Sup y'all...I'm assuming this is the proper board this should go in as it doesn't really fit the criteria for the others...so meh...
I recently bought my first bike (2001 Suzuki SV650...a bit big for a beginner bike no doubt BUT I'm 6'4" so in honesty, its the smallest thing I can fit on comfortably) and in reading my way through the manual (thanks to this site for linking to that, btw) it claims that the "standard tire fitted on this model is a 120/60 ZR17 (55W) for the front and a 160/60 ZR17(69W) for the rear." However, after checking my tires, I came to the startling conclusion that my front tire is not the standard tire...my front tire is a 120/70 ZR17 (58W).
Now...the manual claims that using anything other than the standard tires may result in "instability" so the main question I have to ask is...how much "instability" am I looking at with this 'not-standard' tire? Is it so much that I should really considering dropping another X amount of coin on a new tire despite having just dropped $3300 on insurance/gear or is the 'instability' minimal and fairly easily overcome? Or is not replacing them stacking the deck further against myself as I'm already learning on a more powerful bike than the ideal bike people would learn on?
The lesser questions I should like to have answered regarding tires are pretty typical, I imagine, for first time vehicle owners: what do the various numbers (120/60, ZR17, 58W, etc) mean? And what is the 'optimum' tire pressure for my 'not-standard' tire or is it the same as what's listed in the manual and the tire itself has little bearing on the 'optimum' tire pressure?
tire pressure is listed right on the sidewall, lots of rims can take multiple size tire, my 636 has a different front than stock, bigger front will make your turn in slow but you probably wont notice it.
Tire pressure on the sidewall of the tire is usually NOT the optimum pressure because that varies bike to bike as many of them share tire sizes. Sidewall pressure is generally the maximum cold pressure of the tire.
The optimum pressure will be found in the original manual or on a sticker on the bike. It may be in kPa not PSI so you'll have to convert. It's probably somewhere around 30 PSI.
Size wise, the industry standard is - you can put anything you want as long as the overall diameter is within 3% difference and there's no clearance issues when it comes to width.
In your case the difference would be around 5% so over the standard but not drastic. I wouldn't worry about it that much but if you feel that the front is too soft, spongy or has not enough handling, especially when cornering, it could be actually that. Keep in mind that it will throw your speedometer off slightly. As in you'll be going faster than indicated. 2 other things that are important on the tire are load range and speed rating. Make sure they are both the same or exceeding your manufacturer's specs (they're in your case)
Keep in mind that quite often manufacturers will make a special edition or a gt model or whatever and it will have different tire size from base version.
One more thing, check the DOT code and if the tire's between 6-10 years check it thoroughly for cracks, uneven wear, heel and toe, feathering and such. if you see any signs of severe aging, replace it. if the tire's older than 10 years, replace it no matter what it looks like. Last 4 digits of the DOT are the date.
For example 2705 would mean that the tire was manufactured in the 27th week of 2005.
120 = tire width in mm or 12cm
60 = sidewall width (60% of tire width, therefore 72mm)
17 = rim size - 17 inches
58 = load range (as in tire's load capacity) - 520lbs
W = speed rating - tire capable of withstanding constant speed of 270km/h
It is common to replace 120/60-17 front tires with 120/70-17 because the 60 series is somewhat of an oddball, and there is a much greater selection of tires with 120/70-17.
It will not cause stability issues, but what it WILL do (based on personal experience - I rode an SV650 at a Shannonville track day with this tire size substituted) is slow down the steering and make the steering effort high. This is because it has raised up the front of the bike and changed the geometry - it has the effect of raking out the front end (heading towards the direction of a "chopper" just not as extreme) and adding "trail" - the distance that the center of the tire contact patch follows behind the imaginary intersection of the steering axis with ground level.
You can correct the geometry by lowering the front ride height (by sliding the forks up in the triple clamps) and raising the rear ride height (if your rear shock has this capability). My experience was that it "wants" about 8mm of total adjustment, except that you can't lower the forks on the front of an SV more than 5mm from stock before the top of the fork lowers can collide with the bottom of the lower triple clamp before it hits the internal bump stop ... So, you need to lower the front ride height about 5mm and raise the rear ride height about 3mm.
If this flew straight over your head, take the bike to a good shop that knows race suspension (Rider's Choice, Pro 6 Cycle, Z1 Cycletech) and get them to do it. Once this is sorted out, your "new" correct front tire size will be the more common 120/70-17 ...
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