Unsprung Weight and Horse Power, Final Answer???

[xxtraspec99]

Here's a potentially potent final answer to the much debated issue of -- just what effect, if any, reducing unsprung weight has on increasing horse power.

A machinist with nothing more than a high school education told me recently that there is a simple solution, and I believe him as he has proved to me many times that he knows more than most engineers that I've met.

You simply look at the difference between horsepower ratings at the crank and at the rear wheel. The difference is due to the weight of the unsprung mass and some small loss due to the drag of the gears and clutch (but not much).

So, on the XX, you can add up the stock unsprung mass and assume this is 90% responsible for the difference in horse power. Lowering the mass will regain the horsepower in proportion. Example, add the weight of the stock rear wheel (22 lb) tire (17 lb) rotor (3) sprocket (3) and chain (5) that's about 50 lbs of unsprung mass.

The crank horsepower numbers for the XX are all over the board depending on the source, etc. But Ride Magazine from England had the 1997 XX at the crank at 165 hp and the rear at 142. So, unsprung mass causes 90% of the 23 hp loss which is about 20 hp.

[Note, loaner bikes given to magazine writers notoriously have been lent in a state of high tune, and probably lacked all emissions controls and restrictions, and then there is good ole payola, if you get my drift...]

If you reduce the rear by 10 lbs you will regain about 4 horsepower. Reducing the rear by that much means aftermarket and very expensive wheels, race tires, brakes, chain and sprocket -- at least $1,000 and up to $2,000 -- all for 4 or even 5 horsepower.

Well, there it is. If you're a privateer shooting for an AFM win -- this may mean everything, but for the average rider... That's $200 per horsepower. Man, that's expensive, to say the least.

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[velocity]

*cough* bullshit *cough*

ok tell me just how much unsprung weight affects a bike when strapped to a dyno? (ie. the engine doesn't really need to move the unsprung weight)

I absolutely have no doubt that unsprung weight affects handling and acceleration on a track. But to say that it affect dyno number, no way. :roll:

[xxtraspec99]

If you are correct, then answer this, on a dyno, WHAT ELSE IS THE ENGINE DOING???

Or, more specifiically what is the engine pulling after the crank shaft???

Answer: the engine is moving an added load.

That load is composed of the resistence offered by the transmission, clutch, sprockets, chain, wheel and tire. These combined forces are measured by mechanisms which brake the rolling road of the dyno. The force detected by the rolling road is how the dyno works. The rolling road measures how much force the rear tire is putting out. It does so by measuring resistence. You can think of the resistence in terms of braking force, or braking resistence. This is why it is called bhp, or brake horsepower.

Now, if the engine's crank shaft could act on the braking/measuring instrument directly and not have to carry the additional load then you would be measuring pure crank horsepower or engine horsepower.

But, there is one fudge factor in all this, that is, adding resistence is not purely a matter of adding weight -- such as unsprung weight -- the added resistence is scientifically measured by MOI, moment of inertia. MOI depends on where the weight is located, what speed it is going at and how efficient the barings are and so forth. This is important in practical terms, for example, some aftermarket wheels have five sets of barings in the rear axle, while the stock Honda has only 3 sets. Moreover, a well designed aftermarket wheel will have more weight in the center and less around the rim, which reduces the MOI because the rim weight is moving much faster than the axle weight.

However, for purposes of judging horsepower gains or losses, and assuming everything else is equal, looking at a decrease in unsprung weight as proportional to a decrease in resistence is fair on this scale of things.

That is, the method presented in the beginning of this thread -- of estimating horsepower changes -- is a fair one given the over all circumstances.

[jlheine]

Say, have a good line on Carrozzeria wheels?

:shock:

[jcrich]

Say, have a good line on Carrozzeria wheels?

:shock:

:pointup: Maybe we can get a group buy together!!! :banghead:

[Pete in PA]

One of the bike rags was doing this talking about a worn tire giving you bigger HP #'s on th dyno because it is lighter.

It is true, not so much unsprung weight but spinning weight. Moment of inertia measures the force necessary to get a wheel in motion. Less weight gives you faster response to power applied so more HP.

I'm just not spending $1K on light rims only to have them fold at the first pothole. This IS PA after all. :roll:

[blackhawkxx]

I'm just not spending $1K on light rims only to have them fold at the first pothole. This IS PA after all.

I bet you know where to find the worn out tires though. :grin:

[velocity]

If you are correct, then answer this, on a dyno, WHAT ELSE IS THE ENGINE DOING???

Or, more specifiically what is the engine pulling after the crank shaft???

Answer: the engine is moving an added load.

That load is composed of the resistence offered by the transmission, clutch, sprockets, chain, wheel and tire. These combined forces are measured by mechanisms which brake the rolling road of the dyno. The force detected by the rolling road is how the dyno works. The rolling road measures how much force the rear tire is putting out. It does so by measuring resistence. You can think of the resistence in terms of braking force, or braking resistence. This is why it is called bhp, or brake horsepower.

Now, if the engine's crank shaft could act on the braking/measuring instrument directly and not have to carry the additional load then you would be measuring pure crank horsepower or engine horsepower.

But, there is one fudge factor in all this, that is, adding resistence is not purely a matter of adding weight -- such as unsprung weight -- the added resistence is scientifically measured by MOI, moment of inertia. MOI depends on where the weight is located, what speed it is going at and how efficient the barings are and so forth. This is important in practical terms, for example, some aftermarket wheels have five sets of barings in the rear axle, while the stock Honda has only 3 sets. Moreover, a well designed aftermarket wheel will have more weight in the center and less around the rim, which reduces the MOI because the rim weight is moving much faster than the axle weight.

However, for purposes of judging horsepower gains or losses, and assuming everything else is equal, looking at a decrease in unsprung weight as proportional to a decrease in resistence is fair on this scale of things.

That is, the method presented in the beginning of this thread -- of estimating horsepower changes -- is a fair one given the over all circumstances.

sorry...I shouldn't write about technical stuff when I've been drinking. :oops:

You are right, the moment of inertia (rotational mass) of the wheel will affect whp. I was just thrown by the term unsprung weight which is a bit of a misnomer since there is unsprung weight associated with the front wheel as well. That weight won't affect dyno numbers since the engine doesn't directly drive the front wheel.

[matey_peeps]

unsprung weight and HP? really? i just thought it had to do with being able to handle road surface irregularities better :roll:

[demon]

It's not really unsprung weight, but more to do with MOI. Any weight loss anywhere is good, but weight loss that is rotating at a high rate of speed is better. I read in Sport Rider, I believe, that a worn out tire gave them 2 more HP than a brand new tire. Dyno charts and everything. I believe this has more to do with MOI than unsprung weight. If you reduce your MOI numbers, the bike will probably produce a little more power, but will handle a lot better with reduced stability . Now, where's that Crapozeria BS wheel? I can find a lot better ways to improve my performance without changing wheels. For the cost of a set of Magnesium wheels (VERY light), you can put on a street turbo kit. Which would you rather have? 5HP and better handling, or 100HP and that sweet whine :roll: :roll:

[molson309]

Methinks people are confusing acceleration with horsepower.

Added mass doesn't change the horsepower output. Horsepower is lost to friction and other energy losses. Acceleration, though, is directly affected by how much rotating mass there is. It's like adding an heavier flywheel to the engine - it might rev up slower but it is still delivering the same amount of horsepower at a given RPM.

If you take two wheels of the same diameter and mass, but one with more of the mass at the outside of the rim, and roll both of them down an incline, the one with the mass more toward the outside will accelerate down the incline more slowly than the one with the mass more at the center.

This principle certainly applies to motorcycle/car wheels and is the reason why the statement about reducing unspring mass = more acceleration is definitely valid. What isn't true, though, is that the horsepower output is

changed.

Remember: Once you pull his pin, Mr. Grenade is no longer your friend.

[RodeRash]

All a Dynojet dyno does is measure how quickly your bike can accelerate a 750 lb wheel. Then it uses software to return calculated HP and torque readings. So, anything that you do to your bike to help with acceleration, (lighter wheels/tires, smaller flywheel, etc.) will return better numbers on a Dynojet dyno.

[velocity]

All a Dynojet dyno does is measure how quickly your bike can accelerate a 750 lb wheel. Then it uses software to return calculated HP and torque readings. So, anything that you do to your bike to help with acceleration, (lighter wheels/tires, smaller flywheel, etc.) will return better numbers on a Dynojet dyno.

Ok but if you add weight (increase MOI) to your rear wheel you are effectively trying to accelerate a larger wheel with the same amount of horsepower. So the wheel accelerates slower, causing the dyno to give you a lower number, regardless of how much horsepower the engine is actually putting out. That's why it's called WHEEL horsepower.

[xxtraspec99]

Okay, okay, okay... I get it.

You know what is so confusing is the two terms horsepower and "bhp" or brake horse power.

In fact, there should not be a term called brake horse power or wheel horse power. Why? 'Cause horsepower is meant to measure the power output of the engine, period.

As for dyno and what they measure and computations of torque and horsepower -- think of Spock telling Cpt. Kirk -- "remember your basic physics..." Horsepower is defined as RPMs times Torque. To get the numbers right you then divide by the conversion factor of 5252 (this converts feet and lbs to power measurements). What then is torque? You ask. Torque is a measure of work. Work is moving a weight a certain distance. Moving the same distance over time equals POWER.

So, a dyno that measures resistence or the strength of a load is a very useful tool. Why? Well, you then only need to know the RPMs the engine is turning at to derive HP. No way? Go figure? Yes, Captain, that's how it's done.

But not always.

Example, a top fuel dragster can do a 3.xx quarter mile and has over 3,000 horsepower but getting those horses to all push the rear wheel at the same time is problematic and the name of the game; these dragsters have big problems trying to produce 3,000 hp on a dyno's rolling road. 'Cause the power starts leaking everywhere. So, measurements of top engines are generally restricted to the crank.

When you start measuring at the wheel then things get very sticky. Is it with a rider on the bike? How heavy is the bike... etc., etc.

BHP is actually trying to measure performance of the bike, and not so much the engine's raw power. You might say BHP is measuring how effectively the engine's power is being transferred to the rear wheel.

In any case, I still stand by the original post at the top of this thread. That is, engine horsepower is lost by about 10% to 15% as the power is delivered to the back wheel from the engine. That loss of power comes from the engine carryiing the added load of the parts that come after it, namely, the tranny, clutch, sprockets, chain, wheel, tire, rotor. THEREFORE, the most that could ever be gained (or re-grained) from reducing these parts is that 10-15% loss. And since you need these parts to be strong you can only reduce their weight by a realitively small fraction -- so you'll ever only gain a few percentage of the hp back -- even with big money spent.

As for crapizeria wheels, if you read my whole openning remarks you could see I was advocating against spending money on wheels unless you are a serious racer who needs every advantage possible.

[Note, however, Carrozzeria (or how ever you spell it) is actually about the best deal out there if you can find them at auction cheaply. Why? The company make the mistake of using a name that was already taken by an existing company, and hence they were taken to court over it. Their original name was HI POINT. Since this conflicted with Hi Point Racing, the wheel company had to change it. That is how the name Carrozzeria came to be.

Luckily for a few smart buyers, some of the older wheels in stock could not be remachined with the new name. These mis-named wheels were then dumped on the market at 30 cents on the dollar. Why? Probably by court order. A court will usually give a company a short time to sell out its old stock before the ban on the name begins -- the legal principal here is called "Preventing Economic Waste." And that's a very good argument to use. Consequently, anyone buying those wheels sold by court order would be getting a very sweet deal, indeed. Specifically, wheels that once sold for $2k went for only $600.]

Lastly, forged aluminum wheels are 80% stronger than stock cast aluminum wheels. So if you are in PA you'd be much, much better off with the lighter wheels if they are forged.

[xxtraspec99]

Yes, I like fancy wheels as much as the next guy; it's only too bad that they cost so darn much. When you can find a bargain -- go for it.

Yes, also, I read the Sport Rider Mag article about the 14 light wheels they tested; and yes, there was an aftermarket wheel which weighed around the same as the stock Suzuki but had much less MOI -- because, exactly as you said, the weight was near the axle and the rim was lighter than stock.

Here's a very funny thing about MOI. I could hardly believe this when I first read it.

Moment of inertia is not just the measure of the energy to start an object off from rest, MOI is also the energy to turn an object in motion to another direction. In theory, the object in motion is "at rest" in relation to the new direction (at least in a two dimensional model).

Now, here's the weirder part.

Any of a wheel's vector which is tangent to a wheel's surface is always changing directions because, as we all know, the wheel is constantly turning. Can you imagine that? It means simply that the tire's surface that contacts the road is constantly changing.

That means that there is "in theory" a constant MOI in regard to a turning wheel. That is, since the rim is constantly changing directions there is inertia need at every moment to keep it changing constantly. So, the member who replied that weight loss and MOI only were significant during accelleration is correct, but incomplete. Why? Yes, because a turning wheel is constantly changing directions in relation to the axis, and therefore constantly creating a new MOI. Whew!!! I hope I got that right. Man, this is tough stuff.

[molson309]

That is, since the rim is constantly changing directions there is inertia need at every moment to keep it changing constantly. So, the member who replied that weight loss and MOI only were significant during accelleration is correct, but incomplete. Why? Yes, because a turning wheel is constantly changing directions in relation to the axis, and therefore constantly creating a new MOI. Whew!!! I hope I got that right. Man, this is tough stuff.

Inertia is only of importance when a mass is being accelerated. A wheel turning at constant speed is not being accelerated and thus does not take energy to keep it turning. The energy used (i.e. horsepower) goes into friction, such as bearings, air resistance and other mechanical losses, which are independent of mass.

We are really saying the same thing here: More mass at the outer rim of a wheel will affect acceleration out of proportion to the actual mass - i.e. if you took a 10 pound weight and placed it on the seat, the bike would accelerate faster than if that same 10 pounds were distributed around the rim of the front or rear wheel. If you want to accelerate at the same rate as if the 10 lbs weren't there - in either case - you need more horsepower to do it. I guess 7lbs = 1 horsepower unless it's on the rim of a wheel :grin:

But technically speaking, by putting weight -anywhere- on a bike, you haven't reduced it's horsepower, you've just made the bike heavier, and thus it won't accelerate as quickly.

Measuring horsepower by how fast a bike can accelerate a known mass is only an approximation - because, as was correctly pointed out by a previous poster, there are other rotating masses on the bike being accelerated that are difficult to take into account. The only scientifically correct measure of horsepower is steady state output: i.e. torque at a given RPM.

Personally, I put more stock in 1/4 mile times, roll-on comparisons, and top speed than I do raw horsepower numbers.

[bbxx]

wow i must have come to the wrong site im sure i didnt go to the mensa site

to me it dosnt matter if its moi bhp gpz cbr gsxr or whatever the most important thing is the blackbird is a damn fast comfy machine

[Animal Mother]

Man, all this talk is reminding me of my Weight and Balance charts!

Moments, Inertia (not relative, just added), Datums, Arms, Fulcrums, Weight, Inch-pounds....

Intersting to hear what everyone thinks though :wink:

Steve

[xxtraspec99]

Yeah! Physics and math and school stuff is what I'm trying to get away from when riding.

But, it is fun to try and justify those expensive mods... at least for the wife's sake when she sees the credit card bill. "...it's not just looks, honey, it handles way better... and safer..."

Okay, so I wasn't a physics or math major in college.

But, I swear the following is try and can be found in any high school science text that covers rotation and mechanics.

A wheel that is turning even at a constant speed is ALWAYS ACCELERATING or decelerating, as the case maybe. Why? Because it is always changing direction. The force vector of acceleration points NOT TOWARD the direction of spin as you would expect, BUT TOWARD THE AXIS of the wheel.

This actually means that the material on the rim is always being thrown inward toward the axis, that is, if it wasn't attached. Example, an orbiting satellite will be thrown toward the earth when it falls out of orbit, and it's not just gravity that throws it, it's the mechanics of rotation. You can think of it as a ball on a string that is being rotated in the air, the string is constantly pulling the ball toward you at each moment. While the velocity vector may be tangent to the circumference of the orbit, the acceleration vector is toward the center. In fact, this is the very principle that allows objects to stay in orbit.

Now, you must be pulling your hair out and asking -- how did we get to this baffling stuff?. It seems so counter intuitive. And what does it have to do with motorcycles? If nothing else it shows that a MOI of the rear tire is always present as long as the tire is rotating, whether that means anything in practical terms is another matter. But, I think it means that the unsprung mass of the rear wheel is always important even when at a constant speed -- as with less mass then the less horsepower need to keep up that speed.

Grrrrrhhh, I feel like I'm studying for a high school science test. Help, someone let me off this thread!!!

[molson309]

A wheel that is turning even at a constant speed is ALWAYS ACCELERATING or decelerating, as the case maybe. Why? Because it is always changing direction. The force vector of acceleration points NOT TOWARD the direction of spin as you would expect, BUT TOWARD THE AXIS of the wheel.

I think you meant "a POINT" on a wheel. Because any given point on a spinning wheel "wants" to travel in a straight line, it takes a constant force to keep it on a curved path. But - the point on the exact opposite side of the wheel requires an equal and opposing force to keep it on its curved path - and presto, the two cancel out. No work is done - hence no energy, i.e. horsepower, is needed to keep it spinning.

That's why balance in a wheel is so important.

I think black wheels ADD horsepower. Everyone knows it's the fastest color... :grin: