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Discussion Starter · #1 ·
Somewhere else on this forum I read where a wide tire set-up means a loss of 9 BHP. Is this post in reference to the stock 180 vs the 200, or is it in reference to some super-wide aftermarket set-up?
Thing is, in looking at the 180 photos and 200 photos, I see no visible difference..:moped:
 

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Posting From The Pub
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The 240.

But it is a misunderstanding anyway - the figure is based on dynos that test how fast the bike can accelerate a roller, rather than those that push a roller against a brake.

The former is obviously affected by the extra tyre mass, the latter isn't.

So what is really being said is that a 240 tyres produces a 9BHP lower figure on most dynos. The amount of ACTUAL power that is available changes very little, if at all.
 

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Discussion Starter · #3 ·
So, would the relationship be equal in view to say, my being 240 lbs would equal a per pound hp loss, then when viewed against a 140 lb rider, would display a difference in loss or gain, depending on point of view?
 

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If I've understood that right, then no, not unless you wrapped yourself around the rear tyre :)

I'm no expert on dynos, but as I understand it the common or garden type works by measuring how fast the bike can accelerate a heavy roller. A heavy tyre ALSO had to spin up, effectively increasing the intertia the bike has to overcome. Hence the 9BHP loss.

A brake dyno has the wheel spinning at a constant speed, and measures how much braking force the bike can overcome. Hence no loss.

On the road there will be SOME loss from the bike having to overcome the inertia created by the extra weight of the wheel, but it will be small. In fact it might even be a gain, since the tyre will be that less likely to spin up and break traction.

That's my reading. I'm willing to be told I'm wrong...
 

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HP loss as recorded by some types of dynos will be noticed, but in real world applications, there is no appreciable difference between a 180 and 240 rear tire/wheel, other than the weight factor.

But yes, a 240 lb rider compared to a 140lb rider does make a difference, all other things being equal.
 

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The rolling mass of the tire represents unsprung weight. Unsprung weight requires more energy to change the rate of rotation. This effect is similar to a flywheel and is most noticeable during braking or acceleration. It represents an inefficiency in the mechanical system resulting in reduced power being available at the rear wheel during acceleration and additional force to be overcome by the brakes during deceleration. During normal rolling motion at steady speeds the additional mass of the larger tire is additional weight that must be carried, similar to the addition of any other stuff we might have in the saddle bags.

If acceleration is your goal, you may be able to offset the loss of efficiency by the addition of increased traction available because of the wider contact patch. If you find the right balance point, you can launch hard without spinning your tire but you now run the risk of raising the front end.

All of these effects become exaggerated as the amount of HP decreases. They have little effect on a 1300HP race car but would have a huge effect if you installed them on a bicycle.

In summary - the wide tire looks cool but is only an advantage if you can reduce tire spin on hard launches and maintain control of the bike.
 

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Will make a difference to the performance of the BIKE, but the BHP will (obviously) remain the same.
 

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Nite Rod'n in SC
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I've heard/read somewhere...

consafos said:
So, would the relationship be equal in view to say, my being 240 lbs would equal a per pound hp loss, then when viewed against a 140 lb rider, would display a difference in loss or gain, depending on point of view?

That every 7 lbs difference in weight is equal to 1 HP, all else being equal. So if this is true, the 140lb rider would have roughly a 14 HP advantage over a 240 lb rider...all else being equal.:hidesbeh:

Like I said..don't know how factual that is...anyone else heard of such?
 

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If you were to measure (not really, impute) horsepower by acceleration times on a measured distance, then you would see a difference by changing the total weight of rider and bike. In fact, the dyno drags weight the bike and rider to develop their curves for the simulations.
On brake dynos, first you don't measure horsepower. Horsepower can't be measured directly as it is a derivative. It is defined as so much force used to do work.
1 hp = 33,000 ft·pound-force·min−1 = exactly 745.69987158227022 W
The problems are standards used to measure hp and report it. In recent years, dynomometers have gotten cheaper and as the price goes down, often so does the engineering and quality of the equipment. Just look at repeatability. If you run the same bike on a chassis brake dyno 5 times, you will receive 5 distinct readings. From a statistical view, take the average of the three middle runs and that is a reliable, repeatable estimate of the hp for the bike. What actually happens is only the highest is selected as the true reading because more is better.

What has been lost is the meaning of the measure of power in a standard way for comparison. If I change the compression ratio, or piston shape, what will the result be? If I change the transmission configuration, what happens.

Now that chassis dynos are in use nearly everywhere at affordable prices, what has changed? Well, the configuration of the dyno has an effect on the readings. people don't want to know what hp is at the crankshaft because they were marketed into 'real world, rear wheel' hp readings. These are no more real than any other measure. the efficiency of a transmission and driveline can be measured. It should be (given a reasonable design) be consistent and could be factored in to give power numbers that would show comparable numbers to compare across manufacturers.

The marketing is that if you measure on a chassis dyno you show hp numbers that reflect the real world because you are taking into account the driveline losses. This leaves hp that is left and is applied to doing real work so it is better than measures of hp at the engine alone. There are lots of other factors that are ignored in this case. Wind resistance consumes power just like gears and belts. Tires have rolling resistance and it varies with speed because the tire changes shape.

The issue of tire sizes causing changes in hp is one of the things that come out of this circumstance. If you think about it, how could a tire change the amount of hp produced? Now, the amount of resistance to rolling on motorcycle tires is not greatly changed because even a 'wide' tire like a 240mm doesn't have a very large contact patch. Look at the tire, it is rounded (unless you run drag slicks, and they are flat to produce more contact) and many modern tires have a larger contact patch when the bike is leaned over than when it is sitting straight up.

Then change is in the mass that has to be accellerated may change. The tire weights are not that different. 15lbs vs 17.5 lbs 180/240 in the same man. range of tires. Surely 2.5lbs can't make enough difference to result in a reading of 9hp less.

I think what is needed to remove all these red herrings is a standard wheel/tire specification and the ability to compensate for it in the dyno software. I believe that this may already exist, there have been people that referenced this. Otherwise, reasonable people will make statements like I lost 9hp when I went to a 240. Was the wheel change to accomodate the 240 the cause? probably. My guess is that a 8-10 inch wheel that is used on these conversions weighs more than the stock 5.5. I would be willing to bet that you don't see a change in hp readings if you change from a 180 to a 200 tire on the same wheel. Although, I haven't seen dyno runs on the same bike, same day that were withing 5% of each other. :stilpoke:
 

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All of which is true but, in my opinion misses the simple point: it oisn't rolling resinatnce that is the issue, it is the polar moment (have I got that right?) of the wheel / tyre combo on a non-brake dyno. That is where the loss is coming, and it is not a real loss, because, as you point out in different words, what the dyno is measuring is not BHP but a PROXY for BHP, and the relationship between BHP and that proxy is changed by that polar moment.
 

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Louis said:
All of which is true but, in my opinion misses the simple point: it oisn't rolling resinatnce that is the issue, it is the polar moment (have I got that right?) of the wheel / tyre combo on a non-brake dyno. That is where the loss is coming, and it is not a real loss, because, as you point out in different words, what the dyno is measuring is not BHP but a PROXY for BHP, and the relationship between BHP and that proxy is changed by that polar moment.
yes, the 9 bhp is due to overcoming polar moment of inertia.
 

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And, as such, it is mainly a flaw in the means of measurement and can be, more or less, ignored.

The REAL problem with the weight of the 240 is it gives the rear suspension a hard time, and results in a bike that can wallow and kick your arse about at the same time, as I was discovering on a mountain road this evening.

Still, it isn't TOO bad, and you have to suffer to be beautiful.
 

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Louis said:
And, as such, it is mainly a flaw in the means of measurement and can be, more or less, ignored.

The REAL problem with the weight of the 240 is it gives the rear suspension a hard time, and results in a bike that can wallow and kick your arse about at the same time, as I was discovering on a mountain road this evening.

Still, it isn't TOO bad, and you have to suffer to be beautiful.
So true!
 

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Well stated Hawkstang.
 

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Discussion Starter · #15 ·
Given the unsprung weight of mah beer belly plus or minus a longneck or two would either compensate or re-fixate the entire thought of maybe putting a big tire front runner to match the rear...
 

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Your beer belly is spring weight - it is on the suspended part of the bike, not attached to the wheels. Anyway, if your gut is anything like mine, it has its own suspension system :)
 

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autimbo said:
That every 7 lbs difference in weight is equal to 1 HP, all else being equal. So if this is true, the 140lb rider would have roughly a 14 HP advantage over a 240 lb rider...all else being equal.:hidesbeh:

Like I said..don't know how factual that is...anyone else heard of such?
So the figure of 120 hp at the engine should be 74 hp at the rear (650 pound bike [divided by] 7 = 46.... so 120-46=74)
Don't think the calculation can be correct.

Something I don't think people have taken into consideration, rotating mass has a greater effect than static mass. i.e. every 1 pound you lighten your wheels is like reducing the bike's weight by 10#'s (read that last part here on the fourm.)
 

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Paintless Rider
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To sort of sum this up, the dynamometer measures how fast the engine can spin the tire up to max engine speed. The lighter the wheel, the easier it is to spin. When you take it out on the road, the wheel become part of the weight of the total bike that the engine has to move.
 

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Isn't the real point of all this (other than bragging rights) is to accelerate the bike? In the real world of street and strip the bigger tire bike with all other things being equal ie weight of rider etc. the wider tire bike will be slower. So no matter what kind of dyno you are using or what the calculations are you would lose the race with the big tire all other things equal.
 
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