Trail Question

Forewarned said:

I wasn't shooting for a specific trail number, I just needed my bike to finish the corners better on the gas. And I was also rubbing my front tire on the radiator shroud under braking at turn 14 of Thunderhill resulting in the rear tire dancing and coming up too much. So I was solving multiple problems when I did all those mods.

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SuperDomestique said:

What trail figure did you ultimately arrive at? And was this for a V4?

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Forewarned said:

I didn't calculate it because I don't have a chassis program, and I don't know how to do it other than that way. I was going on feel rather than numbers because there's no real info out there to compare to. I'm happy with the results though, now I need to dial in the rear. The bike is a 959.

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Forewarned said:

That calculator says 103.6 Trail, 94.2 Normal Trail. But that doesn't calculate where my forks are in the triples which changes trail.

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SuperDomestique said:

Not necessarily. Depends on who you talk to. Adjusting fork length will affect rake relative to stock position. I’d assume stock position I had 8-10 mm fork (5 mm anodized) exposed. Move the forks to flush with triple increases fork length and therefore rake which then increases trail.

I’m also assuming that all these figures are determined with no sag factored in.

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Forewarned said:

Honestly because I have aftermarket triples that are thicker that stock, I have no idea where the forks are compared to stock. For all I know flush on the top triple could be the same as 5mm showing with the stock triples.

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andyb said:

yep me too....exact same conclusion and changes!

But as us amateurs some cynics would question can we really feel these millimetre differences...

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baggerman said:

What I think is your change may be too subtle for them. I know you must have run the bike at some point with the front dropped to give you the 24 degree head and the associated trail. But instead you made bushings, which pretty much leave the CG and ant-squat alone. I'm thinking that wasn't by accident. I actually think your set-up would be good for SD as he's light so his bikes CG is lower to begin with and goes lower dropping the front. Ducati moved the CG up at the 22 (?) revision for a reason.

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BP PanigaleV4S said:

Ducati gives a offset of 24.5 degrees with a 100mm trail. I measured the offset at 30mm. I measured the radius of the front tire at 295mm, that is center of the axle to the ground, taking into account the tire deforming under the weight of the bike. This gives a mechanical trail of 101.5mm. If my wife sits on the bike, ~140lbs, the front tire radius is now 292mm and the trail is 100.1mm. I'm probably 60-70lbs heavier plus another 25lbs with riding gear. I can see the tire deforming slightly more, but I would think that a sliding scale which depends on the air pressure.

Are trail numbers given when there's a rider on the bike, or when the bike is merely under it's own weight?

I've also come across trail calculators which take into account fork length, but only for calculators that also output a wheelbase number.

This was the calculator I was using for purposes of this post.

https://www.calculatoratoz.com/en/mechanical-trail-calculator/Calc-34933
In my quest to understand bike geometry, I also came across an old Ducati Corse document, from the 1990s on Ducati.ms which basically said trail should be the last thing considered after all other areas of the suspension was addressed.

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1. Trail Numbers: Rider vs. No Rider​

• Trail numbers are typically given for the bike under its own weight, without a rider. This is the standard reference point used by manufacturers and in most technical specifications.
• However, as you've observed, the trail changes when a rider is on the bike due to the front tire compressing under the additional weight. This is why your measurements show a slight reduction in trail when your wife sits on the bike (from 101.5mm to 100.1mm). The same effect would be more pronounced with your additional weight and riding gear.
• Manufacturers don't usually provide trail numbers for a loaded bike because rider weight, riding position, and tire pressure can vary significantly, making it difficult to standardize.

2. Tire Deformation and Air Pressure​

• You're correct that tire deformation is influenced by air pressure. Higher pressure reduces deformation, while lower pressure increases it. This directly affects the effective radius of the tire and, consequently, the trail.
• The relationship between tire pressure, load, and deformation is not linear, so small changes in pressure or weight can have noticeable effects on trail.

3. Fork Length and Trail Calculators​

• Trail calculators that take fork length into account are useful because fork length (or more specifically, the position of the front axle relative to the steering axis) is a key factor in determining trail. These calculators often output wheelbase because fork length also affects the overall length of the bike.
• The calculator you linked is a good tool for basic calculations, but it may not account for all variables, such as tire deformation under load or specific bike geometry nuances.

4. Ducati Corse Document: Trail as a Secondary Consideration​

• The Ducati Corse document you referenced aligns with a common philosophy in motorcycle design and tuning. Trail is indeed an important factor in stability and handling, but it is often adjusted after other suspension and geometry parameters (such as rake angle, fork stiffness, and rear suspension setup) are optimized.
• This is because trail is a result of other geometric factors (rake angle and offset) and can be fine-tuned relatively easily by adjusting offset or tire size. However, it should not be ignored, as it plays a critical role in how the bike handles, especially at high speeds or during cornering.

Practical Implications for Your Ducati Panigale V4​

• Your measurements show that the trail changes by about 1.4mm (from 101.5mm to 100.1mm) when your wife sits on the bike. This is a relatively small change and is unlikely to significantly affect handling under normal conditions.
• However, if you're looking to fine-tune the handling, you could experiment with tire pressure to see how it affects trail and overall feel. Keep in mind that tire pressure also affects grip, wear, and comfort, so it's a balancing act.
• If you're considering more significant changes, such as adjusting the offset or changing the fork length, you should consult with a professional or use a more advanced calculator that takes into account all relevant factors.

Conclusion​

• Trail numbers are typically given for the bike under its own weight, but your measurements show how they change under load.
• Tire deformation and air pressure play a significant role in trail, and these factors should be considered when fine-tuning handling.
• While trail is important, it is often adjusted after other suspension and geometry parameters are optimized, as suggested by the Ducati Corse document.