed, but there's 4 or 5 inches of suspension travel front and back, so the whole frame can rotate relative to the ground due to braking or acceleration. If you hit a dip and both front and rear compress equally, no change in trail, but braking and throttle can cause the front and rear to do the opposite of each other, and there will be a change in trail.

 

Perhaps considering an extreme example will make it more clear. Consider a guy doing a "stoppie" draw that on paper, draw your lines, now how much trail is there? At some point it will even become negative (the tire contact point will be in front of the steering pivot).

 

Back to the original subject, I have an SV650 that has a slight tendency to want to stand up in turns. So now that you got me thinking about this "camber thrust vs. trail" theory, I'm thinking of doing a bit of testing to see if I can use that to make it better or worse.

 

You said your bike does not have adjustable suspension, but I don't think I've ever seen a bike that didn't at least have preload adjustment on the back (to deal with the possibility of riding with a passenger). So if you have that, try cranking that all the way in one direction, then ride, then all the way in the other direction, then ride. I wonder if there'll be a noticeable difference in the bikes tendency to want to stand up, for better or worse. Also there's the possibility of sliding the forks up in the tripple clamps a little bit. Both of these changes would pitch the bike slightly forward or backward which would change the trail a little.

 

AHA! Did you hear the penny drop? :)

Thank you, that it is the piece I was missing, that the forks DO rotate, RELATIVE TO THE GROUND, when the bike tips forward. I got it now. Phew!

 

Things get so much more difficult when I try to understand all the formulas instead of just going by the seat-of-my-pants intuitive feeling. But, your patient explanations helped me reconcile the two and I feel MUCH better now!

 

I'll be very interested to hear what you find in your experiments. You're right, apparently there is a preload adjuster on my cruiser, and I'll try stiffening that... but I'm not sure when I'll be out riding it again to test it - THIS weekend I'm going to go ride that snazzy new BMW 1000RR! Wheeee!

 

Thanks again.

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I guess the formulas are good when you want to get an idea of *how much* difference a certain change can potentially make. Like in that article I just linked to, they show that tipping the bike forward just 0.2 degree by lowering the front end can make a noticeable difference in the amount of trail. Which means that front braking probably makes a very significant difference in trail.

 

Also try setting the preload to have the most sag (what post people refer to as "soft"). Per my "camber thrust vs. trail" theory that should make it better because it would increase trail, helping it overcome the camber thrust, moving the handling more toward neutral instead of wanting to stand up. Although I could be totally wrong about this theory. Also it would reduce the jacking up of the rear on throttle if that's what's happening, per Eirik's explanation.

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it would increase trail, helping it overcome the camber thrust, moving the handling more toward neutral instead of wanting to stand up.

 

If that is indeed the case, why is it that cruisers generally handle very poorly despite lots of rake and trail? I'm not saying your theory is wrong, I'm just trying to understand it. Perhaps the problem is the rake? The more you rake out the front end, the further the contact patch will move away from the bike's line when the wheel is turned. To make tings even worse, trail is often reduced by raking the forks more than the steering stem, enhancing the tendency for the contact patch to move. Hence you can see cruisers with virtually zero cornering clearance that have worn their front tyres to their edges; when turning sharply at low speed, the front tyre is leaned over a lot even if the bike is upright.

 

Sorry, I guess this only helped to confuse things more :blink:

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Good point. I'm totally open to being completely wrong about this. I hope I can test with my SV this weekend. Actually, until this thread I've always just assumed without much thought that since steeper head angles make for lighter steering that it would also make for a greater tendency to fall in. Which is opposite of my camber thrust vs. trail theory.

 

Cruisers seem to come in a really huge variety of tire sizes and rake/trail settings. Some have really fat front tires and a lot more weight squishing the front tire down, so that would create a larger and wider front contact patch, which would create more camber thrust, which would have to be offset with more trail.

 

The ones with a very shallow rake angle, chopper style, they're going to have a really "floppy" front end - turning the handlebars will make front of the bike drop a lot. Gravity is trying to turn the steering. I don't know how much this plays into it but maybe it's another variable. I couldn't find rake/trail specs on the savage but it's clearly not all that extreme.

 

You can have a shallow head with a lot of forward fork offset to compensate, or a steep head angle and less fork offset, and either way have the same amount of trail. So your concern about the sideways movement of the contact patch - it's not necessarily any worse with the chopper. Here's a pic to demonstrate:

http://picasaweb.google.com/lh/photo/HXj7w...feat=directlink

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I experimented a little with my SV650 this weekend. The bike has an aftermarket shock with adjustable ride height and preload, but both are difficult to adjust so I didn't mess with that, but I suspect the back is sitting considerably higher than stock. I started out with the forks raised 6mm in the triple clamps, so the front sits lower than stock, and then on top of that I let all the preload out in the forks, so the front has a lot of sag. So all this means the bike is tipped forward a lot, so steeper head angle and not a lot of trail. Then I went for a ride. Off throttle it had a very strong tendency to want to stand up. Steady throttle was a little more neutral but not much better. Actual acceleration made it become neutral, but it took considerably more than just the usual stabilizing cornering throttle to make it neutral. Heavier throttle didn't change anything. I suspect that once the forks top out on throttle there's no more geometry change due to throttle and that's where more throttle stops affecting the steering neutrality.

 

So then I cranked the preload all the way up, so the front sags a lot less, and moved the forks flush with the top of the triple tree. Altogether this would make the front sit about 21mm higher, thus increasing trail. Then I went for another ride on the same turns. It still wanted to stand up off-throttle or at steady speed, but it wasn't as strong and took took less throttle to make it neutral. So this was an improvement. On top of that it seemed much less "twitchy" on bumpy stuff, which is interesting to me, because I would not have described the bike as twitchy in the first place, and because it is the first time I've noticed that kind of change from suspension adjustments on any bike. For public road riding these changes seem really nice. Theoretically the steering would be slower but at public roads I can flick any bike super fast, so it's a non-issue. This makes me realize that one of the previous owners, having dropped the front a little and raised the rear, no doubt thinking he was making his bike "more sporty," actually just messed it up, gave it that desire to stand up in turns.

 

After that I changed the front tire pressure from 29 to 36 psi, and this made the biggest difference, made it more neutral (less camber thrust). I would like to repeat all these experiments eventually with a brand new front tire, because this one already has some of that flat spotting in the profile that I mentioned in an earlier post, which I have little doubt is another source of the bikes tendency to stand up in turns.

 

I would also like to repeat the geometry changes making more drastic changes, to take most of the subjectivity out of the test. I might take my KLR650, for example with 9 inches of suspension travel, and drop the front end like 3 inches, see if it changes more toward neutral instead of it's current tendency to fall in. Obviously that would have to be temporary, but if it works, I could considering adding a preload spacer to the rear shock to get the same result, which would have the added benefit of more ground clearance.

 

As a result of this thread you really got me think of ways of improving all 3 of my bikes!

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