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Tire pressure by Dave Moss

Old 05-09-2013, 09:35 AM
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Tire pressure by Dave Moss

Day 4: tire pressure affecting chassis balance

No need to preach about carcass temps but let's take a closer look at the effects of pressure on the carcass. The cold pressure will ideally grow 5-7psi while on the track or 3-5psi on the street. What does that growth do to the contact patch size? How does that affect rake and trail and weight transfer?

Small bikes (GP250, 400, 650 twins) use very high corner speed and therefore load the outside and sidewall tire heavily throughout the corner.
600's use heavier trail braking going in and high corner speed.
1000's brake hard going in, minimize mid corner and hammer the throttle going out.
All of this behavior radically changes the carcass shape and therefore air pressure is a vital factor in controlling sidewall and upper surface flex.

Generally you use more air in a soft carcass to maintain structural shape under heavy braking and acceleration, or you use less air pressure to increase the size of the contact patch and therefore tire surface and grip. Depending on the bike and riding style there are g-load needs to be met as well as braking and accelerating forces. What pressures are you running, but more importantly, WHY? Have you gone through a range of pressures to find the pressure that gets the right carcass heat range and gives you the correct carcass flex/stability you are looking for?

For example, in the previous generation Pirelli DSC1 DOT race tires I ran 38 hot front and 33 hot rear on my VFR400 race bike. That gave me the structural roll profile I needed going into the corner and the sidewall rigidity I wanted for very high speed mid corner stability. It took me 3 days of testing to get to that point but it took my lap times down to race pace and most important of all I was comfortable and trusted the tires. I got the tire to meet MY needs. Your needs may be completely different.

Why so high in the front? Let's look at this dynamically. Hard braking produces weight transfer, That loads the tire and creates carcass flex. If the sidewall flexes too much, two things happen:- you lose tire shape and therefore some degree of steering ability from the profile loss AND you lose rake and trail numbers substantially enhancing that loss of steering. Take a second to stop right here and visualize that in sequence as braking occurs.

We can manage brake pressure to deal with this, but should we have to do that on the street or track? If we do, the bike is managing us. That's very poor attention to this part of set up. The bike should do exactly what you want it to and that is why carcass flex/tire pressure is absolutely critical with soft carcass tires. How many street crashes are related to not checking tire pressure before going riding and the tire folding under braking? How about crashes on the track from poor/excessive tire wear at the track from incorrect pressure?

Stresses under acceleration are obviously very different by engine size, configuration and torque/peak power. Again sidewall flex has the same roll to play as does pressure in controlling flex. If the sidewall flexes too much, can you control you exit line from any corner street or track? Where will you ultimately go exiting a corner (and as a result, what do you do with the throttle?). Now stop reading, visualize that scenario step by step starting with brake release and throttle roll on.

Now add in weight transfer and the effect on the front end geometry.

As you have those mental frame by frame references in place, when you change brands of tires you need to understand what degree of flex you have to manage (this builds on yesterdays post) and therefore the tire pressure that YOU need.

We are all different riders in so many ways no matter what we ride, where we ride and how we ride, and tire carcasses are as different as we are. If you understand flex and pressure working together, you can tune the tire to get what YOU need out of it. That in and as of itself is priceless as we all know how small that contact patch is, so why not get the best out of it but more importantly, get the most out of the rest of the tire?
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Old 05-09-2013, 09:45 AM
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Just got done reading this on FB, good read great info. We all see ridiculously under inflated tires on the street all the time. I hate seeing "wanna be's" running that way knowing full well they're a crash waiting to happen, and then they'll wonder why they went down!
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Old 05-09-2013, 10:58 AM
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on my old BMW cafe I ran a higher pressure in the front because I got better stability. I had to get there by trial and error, but once I got it dialed in my stability problems improved dramatically. Different bike and tiers but I did get a lot of skepticism, it worked for me.
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Old 05-09-2013, 09:08 PM
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When Miller opened I was running Michelin Powers, and the bike was handling like crap. The tire vendor really only knew "race" tires at that time (6years on he is extremely sharp now). Freddy Spencer came by and saw his board with high 20's for PSI and flipped.

Bumped up to the high 30's at Freddies suggestion and the Shawk was rockin.
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