Day 5: Assessing tire wear.
By now you have realized that tires are a key element in chassis and suspension tuning, and from that perspective lets see how tires help us recognize wear patterns that can guide us to the correct chassis or suspension solution.
For street riding, it is very unusual to see poor tire wear based on poor tire pressure. The most common is the ubiquitous ‘flat spot’ on the rear followed by the one side worn out more than the other on a front tire, also from low pressure and crowning of the road. It may be the left or the right side, depending on which side of the road you are forced to drive on in your country!
More commonly with street tires, wear patterns reflect suspension set up most noticeably with rebound damping. Lets start with the rear tire as that is the easiest to look at. When you follow the direction of rotation of the tire and on the outside edge of the tire, you will see the first or leading edge of the tread/sipe rounded down or gone completely. Why?
Lets break this down:
- you are braking and turning, so given weight transfer, where is the chassis load being placed?
- that being said, how much load is on the rear tire?
- if there’s a limited load on the rear tire, how much load is on the rear shock?
- given this load, how does the tire react to every little surface irregularity?
- are you seeing why the leading sipe is wearing down so fast?
- think about deflection and flex (there’s a groove behind the first edge)
If you move further toward the middle of the tire, you might notice that this rounding effect switches sides and moves to the rear edge of the sipe. ??!!??
Go on – go look at your rear tire to see if this is the case…… yes, now
In the middle 2/3rds or perhaps ¾ of the carcass you have control of the rear tire via the throttle, therefore the rear tire and rear shock are under load. What degree of load depends on many factors (tire pressure, shock settings, gearing, torque, peak power, throttle position and roll on, 1 or 2 passengers, rear axle location etc). With this load on the swing arm, shock and tire, the carcass flex will be completely different in every aspect from braking and turning.
With that being said, on the outside edge of the tire under braking and turning, how fast is the tire reacting to every bump it encounters. When loaded, is that reaction speed different? Why?
Welcome to the world of high and low speed damping, all managed via piston design, flow rates, valving shims and oil viscosity. That’s why suspension experts and companies take so much time with this level of engineering to provide you with better alternatives to stock components but we cannot digress in this post.
You will see in EVERY stock motorcycle this form of tire wear. Don’t bitch about it – you get an enormous amount of engineering in modern bikes – learn how to tune the suspension to optimize the tire wear.
You only tune the tire where you have control of it via the throttle or brakes as you have no control over high speed damping!
Track and race DOT and slick tires:
Because these tires are subjected to a significant amount of load well above and beyond street bikes, these tires tend to show you all kinds of wear patterns in very short order ie: 5 laps/10-15 miles (bear in mind that they are meant to last a few hundred miles, not thousands!).
With DOT race tires, you can see the above sipe rebound markings clear as day, so you can make corrections to rebound damping to a degree, but it will never be perfect. Why not – we are not binary robots that do the same things every lap in every detail. Good luck on that quest…….
Here are the most common wear patterns that I deal with!
Should you get the hot pressure wrong, you will experience two things:
- when the pressure gain is 3psi or less or the carcass is at 120F, the carcass cannot get to operating temps so it tears itself apart creating the classic “cold tear
When the pressure gain is 8psi or more and the carcass is over 200F the carcass is over heating creating the classic “hot tear” This is very easy to diagnose with a pyrometer probe and less accurate with a pressure gauge.
You have to let the tire go cold and reset the cold pressure, then start over, or you stay in the hot pit chasing carcass temps for far too long!
Do you have the right compound?
If you have the wrong compound it will tear itself apart in 3 sessions, so do your homework and contact the tire vendors that frequently travel to that track to get their advice. They don’t want to ruin their reputation by selling you the wrong tire – they want you to have a great experience on their product so you will buy it again. Phone call, email, local racer comments – research them all.
Track temperatures and compounds
You have to understand by brand why each compound has a heat range that it works best with. For the most part, soft tires need hot track temps of 100-120F or more. If those temps are not present, DO NOT use that tire. A medium range compound tire is much more durable and will generally work much better in the 50-80F range. Obviously hard tires need heat but they have only so much grip as they are meant to be endurance type tires capable of sustaining 1-2 hours of track speed so that carcass has a design principle embedded into its construction. Does every tire work the same way in each brand? No it does not. Ask you vendor, top club or national racers, or other riders that know this information.
Tire carcass designs change regularly so what was true one year is upside down the next.
If there’s too much or too little weight bias on the rear tire you will get a band of wear that resembles a cold tear. Your pyrometer probe will give you immediate clues as to why the tear is there. Too little heat, the rear end is too high – too much heat there’s too much weight on the tire. Remember, nothing is black and white, so you need to recheck sag and hydraulic settings!
If the edge of the front tire looks like it lost the fight to a cheese grater, you have a geometry tear from too much or too little weight on the tire. Your pyrometer probe will give you immediate clues as to why the tear is there. Too little heat, the front end is too high – too much heat there’s too much weight on the tire. Again, remember that nothing is black and white, so you need to recheck sag and hydraulic settings!
Tires: - Dave Moss Unsprung: Tires, Dave discusses one of the most forgotten pieces of motorcycle suspension, the tires. He explores tire profiles, carcass construc...
Tire wear:- Dave Moss Unsprung: Tire Wear, how to "see" what your tire is telling you in terms of wear patterns that can easily be recognized and then attended to. Th...
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Please share with the forums and web sites you placed the tire tech on so everyone can debate this and come up with their solution.
You have bought tires that have a stiff sidewall but soft carcass top to the tire with the same size as the ones you removed. These will be going on your street bike that also doubles as a track time at times throughout your riding season. Organize this list into the correct sequence of events:
1. Fork height
2. Setting sag
3. Cold tire pressure
4. Chain tension
5. Shock length
6. Adjusting hydraulics
7. Hot tire pressures
8. Track geometry amendment
9. Circumference measurement
10. Track hot pressures