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Old Oct 9th, 2009, 11:40 am   #1 (permalink)
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Stability and Steering Angle, Trail, Fork Offset and Wheelbase

The central issue with making changes to steering angle is stability. A motorcycle is designed to return to its straight-ahead condition after hitting an object or bump in the road that causes the front wheel to deflect slightly to the right or left. In other words, it has to remain stable for a variety of road conditions, and motorcycle stability is foremost a safety issue.

The way dynamic stability is assured is to design a bike with enough distance between the point where the front wheel touches the road and the intersection point between the steering axis and the road. This is called the trail dimension.

A longer trail dimension increases the motorcycle's stability on straights, but will also negatively affect the motorcycle's turning effort, i.e. more rider's strength is required in corners and transitions. However, the more trail, the greater the ability of the bike to self-correct it's steering. It's a longer trail dimension, for example, that allows you to easily ride with no hands on some bicycles, but not others.

A shorter trail dimension, on the other hand, produces a lower opposing force to steering inputs. It's kind of like power steering. So the steering requires less rider strength, but larger handlebar displacements from bumps in the road and corners are fed back to the rider. Said another way, the shorter the trail, the more rider input that is needed to hold a line and the more responsive the bike feels since it is more sensitive to steering inputs.

The two superbike steering angle positions, 23°30' and 24°30' produce trail dimensions of 91mm and 97mm respectively. The wheelbase, also an important factor in stability, remains unaffected when you change it. As a comparison, the Monster steering angle is fixed at 24° and the trail dimension is 94mm. Adjusting the trail dimension on most manufacturer's bikes is not an option.

As an aside, when you change to the steeper 23°30' position you loose a significant amount of steering lock making low speed U-turns more difficult. Also, the ignition steering head lock doesn’t engage in the steeper position.

Now, here's Ducati's warning: "Trail should only be altered after all the other (geometry and suspension) changes have been made and you are comfortable on the bike. If the bike displays any instability problems they need to be sorted out first, as this steering head angle change will magnify these characteristics."

(One reason, for example, is that part of its effect mimics changing the rear ride height.)

The Haynes Service manual goes on to say "Warning: The steering head angle must be set to the road position (longer trail) whenever the bike is used on the road. If the steering angle is set to the race position (shorter trail) ... the handling of the machine could become unpredictable on uneven road surfaces."

So, shortening the trail is considered unwise for street riding (unlike tracks) where bumps in corners, potholes and other road hazards repeatedly challenge your bikes steering stability. Here's a case where inexperienced or uninformed riders who set-up their street bike chassis geometry as racebikes are just looking for trouble.

Trying to mimic factory race bike set-ups can get you into trouble. It's central to racing that race bikes often need to sacrifice high-speed stability to handling. Riders may initially run the steeper steering head angle, but often, as they get faster, they realize they want more stability, not less.

To get more stability there are two things that Ducati typically changes on their racebikes: the triple clamps and the swingarm. They use triple clamps with less offset, typically 27mm instead of the stock 36mm, and use a 25mm longer swingarm to increase the wheelbase. These changes to the triple clamps or the swingarm have the effect of moving the center of gravity forward which is the typical starting geometry of the Corsa race bikes. Remember, changing the steering head angle does not, by itself, change the wheelbase or alter the center of gravity.

According to an earlier post by Jeff Koch, for superbikes, for every 1mm decrease in fork offset:

Trail increases 1.1mm
Wheelbase decreases 0.9mm
Height of the bike’s center of gravity increases 0.2mm
Percent of the bike’s weight on the front wheel increases 0.05 percent

Some here have suggested that you can get the same effect (reduction in trail) with a finer adjustment by increasing the rear ride height instead. However, you'll need to raise rear ride height 16mm to get an equal amount of trail reduction, and in doing so you'll also end up increasing the height of the bike's CG by about 12mm that (among other things) will increase loading to the front tire, so when hard on the brakes, the rear tire gets/feels very loose.

Again quoting Jeff, for every 1mm increase in rear ride height:

Trail decreases 0.4mm
Wheelbase decreases 0.2mm
Height of the bike’s center of gravity increases 0.8mm
Percent of the bike’s weight on the front wheel increases 0.03 percent

One improvement with the steeper angle that’s been observed is in trailbraking - The bike stands up less on the brakes, which can be a benefit on backroads where you never quite know what might be coming up around the next bend. Also, some feel that the steering is more neutral at large lean angles.

The area of major concern is tankslappers. Reducing trail by reducing the force that centers the front wheel will give you more headshake, especially when accelerating (less weight on the front wheel) out of bumpy corners.

You won't get a tankslapper out of most corners if you change to the steeper steering head angle, but you will make them more likely, and more violent when they do occur. Some will say to crank-up an adjustable steering damper to settle the steering, but dampers will only resist changes in steering direction and don't provide a restoring force to re-center the wheel like trail does. The higher damping rates also spoil your quick steering and cause weave instability problems when cranked-up too high.

For those of you who haven’t experienced this phenomena, see one here:

http://www.randtclub.com/Video/cedwa...pper_tt99.mpeg

I’ll choose more stability over quicker steering any day.

One more thing.

Keep in mind that lowering the front ride height, or raising the rear ride height, are not equivalent adjustments. Lowering the front serves to lower the bike's center of gravity. With a higher front, raising the rear, raises the C.G.

Again, according to Jeff, for every 1mm that you raise the forks in the triple clamps (lowering the front end):

Trail decreases 0.2mm
Wheelbase decreases 0.5mm
Height of the bike’s center of gravity decreases 0.4mm
Percent of the bike’s weight on the front wheel increases 0.06 percent

Ducati Corse, in a 1996 memorandum that was posted on the old Ducati.com web site, recommended raising the front 10mm to increase "flickability" in turns. Yes, I said raise, not lower. Raising the front end raises C.G., and a higher C.G. makes the bike go to the tire edge quicker according to the memo. I was told that the same advice is given in the factory race bike setup manual.

Familiar with the Mille SP? It has the capability to raise the engine in the frame to increase C.G. to improve flickability. Same effect. Even the Mille R has the engine mounted higher in the frame to do the same thing.

From a chassis design point-of-view you generally you want the C.G. to be a distance equal to half the wheelbase above the line connecting the axles. Raising the C.G. above this point makes the bike easier to turn.

Once in a turn, a higher C.G. biases the weight more to the inside of the corner which helps the bike turn.
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Old Oct 9th, 2009, 2:45 pm   #2 (permalink)
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Once again....priceless info to me as I am exploring ways to reduce trail-braking stand-up. My 999 is especially sensitive to this situation with the current set-up I am using. Coming from a '02 R-1 which is considered a nervous bike (short wheelbase as well as other factors) I am fairly tolerant of bike movement so this might well be the justification for changing rake. My thanks to you once again for hard earned knowledge given freely to us all.

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Old Nov 21st, 2009, 6:41 am   #3 (permalink)
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Great read, top work once again.

Cheers

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Old Nov 21st, 2009, 7:20 am   #4 (permalink)
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The 999 retains use of the steering lock in steep mode, as the steering lock tab is movable.

Also, I lost no steering lock when I switched to steep on my 999R.

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Old Nov 21st, 2009, 2:20 pm   #5 (permalink)
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Shazaam,

If by going to lightweight wheels or to the extreme by going to carbon fiber wheels, would this upgrade act to compensate for lack of flickability if you are set up in the stability mode?

I just bought the Buell 1125CR and the owners manual has a nice instructional piece dealing with these adjustments, flickability and stability. The stock suspension settings were unsatisfactory so after reading a comparison article between the 1125CR and Duc Streetfighter, I went with the recommended settings by the magazine writers (Sport Rider). I was amazed at the improvement and now its just a matter of making small increment changes depending on the conditions. I had fun more recently, with I might add some frustration when I decided to mess with the suspension on my Multistrada. I got it right on once then lost it when I made some other adjustments. The nice thing about doing that you can't stay lost forever because you can always return to factory settings and start over.

I have one question regarding the front fork spring preload, this has to do with the Buell but it applies to all bikes. Sport Rider recommended "14mm showing". On a Ducati one turn equals 2mm and since the Buell forks are Showas, maybe one turn is also 2mm. That would mean seven turns. In the Buell manual they recommend seven turns in from soft for riders in the range of 170 to 230 lbs. So, I believe the Sport Rider recommendation is the factory setting in this case. But, what do they mean by "showing", whats showing, the fork lines?

Thanks for your help.
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Old Nov 21st, 2009, 3:33 pm   #6 (permalink)
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The Best Performance Modification You Can Make

You can increase acceleration by increasing engine power, but you can increase acceleration AND improve handling, and improve braking by reducing weight. Further, when you reduce the weight of rotating components you get an additional acceleration benefit from reducing rotational inertia.

So yes, you'll get quicker steering transitions from reduction in these gyroscopic forces.

The best performance modification you can make is a switch to lightweight wheels. Lightweight wheels don't have the stalling and drivability drawbacks of a lighter flywheel, and because wheels have a much greater rotational inertia than a flywheel, weight reduction here results in a much greater improvement in acceleration (and braking) with an added benefit of reduced unsprung weight for improved handling. Lighter front rotors have a similar benefit. There’s even a significant difference in tire weights between brands to consider when you need new tires.

Lines Showing

Take a look at this picture and note the number of lines showing above the red nut and directly below the crescent wrench


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Old Nov 21st, 2009, 9:38 pm   #7 (permalink)
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As always Shazaam. A read worth a re-read, then another read. Once again, thank you.
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Old Jan 12th, 2010, 3:37 pm   #8 (permalink)
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Quote:
Originally Posted by Shazaam View Post
From a chassis design point-of-view you generally you want the C.G. to be a distance equal to half the wheelbase above the line connecting the axles. Raising the C.G. above this point makes the bike easier to turn.

hi there.

i think you got confused while writing this great text about trail and off-set,
shouldn't it read "you want the C.G. to be a distance equal to half the wheelbase above the ground" ?

wheelbase on this great Supersport is at ~143cm, half of it is at ~71.5cm,
see C.G. II, which is 71.5cm above the line connecting the axles, that can't
be it, as it is above the entire motorcylce.
see C.G. I, drawn at 71.5cm above the ground, that seems to be more
realistic, still pretty high though, with the L-Cylinder design, should be some-
what deeper on our bikes.



how can it be dermined in the end ? at home ?








kind regards from germany
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Old Mar 4th, 2013, 7:29 pm   #9 (permalink)
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Talking

Quote:
Originally Posted by Shazaam View Post
The two superbike steering angle positions, 23°30' and 24°30' produce trail dimensions of 91mm and 97mm respectively.
Shazaam, I don't know if you're still monitoring this most excellent post but I'd like to know how you calculate the trail dimensions (length) from the steering angle positions.

It seems that the above superbike example shows a pattern;

23°30' = 91mm
24°30' = 97mm

If I understand this right, the greater the angle the longer the trail length?

My bike's specs are as follows;

Stock Caster Angle = 25°06'
Stock Trail Length = 88.8 mm
Stock Wheelbase = 1295mm

Unless the Caster Angle is not the same as the steering angle this appears to indicate to me that the factory specs for my bike show that the greater the steering angle, the shorter the trail length?

Or, maybe I'm just completely out to lunch? (wouldn't be the first time )

Either way, this is an excellent post and, though a new rider, I have experienced firsthand what you described in the first few paragraphs after changing tire sizes.

With my new tires (different size), my bike drove itself in a straight line. I mean, it was like driving a truck. You could literally read a book with both hands while driving. Zero lean in corners - it just wanted to go straight all day long.

This was in direct contrast to the stock tires. On those, the bike was very 'nervous' or 'twitchy'. Touch the handlebars just a bit and that was all it took (power steering). This made it a bit difficult in the corners though as it tended to oversteer a great deal. If you entered a corner on the outside (closest to the centerline) it was very easy to exit on the inside of the corner close to the ditch. I found it turned way too sharply.

So what I did was raise the front shocks by a few mm and then test rode it until it got back to the over-steer condition. I then backed it off a tad to where I liked it. It now handles incredibly well - I can just about scrape the pegs when I weave back and forth in my own lane and it's easy to keep a nice smooth weave whereas before it was very difficult to weave in a consistent pattern due to the over-sharp steering.

The best way I can describe the difference is that now, anyone, and I mean anyone, could ride it safely. It's a whole different bike.

But my question remains. According to the specs for my bike (if I have the right ones) my steering angle is/was greater than the two superbikes you mentioned, yet, my trail is shorter (in the stock form). This seems to be an odd discrepancy when weighed out against your examples.

What am I missing?
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Old Mar 4th, 2013, 8:38 pm   #10 (permalink)
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You have to also consider the amount of fork offset designed into a particular model.

For a given castor angle, increasing offset (moving the hub forward) reduces trail, while moving the hub backward increases trail.

The more offset the less trail, less trail gives quicker steering. Less offset, the more trail, more trail gives more stability.

Offsets
Monsters are 25 mm
Supersports are 30 mm
Superbikes are 36 mm
Attached Thumbnails
Stability and Steering Angle, Trail, Fork Offset and Wheelbase-screen-shot-2011-04-09-9.45.48-pm.jpg   Stability and Steering Angle, Trail, Fork Offset and Wheelbase-screen-shot-2011-04-09-9.46.14-pm.jpg  
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