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Tire size/shape vs. bike geometry

Discussion in 'Tech' started by prm, Jun 8, 2022.

  1. prm

    prm Well-Known Member

    I put some new Pirelli SC3s on my Duke 890 R. They replaced Michelin Power Cup 2s. Same marked size front while the rear went from 180/55 to a 180/60. The change raised the back 7mm and the front 2mm, for a net change of the rear +5mm. Though both 180s, the 180/60 is actually about 10mm wider. My intuition was that it would initiate turn quicker as a result of the raised rear. In reality, it felt much more stable when upright. More effort required to initiate turn. It would reach a lean angle and then feel like it would free up. I was only running local back roads so difficult to be real precise or hold lean for any length of time. Not a negative feeling per se, just very different. I plan to raise the front a little to get it back closer to where it was just to see how it changes.
    For any setup experts, what changes are typical when changing tires with different size and shape?
     
  2. nlzmo400r

    nlzmo400r Well-Known Member

    I'll just speak in generalities and you can deduce how this applies to you/your bike from there:

    The front tire height will have a direct effect on ground trail number. The taller the front tire, the more ground trail. An increase in trail will increase stability and self steering effect. A smaller diameter front tire will have the opposite effect.

    The taller front tire will also raise the CoM slightly which will increase capsize. Capsize is the bike's ability to go from full lean angle one one side to full lean angle on the other size. More capsize makes the bike want to 'fall' into corners more and will help it maintain an line throughout the corner because the CoM is higher up and therefore closer to the inside edge of the corner during lean. Raising ONLY the front doesn't raise the CoM much, but it would a little bit. 2mm of front tire height change probably won't be immediately noticeable.

    A taller rear tire will lower numerical gearing meaning less torque will be applied to the ground. However because it also raises the bike of the swingarm where the sprocket is carried, it increases chain pull angle and therefore introduces more anti-squat effect while accelerating. This helps keep the front tire down during acceleration and can aid in stability. However because the rear tire is taller the CoM is also raised which will INCREASE the motorcycles tendency to wheelie during acceleration.

    A taller tire will also have the same effect on capsize as the taller front tire because it affects the CoM in the same manner but to a lesser degree (because the rear wheel is farther from the CoM).

    A wider rear tire won't have much affect aside from edge grip, drive grip while leaned over etc. But if the tire is a different profile (likely) AND different width, then obviously that could hugely affect how it feels underneath you as you transition from upright to tipping in to full lean at apex.
     
    backho and prm like this.
  3. Badger911GT3

    Badger911GT3 Well-Known Member

    I'm having a hard time visualizing how this could be. If the only thing that changes is an increase in the rear tire diameter, then I would think that change would increase the distance from the ground (tire contact patch) to the rear axle. So the angle of the swingarm to the GROUND would change. But everything else related to the chain vs swingarm angle would stay the same. In my head, I picture the entire rear of the bike lifting up a bit, but none of the swingarm, chain, shock length components changing relative to each other. Hopefully I'm stating this clearly. Would you mind telling me where I'm going wrong? Thanks,
     
  4. stangmx13

    stangmx13 Well-Known Member

    The over-simplification of 'net ride-height' ignores too many variables to justify changes before riding the bike. Those ignored variables also make it very hard to apply generalizations about setup. The moto spends a lot of time around a track not on the center of the tire. If the profile is dramatically different, it would be very difficult to know how the "ride-height" change (and all subsequent geometry changes) for every lean angle. Perhaps at 0deg lean angle, the rear is +3mm. But perhaps at 40deg, the rear is now -2mm because of the profile change. And any guess you had about that may be made worse by tire width changes. A taller wider tire may be taller at every lean angle, but by how much? And what if it's not...

    I ride the bike first, then make adjustments to alter the feeling. I tend to just tune the bike as-is, without letting the difference in dimensions impact the changes I want to try. No adjustment is off the table when changing tires, especially when changing brands. Springs, ride-heights, and damping changes all might be necessary. But I only find myself in this scenario when Dunlop releases new race tires - and usually ride-height changes are a great start. If I were to switch to Pirellis or something, I'd probably call my suspension tuner and pay for a completely new setup.
     
    prm, nlzmo400r and Badger911GT3 like this.
  5. nlzmo400r

    nlzmo400r Well-Known Member

    I completely agree. Motorcycle dynamics are probably the most complicated system I've seen in my experience. However, I feel that knowing the 'generalities' gives the average intelligent rider a good understanding of what to expect. Part of truly understanding is having a hypothesis and then seeing if your results match your hypothesis. There's almost infinite variables in the equation, but getting the biggest ones understood gets you like 85% of the way there. We're obviously not competing on a MotoGP level, so I think too much more complication just becomes confusing and not worth the effort to get the smaller % of improvement.
     
  6. prm

    prm Well-Known Member

    100% agree what I wrote is very simplified. Discussion with generalities is exactly what I was hoping for. I kind of geek out on this stuff so fun to play with. Also, there’s not a lot of track setup info out there on an 890 R. Definitely a change from the R6 I had before where every possible nuance was well documented.
     
  7. stangmx13

    stangmx13 Well-Known Member

    Understanding doesn't produce a fast motorcycle. Understanding the concepts is <10% of setting up your own bike. The actual work is correlating suspension adjustments with changes in feeling on the bike AND figuring out what you like. Researching trail, chain pull, and anti-squat is practically a waste of time compared to just changing your shock length and testing. After all, what does something like 100mm of trail feel like....

    I used to spend hours mulling over suspension changes in the week before a race weekend, trying to plan out the perfect list of changes and to find the "why" in every adjustment. Unfortunately, the mind is really good at making shit up, especially these little feelings that we have on a motorcycle. If you think X change is supposed to produce Y feeling, you are more likely to feel Y or feel it in a larger magnitude. I eventually realized that this method for tuning added so much bias and prevented me from trying many things.

    Now I don't really bother with "why". I try something, write down the results, and try something else. It's faster and focuses far more on results. It takes half a trackday to run the whole reasonable range of ride-heights and you'll learn far more doing that than anything else. Do that with the major settings over 1-2 trackdays and you'll be lightyears ahead of the guy that spent too much time researching because your setting changes actually work for you.

    You could try to do both. But there are only so many hours in a day.
     
  8. ducnut

    ducnut Well-Known Member

    Freddy, at Superbike Unlimited, has an 890 he’s been tweaking, uploading YT videos about, and is worth checking out. Whatever questions you may have, he’s a great resource.
     
  9. Pneumatico Delle Vittorie

    Pneumatico Delle Vittorie Retired "Tire" Guy

    Normally I would check, measure, and write down the set up with current tires then I would measure and check the same with the new tires then adjust to accordingly, but what the hell do I know?
     
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  10. prm

    prm Well-Known Member

    I did that, it’s the “then adjust accordingly” part. Just curious if there are any rules of thumb to start in the right direction. Or, just leave it as-is and ride it. :beer:
     
  11. Pneumatico Delle Vittorie

    Pneumatico Delle Vittorie Retired "Tire" Guy

    Adjust to get the same general setup. So if a new front tire raises the front 6mm vs previous front reset to get the same chassis height. Then ride the thing to see what you think
     
    prm likes this.
  12. nlzmo400r

    nlzmo400r Well-Known Member

    Both is definitely the way. But to me having an understanding is pivotal. I started by studying what to expect and why, then getting out there and turning the preload ALL the way in for a few laps. Then turning it all the way out for a few laps and getting to actually feel what happens underneath you. I found doing things like this super helpful and takes some of the 'scary' out of being worried if you go 3 clicks too far once you realize the bike will still be ridable no matter what. It'll just feel like relative garbage.
     
  13. stangmx13

    stangmx13 Well-Known Member

    Next time, start at your base setting and remove preload systematically until the feeling is garbage. Then go back to the base and add preload until it's bad again. This would have revealed your useable range for that setting.

    Most fork cartridges don't put enough preload on the spring with the adjuster all the way out for the cartridge to work properly - topout springs and such. So "no one" runs it there. But I think you'd have a hard time finding that info in books or on the internet.
     
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