Interesting thoughts from everyone. Im gonna do as was suggested and put my phn in my tank bag with my gps app set and make several different runs. 40, 60, 80 mph and odometer check also gonna try to get a top speed run. Hard part with top speed run is viewing the gps, speedo and tach all at once. This should be fun!!!!

 

Well as once again I get the ...oh you're making stuff up line....

How about this: Tire Technology, excerpt no. 2 from The Racing & High-Performance Tire

Longitudinal Forces

The forces on a tire during acceleration and braking deform the sidewall enough that the contact patch moves a noticeable amount. The three schematics in Fig. 6.6 show how braking and driving forces can move the contact patch compared to static conditions.
Fig. 6.6

During braking and acceleration tires generate longitudinal force, and there is some longitudinal slip between the tread and the road. This shows up as a difference between the actual rotation of the tire and the rotation needed if there were no slip. Under hard acceleration the tire turns a little faster, and during hard braking the tire rotates less than it would if there were no slip. These two graphs show driving (Fig. 6.7) and braking forces (Fig. 6.8) vs. percent slip.
Fig. 6.7 Fig. 6.8

The shape of these two curves suggest the tire reacts to braking and driving forces in different ways. As soon as driving slip approaches 50%, driving force falls off rapidly. Braking slip falls off at only 25% slip, but the force reduction is more gradual. I don't know the source of the data for these graphs but they might look more alike if the percent-slip scale were the same.
Another possibility is that the driving-force curve drops off and flattens out because the tire is still spinning and the tread surface has a chance to cool, where the braking tire is locked at 100% slip and slides on the same contact patch. This heats up the rubber, lowering its friction capability. I'll bet the braking curve continues to fall off after 100% slip, off the scale of this graph.
Of course these curves represent generic data from passenger-car tires. Testing at high slip is difficult due to the forces involved. Probably the only valid test vehicle for racetires is a competitive, fully-instrumented racecar driven by a world-class driver. Even then the tire/road/driver system varies continuously creating noise in the data.


Still don't believe me???

 

Good info Hawk!!! Im gonna still try to to my GPS testing this weekend. I guess the only other measuring device one could use to measure there speed in relation to there speedo would be radar. Dont have one of those so Im gonna stick to my GPS.

 

Here is another discussion on it:
Must a tire slip to generate force? - Forum - F1technical.net

 

I still don't believe you and have a sneaking suspicion that this forum is comprised of a lot of people from Missouri!

 

But you did remember it is around 10% over 100mph (well you were pretty close) so I don't have to ride over and slap you....

 

Happy now?

 

So now what do you think??

 

So, just tested my car, and the same GPS that shows a 7% speedometer error on my SuperHawk, was spot on in the car. The speedo in the car matched the Gps at 20 mph all the way to 120 mph, over an 80 mile trip out to my super secret high speed testing area. Interestingly, the car's Odo is off (short) by ~1.6%, reading 76.9 miles to the Gps 78.1. So, the SuperHawk's Odo is at least as accurate as my car's, I got a smaller error on the bike over multiple 100 mile test loops, and the speedo maintains a 7% error over its entire range, off the same signal as the Odo. It has to be intentionally programmed in, or the slippage being discussed is not accounted for on the bikes, but is in cars, unless it doesn't happen in cars due to their greater contact patch.

 

Actually, thinking about it, the slippage cannot be the cause of the speedometer error. Since both the Odo and Speedo work off the same signal. It has been mentioned here many times that they differ. I am not the only one who has recorded the fact. So while the slippage may be happening, it is either so small as to not be a factor, or the factory accounts for it. By the way, one of my 100 mile loops was done while holding a steady 85 mph average (~80 actual). If tire slippage is a factor at 140 mph, it should have been a factor at 80, and the Odo matched the Gps.

 

It also has to do with the aerodynamics of a motorcycle.
A bike is a much dirtier shape and therefore requires more effort to push it through the air. Add in a single drive wheel and there you go.

The 10% figure applies only to bikes. I did not clarify this before as this is a bike forum and didn't even think of talking about cars.

 

Physics is physics. Bigger contact patch or no, you can't escape it. What throws a wrench into the slippage error theory (not the slippage theory), is that it doesn't, or the designer doesn't allow it to, affect the odometer. And if the odometer calculation corrects for the slippage, and the speedometer calculation does not, it is intentional. Even with the speedohealer, you can't have both be correct on our bikes.

 

Yes physics is physics... and if you actually read the posts...well I'll rephrase that. Tire slipage does change constantly it varies by the load applied.

The difference between 80mph and 180 mph is greatly different. That is why I said at over 100mph on a bike the rule of thumb is a 10% error due to slippage.

The fact that you are changing around what has been said and the qualifiers makes me believe you just want to argue.

So you tell me, Why on a VTR1000 with a 77.9" 180\55 17 rear tire
in factory stock trim and set up in which the bike will only pull to aprox 9800 rpm or so in 6th
Using these numbers the bike should be able to have a top speed of 174.5 MPH
You can use this site Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator

Go to any top speed test on this bike and you will get certified radar gun readings of between 155-160 max.

Take 175.4 and subtract 10% or 17.5 and you get 157.9 or right in the radar gun average.....

So you tell me why the bike runs 10% slower than the number you get by doing the math calculations? I mean the tire should grow at speed and you know the RPM so the math says, if anything, you should be going faster than the calculation, not slower.

So why does the radar say it is 10% slower than that. Where does that 10% of your speed go?

The math is quite simple and the numbers don't lie, so if the tire isn't slipping where does the speed go?

 

You guys really missed it. Back in the late '70's, most cars and many motorcycles sold in the US had their speedometers limited to 85 mph. Meaning, the top speed indicated on the speedo was 85.

Just another one of our Government's brilliant ideas to get us to slow down and save gas. Only problem was, if you were going over 85, you didn't have a clue as to how fast you were going.

 

Lol, I'm not arguing anything, I'm just trying to put the numbers together that I have recorded via my instrumentation. If anything, I'm simply ignoring your argument, sorry (I did like your graphs though). We have already proven that there is a speedo error at slow (30mph) speeds that linearly increases as the speeds do. That error does not seem to affect the odometer at speeds up to 80 mph over a long test. I have a pretty good feeling that the next time I am able to compare the speedo at 150 mph, I will be doing a GPS verified 139-140 (I have seen this casually before), which equates to...a 7% speedometer error. What will also be interesting to see, will be if the odometer picks up any error from the high speed test. I'm looking forward to finding out, but I don't think I'll be able to hold that speed long enough for it to make much of an impact.

 

OH BOY!!!!! I opened up a can of worms.......LOL.

 

Actually, you are all overlooking on small fact... Tire slip is almost non-existant ona steady state... Ie you can drive at 10 mph or 200 mph, and the tire will not really slip at all... But hammer the throttle from low speeds, and the tire will start fighting for grip and slip to some extent...

So, in this case, at around 140-150 mph, when keeping a steady speed, the tire slippage should actually be in the 1-2% category, and may vey well disapear in the OEM speedo error or the GPS lag...

However, drop one gear and open the throttle at anything above walking speeds, and the slippage increases to much more than 7-10%, tapering off down to smaller values as the tire creates grip and the speed increases... It however remains to some extent though all acceleration...

 

This whole debate made me want to do a little research of my own. Apparently some crazy engineers at Ohlins think 8451 may be onto something.

2WD Yamaha R1

"...the only time you have no rear-wheel slip on motorcycle is when you are pushing the bike. If you are riding, even at a constant speed, then you have a small slip. Before rubber transmits any drive at all it deforms. At 80km/h you have maybe 0.5% slip. At full throttle at 200km/h you have as much as 5% slip. At 5% spin there is roughly 160bar pressure in the system [system is pre-pressurised to 2-3 bar at rest]."

 

Pretty cool! But then, what da heck do they know, lol.

So Tweety, at a steady state at WOT in top gear, we may see a 1% error tacked on to the existing speedo error, which if maintained for a long enough period, say on the Autobahn, would become a recordable error compared to the odometer.

Interesting to see that there is always a 0.5% or greater slippage or more, depending on throttle being applied and gear running in.

 

What I see is a group of people saying I'm wrong but not one of them has tried to answer my question.

It is basic math....

So I'll put it in its simplest for.

If you have a tire of X diameter and spin it at Y RPM you should be traveling at Z MPH, correct?

If you limit the RPM to 9800 (which a bone stock VTR will hit) you should have a top speed of 174.5. Not indicated as I am not talking about what the speedo says but a true speed of 174.5 The math says so, it is a very simple calculation.

So why is the bike only able to hit a true speed of 160? What factor is causing the true speed (once again this has nothing to do with the speedo or odo) to be 10% lower than what the math calculation says it should be?

 

Either the rear tire is literally spinning 10% faster than the front, at an actual 160mph (seems unlikely, not saying it's not possible), or your math is wrong. Applying Occam's razor, I think we should examine your equation more closely.

 

So I tested the SuperHawk's speedo and odo against the GPS this evening. Since the last test was done while trying to achieve maximum gas mileage, using very little throttle, I was excited to think lots of real heavy handed riding, at high speed, could introduce some wheel slippage error into the equation. Unfortunately, my Hawk isn't powerful enough (very lean) to get much of a recordable error from that.*

So my results from tonight's test were: GPS - 81.0 miles, odo - 81.4 (0.5% fast). I did get some wheelspin as I was leaving from dinner hard on the throttle leaned over. It was pretty cool. Then I did my high speed run to see if the speedo error increased at top speed: speedo = ~150 mph, gps 138.0 mph (8% slower). The speedo error does seem to be a fraction higher than at lower speeds, maybe 1/2 of 1%, and the odo read approximately 1/2 of 1% high, so that could be evidence that something is going on at higher speeds and lots of WOT. Still, the rest of the speedo error, or 7.5%, is in line with the speedo error at crawling speeds.*

To sum up my findings, my car's odo and speedo are very accurate compared to GPS (basically spot on), the Superhawk's odo is similarly accurate in this all-stock configuration, whereas my SuperHawk's speedo has a 7.5-8.0% error over the whole rev range, or can be said to read ~8.7% higher than actual speed, depending on which way you read it.*

 

well here is the math:
Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator
you tell me where the error is....

 

Ok, great, thanks. I think we may have it then. Assuming all their values are correct, I think the answer lies in the circumference value you use. Taking a quick and dirty, no rider weight, axle-center to ground measurement, I came up with 11.75", which is an effective cir of 73.83". Their value of 77.9" correlates to an a-g distance of 12.40", a significant difference. Try my numbers for circumference and see if things don't match my GPS recorded values.
Btw, that is a very cool tool!

 

Actually, I'd say your numbers might be a bit skewed as well... The best way to get that number accurate, would be to measure axle to an edge of the tire not touching ground... That number will be about what you end up with once the wheel is spinning, factoring in both growth, and the load on the tire... But I guess that number will end up somewhere inbetween the two?

 

There's no doubt the effective cir value is going to change based on rider weight, tire pressure, speed, tire wear, etc. But, no matter how fast we go, there is always a contact patch and tire compression, not to mention, the distortion of the tire under acceleration, that serves to reduce the effective circumference in relation to an unweighted wheel. This lowers our effective overall speed potential.

 

So I’m not the type to just believe graphs that someone posts without checking the source. It looks like those graphs come from a book called The Racing and High Performance Tire which is $60 on Amazon and I’m not going to purchase the book just to fact check this but it looks like it’s based more on a graph that he’s showing as an example to his theory than based off actual numbers.

That being said it’d be interesting to find out what he’s basing his percentages off of. To me looking at this graph 100% driving slip would be the tire is in full burn out mode going up in smoke. And full braking slip is the tire is locked up and on a full skid. However the graphs go much past 100%, on the driving slip going past 200%.

As for the radar gun vs. calculated speeds based off gearing and tire size you have to factor in that every tire is slightly different. And that assumes you have a perfectly round tire and that the circumference is consistent across every tire. However every tire model is going to be slightly different.

CrankenFine I can much more believe what you’re saying the guys at Ohlins found of .5-%5 depending on load.

RZ your comment about the front vs. rear tire speeds make me think of the Trailtech speedos for dirtbikes. If someone happens to have one of these things put it on the front wheel and calibrate it at lower speeds so you know it’s accurate and mount it to the factory speedo so you can easily watch both. This would be one definitive way to calculate it removing many variables.

As for measuring the tire size the most accurate way is going to be to mark the tire and the ground below that mark. Roll the bike forward a full 3 revolutions of the tire and measure this distance and divide by 3. This will reduce fudge factor and give you an accurate circumference of the tire.

Also the contact patch being flat has no real effect on the circumference or how it sees the tire on the road. Think of it as a tank track. It’s not round by any means but it has a circumference and the distance the circumference travels in one revolution is the same amount it travels on the road.

The only caveat to that is with a motorcycle being round across the tread as the contact patch flattens out you have several circumferences hitting the ground that are pushing and pulling against each other. The very center of the contact patch has a larger circumference where as the outer portions of the contact patch have a smaller circumference so you’d probably have to take an average of the differences which is impossible to calculate.

On that line it’s got me thinking one thing to factor into your long distance odo readings is when you are cornering you are rolling to a smaller circumference area on the tire. So the tires will have to spin faster to maintain the same speed. The harder you corner the worse this is exacerbated.

My overall pinion on this after seeing what RZ’s come up with in his testing, reading what the Ohlins guys are saying and just pondering the possibilities is I think there is some slip. But I think it’s more in the lines of .5-5% the Ohlins guys found vs. 10% that Hawk is coming up with.

Sorry if some of what I said seems like rambling. I was just typing while the thoughts were coming to my head.

 

OK, I did the circumference measurement per GTS and came to 19.25 feet unweighted, and an even 19 feet with me on it wearing light gear. That 19 feet breaks down to a 76 inch effective circumference. Resulting gearing chart attached.

 

Well my only comments and my last comments on this are these:

1) I have always said "rule of thumb" as 10% not it is exactly 10% or any other claims of an exact number..... just that tire slip does exist and does constantly change due to the outside forces acting on the bike.

2) if Tire slip does not exist, then why does the center of the rear tire wear at a fast rate than the center of the front tire? The front tire has a smaller diameter, therefor is spinning faster and also has a smaller contact patch, so if the wear is caused just by the tire rolling on the ground it should wear out faster than the rear even if it is carrying slightly less weight than the rear.

3) I have listed sources that describe the physics of what is going on and how friction must be created in order to create traction which is necessary to move the bike forward. All the nay sayers have not put forth one link or reference in their argument that I must be wrong, just their opinion.

So think what you want because IMHO it is like trying to prove to you that the sky is blue or the earth is round. I can list references and also post links but if you still say "I don't believe it" and have no data to support your side of the discussion, then it is just a silly argument and what is the point.

If any of you can find any references saying that tire slip does not exist, then we can have a discussion on the subject. Otherwise I'm done with all this as it is just a waste of time trying to educate people on the subject.

 

I guess I do have one more comment..... do you have Dunlop 204s on your bike? If not then yes you will have a different number that the gearing chart has listed and also what the mag. radar test were done with.

So unless you also want to have a radar true top speed test done with the different tire, then you are comparing apples to oranges in some attempt to prove that I have to be wrong.