BLUEKNIGHT911
Sadly Passed Oct 2024 - RIP
OK, sorry about the length of this epic tome to all who aren't really interested in this stuff, but for those who are, here goes!!And I'd like to start by pointing out.....
While using Drag tires can demonstrate some aspects of tire 'growth' pretty well, you need to remember that Drag tires are specifically designed to run quite low pressures (considering their volume) and the design features built into their construction, especially in the area of the treads/plies, actually works to enhance that 'tire growth' for the centre of the tread. The width of the Drag tire at low revs/low pressure/optimal temps provides for massive traction initially, helping to get things moving quickly; then as the revs increase, by design the tire 'grows' in rolling circumference & narrows in width (thereby reducing rolling resistance which can sap power at these extremes) to give better gearing & enable the tire to enhance a rapid transition from delivering 'high traction' to allowing 'higher speeds' down the track.....hyea:
So while what you say about Drag tires is true Dennis, in this instance that is more applicable to the OE Spec Kendas than it is to your Pirelli, which is a High Performance Road Tire that's been designed with stiffer sidewalls and a fairly rigid tread section that won't 'balloon out' very much at all. BTW, for those not yet aware, the OE Spec Kendas are very lightly constructed, both in the sidewalls and in the tread area, with far more in common with Drag tires than any 'real' auto/road tire, and their tread DOES balloon out - a LOT! Back to your Pirelli.... as a 'real' auto tire, it starts out with stronger sidewalls and a more stable tread area than the Kendas, which is good for our ride, handling, & tread wear concerns, but looking deeper it gets even better! High levels of on road traction require as much tread surface as possible to remain in contact with the road, both in a straight line and while being subjected to cornering forces, so as a High Performance Road Tire, not only are the Pirelli's compounds specified to enhance traction, but also your Pirelli actually has features built in to its construction that work even harder than most auto tires to significantly limit that 'centre growth' or 'ballooning' that you think is happening! Sure, it does happen to a very small extent (as it does on most tires) but I'm pretty certain that by design, this is not all that much of a significant contributor to your tire wear in the centre in this case.
However, your tire pressure very well might be contributing at least a little to that centre tread wear - doing a rough calc using the info on the sidewall you gave us, the specs I can readily find for that tire, & using an 'average Spyder load on the rear tire' figure; then all other things being within 'expected parameters', it looks like that 22psi you are running is juust a little on the high side of the optimal pressure for that size Pirelli. The rough calc I did suggests that you should start your 'fine tuning for your optimal pressure' to suit all your ryding conditions and variables at 18 psi, with a lower limit of 16psi (best for traction & ride) and an upper limit of 20psi (which strangely enough, is best for straight line stability & minimal rolling resistance.....
And this is where my 'growing suspicion' comes in. Bearing in mind that the 'nominal sizes' shown on the sidewall of tires are really only 'approximations', most tires vary from those 'nominal sizes' to some degree - some more than others. The OE Spec Kendas are a bit on the 'small' side of their stated 225/50R15, while your 215/60R15 Pirelli is a 'reasonably large' tire for something wearing that size descriptor. From the specs I can find, it seems that while the tread width of the Pirelli is reasonably close to 215mm or about 8.5", the Section Width of the tire is closer to 9"/229mm, and our Spyders run a 7"/178mm wide rear RIM!! While that 7" wide rear rim is still within the 'suitable' range for a 215/8.5" width tire, it is really juuust a little on the narrow side for a 9"/229mm section width tire - so when your 9" section width tire is fitted, the beads are pulled in a little more than is ideal and as a result, the centre of the tread is bowed UP & OUT just a little, creating a slightly raised middle of the tread. :shocked: And THAT is my 'growing suspicion' as to why some 'otherwise seemingly ideal' tires are still wearing more in the middle of their tread when they shouldn't really! :lecturef_smilie:
How to fix it? In your case Dennis, if the tire is otherwise working well and doing everything it needs to for you, maybe you could try lowering its pressure a couple of psi? That might make it 'feel' a little different until you get used to it, but it could also reduce that 'centre of tread' wear at least a bit, and it should improve your traction a little too, even under hard cornering, despite the potential 'feel' changes! Can you put up with/learn to enjoy the changed feel?! :dontknow: Or maybe you could try the same or a similar tire in a 205 width next time? I don't think you'd want to go much higher than a 65 profile tire tho, and you might need to put a tire that size back up to 22psi (if you've tried going lower?) but it should reduce the section width a little and in doing so, minimise any 'bow up' in the tread face which in turn should reduce the wear in the centre of the tread. Worth a shot? :dontknow:
Your call on that, but there you have it... my 'suspicion' about how/why some 'real' auto tires are still wearing in the middle - in this instance, the 215/60R15 Pirelli P4 Dennis is running - it may well be somewhat different for other make/size tires?! Still, before I sign off, it's worth noting that once again, the rough calcs for optimal pressure for this tire also falls within that 16-20 psi range a few of us have been suggesting.... And in closing, as with all this sort of stuff on public forums, you only get a money back guarantee on my free suppositions and surmising; and as always, YMMV, but for what it's worth, there y'are! :thumbup:
Dennis I agree with everything that Peter said ..... I have the 4 season version and run it at 17psi ( hard in the twistie's & launches ) .... so far mine is wearing evenly across the tread ...... listen to Peter ..... Mike :thumbup:
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