Gpz550 Wheels

Anyone know if 5 spoke gpz550 ('82-'83) wheels weigh less than the 7 spoke gpz('81) wheels. I would assume so bc they are the same size with less spokes but I'd like to confirm. I've searched all over for actual numbers but no luck.

Rumor is they are heavier because the rim had to be beefed up to account for fewer spokes. And even worse, since the extra weight is at the rim, the moment of inertia is greater, which reduces the ability to accelerate. But I never confirmed any of this. They do look cool, so if you have a line on some cheap ones, it may be worth a try.

Be aware, you will need the disks from the 5-spoke wheels. The disks mount with 5 bolts instead of 7.

The 84/85 (Zx550A1/A2) had a 3-double-spoke setup. They might be 18 inch front and rear, instead of 19 inch front.

Thanks loudhvx. I have an '81 that I ride regularly and an '82 parts bike that I've had sitting in my shop for years and it just occurred to me that it might be a good wheel swap if they are in fact lighter. I figured there was probably some quirky thing that would make it not worth doing or else others would have done it already.

However, the fact that I cannot find concrete info on this anywhere on the internet is kinda driving me nuts. Removing the wheels off both bikes and unmounting the tires to get the weights and then finding out it's not worth it sounds like a PITA.

Yeah I hear ya. That's probably why there isn't much info out there.

And to make it even more complicated... even if the red wheels are lighter overall, they may still accelerate slower, if their rims portion is heavier. That's where you get into a gray area. Obviously, if the bike has less mass, it should accelerate faster, but wheels are a special case. A lighter wheel may actually accelerate slower if the mass is concentrated closer to its rim.

If a hoop and a disk are rolled down an incline without slipping, the disk will accelerate faster. (Interestingly, in this case, the mass and diameter do not affect the outcome. The same way that mass does not affect how fast an object falls due to gravity, as long as you ignore air resistance.)

On the motorcycle:
If the 5-spoke wheels are heavier than the 7-spoke, the 5-spoke will accelerate slower.
If the 5-spoke wheels weigh the same as the 7-spoke, the 5-spoke will accelerate slower.
If the 5-spoke wheels weigh a little less than the 7-spoke, the 5-spoke may possibly still accelerate slower.
If the 5-spoke wheels weigh a lot less than the 7-spoke, the 5-spoke will accelerate faster.

Its has nothing to do with the weight of the rim, these wheels aren't rolling under their own power, the bike powers the wheels, you take weight off any part of the bike, doesn't matter where, and retain the same power, you've improved its power to weight ratio, which means the bike will be slightly faster given the lesser weight the engine has to move.... Sprung verses unsprung weight is another thing effected by lighter or heavier wheels, but only effects handling or suspension response, lighter wheels are better for both power to weight and suspension, regardless....

750 R1 wrote: Its has nothing to do with the weight of the rim, these wheels aren't rolling under their own power, the bike powers the wheels, you take weight off any part of the bike, doesn't matter where, and retain the same power, you've improved its power to weight ratio, which means the bike will be slightly faster given the lesser weight the engine has to move.... Sprung verses unsprung weight is another thing effected by lighter or heavier wheels, but only effects handling or suspension response, lighter wheels are better for both power to weight and suspension, regardless....

The gravity experiment was to illustrate the effects of mass distribution on angular acceleration, which is a common, textbook method of showing that.

Angular acceleration of a body (the wheel) is the total torque applied to the body divided by the moment of inertia of that body. The moment of inertia is dependent on the mass distribution of the body. That is also standard textbook stuff.

As I said, less mass means more acceleration, but the wheels (and all of the other rotating parts) have the additional factor of mass-distribution affecting acceleration. It doesn't matter what is providing the torque. The mass-distribution of the rotating body determines the amount of acceleration the torque can produce on the body.

This means, given two wheels of the same mass and diameter, if the mass of one is concentrated at the rim, and the mass of the other is concentrated at the hub, the latter one will accelerate faster, and thus the bike will accelerate faster, assuming all else is the same. As I said, if the wheel is much lighter, it's the better wheel since it can be assumed the rim is also lighter. But when the total mass of both wheels is similar, you need to consider mass distribution to see if the change will be beneficial or detrimental to acceleration. And if they are similar, then the unsprung weight issue will be minimal.

With all due respect, you are completely over thinking it my friend, we are talking an ounce or two at best, I understand the "gravity" side of the equation but its irrelevant here. If this was such a problem, I think manufacturers like Suzuki wouldn't have made their outer rim sizes larger in the early 1990's using the exact same center sections to their wheels, the 1988 J model 750 had a 4.5 inch wide rear wheel, the next model had the exact same wheels except the outer rim grew to 5 or 5.5 inches, therefore with the same HP, and if this was such a measurable problem, caused by such small variances in weight, the next model should have been slower, it was not. Suzuki wasn't the only manufacturer to do this. The forces you refer to here need larger variances in weight to make any measurable difference. As I said above, the biggest difference for a lighter wheel, regardless of where its mass resides, is for the suspension, any reduction in unsprung weight is a benefit to the suspension. I would use the 5 spoke wheels, in the same widths in a heartbeat, without a thought for their slightly differently distributed mass, if at all there is any {all speculation at this point}, I wish Kawasaki made 5 or 7 spoke wheels with wider rims, the morris and campagnolo clones are bloody expensive . .... I'm looking into widening my 7 spoke Kawasaki rims, therefore the rims will be heavier than the originals at the circumference, plenty of people have done these mods, never seen anyone complain about it making their bike slower, look at drag racing for the most extreme example.. To be honest, a change from a 90 profile to a 80 profile front tire would have more of a difference in weight to these wheels...

Obviously Kawasaki was not concerned, so the differences are minimal, but the original poster was concerned, so I'm just stating the associated physics. I never said it was "such a measurable problem". I was making the point that even if the newer wheel were slightly lighter, there may still be no advantage to swapping it, and there is a chance the acceleration will actually be lower, measurable or not. So basically, the decision is purely a cosmetic one (as stated in my very first post on this thread), unless the new wheel is much lighter, and I haven't heard that to be the case regarding the KZ-based Gpz550's, (and the subject has come up several times in the last decade).

To say "it has *nothing* to do with the weight of the rim" implies you are disputing the angular acceleration equation:
angular acceleration = torque / moment of inertia
Or
that you are disputing the fact that mass distribution affects the moment of inertia.
Those are really the only two factors involved since all else is the same.

Besides being in any classical mechanics text book, it's all on wikipedia if you want to see it.
en.wikipedia.org/wiki/Angular_acceleration

The decision of a manufacturer to make the wheel a different size or weight has many factors involved. But the original poster was talking about two wheels of the same size with different mass distributions and possible mass differences, so that is the only issue I was addressing.

loudhvx wrote: Obviously Kawasaki was not concerned, so the differences are minimal, but the original poster was concerned, so I'm just stating the associated physics. I never said it was "such a measurable problem". I was making the point that even if the newer wheel were slightly lighter, there may still be no advantage to swapping it, and there is a chance the acceleration will actually be lower, measurable or not. So basically, the decision is purely a cosmetic one (as stated in my very first post on this thread), unless the new wheel is much lighter, and I haven't heard that to be the case regarding the KZ-based Gpz550's, (and the subject has come up several times in the last decade).

To say "it has *nothing* to do with the weight of the rim" implies you are disputing the angular acceleration equation:
angular acceleration = torque / moment of inertia
Or
that you are disputing the fact that mass distribution affects the moment of inertia.
Those are really the only two factors involved since all else is the same.

Besides being in any classical mechanics text book, it's all on wikipedia if you want to see it.
en.wikipedia.org/wiki/Angular_acceleration

The decision of a manufacturer to make the wheel a different size or weight has many factors involved. But the original poster was talking about two wheels of the same size with different mass distributions and possible mass differences, so that is the only issue I was addressing.

The OP wanted to know if the wheels are lighter , thats all , i'd go out on a limb and say they are , they have less mass than the 7 spoke wheels for a start, I have 3 different sets of 7 spoke wheels and they are heavy, the 5 spokes are on a smaller bike for a start, I'd be almost certain that Kawasaki put the 5 spoke wheels on the smaller bike because they are lighter, why put heavier wheels on a lighter bike..? It would make sense wouldn't it..? . I'm not disputing what you said at all and I don't need a lesson in physics , just to point out an almost immeasurable point, just for the sake of making it, again, Its so small its irrelevant , Here's something you left out, the rim portion, if they are the exact same width probably weigh very close to the same, now that there is 2 spokes less, that join at the rim as well, there is actually less mass at the rim of the 5 spoke wheels, all speculation as well but do you see my point...? You've neglected to respond to the fact that its still an advantage to use lighter wheels, regardless of where the weight is because of its effect on suspension, which would be far more beneficial than any almost impossible to measure rim weight difference and its effect on acceleration. Seriously mate, I can't believe why I have to explain why what you're saying is a non issue here, motorcycles by design are full of compromises... Again, no offense intended...

To the OP, just use the 5 spoke wheels, I am about as anal retentive as it gets when it comes to motorcycles and I wouldn't think twice before using the 5 spokes, especially on a 35 year old bike, you will never notice the difference and you are getting what you want. while you're at it, weight them and you're old wheels....

Thanks guys. I've actually picked up quite a bit of info here from the both of you. A little bit of textbook physics I was not taking into consideration when I originally approached the idea, and some practicality to my specific situation.

I actually just had a pretty insightful conversation with a close friend of mine who is a mechanical engineer by trade and received the same lecture I got from loudhvx (not that I doubted you) and pretty much the same as 750R1's conclusion. If these wheels do weigh the same or close and the 5 spoke had more of it's weight in the rim, it would take more energy (from my machine) to move these wheels. Therefore to swap wheels for the purpose of performance or as an 'upgrade' would be pointless. (Now if one was swapping for a different purpose like for a wider rim area for different tire options then that's a different story.) If the 5 spoke wheels are considerably lighter, they may have an advantage even if they carry a little more of their weight in the outer rim.The kicker is that it's all speculation bc even if I got weights for both wheels, to do a test to find the moment of inertia would be very difficult.

In conclusion, the wheel swap would prob not make a noticeable difference in the performance of my bike. Maybe if I was racing and was concerned with fractions of a second it would matter more. I think I am going to dig out those 5 spoke wheels and weigh them out of curiousity. Prob won't swap them right away but next time I need new rubber I'll throw them on just for fun. Thanks for the info and insight ✌

PtownSLUG wrote: Thanks guys. I've actually picked up quite a bit of info here from the both of you. A little bit of textbook physics I was not taking into consideration when I originally approached the idea, and some practicality to my specific situation.

I actually just had a pretty insightful conversation with a close friend of mine who is a mechanical engineer by trade and received the same lecture I got from loudhvx (not that I doubted you) and pretty much the same as 750R1's conclusion. If these wheels do weigh the same or close and the 5 spoke had more of it's weight in the rim, it would take more energy (from my machine) to move these wheels. Therefore to swap wheels for the purpose of performance or as an 'upgrade' would be pointless. (Now if one was swapping for a different purpose like for a wider rim area for different tire options then that's a different story.) If the 5 spoke wheels are considerably lighter, they may have an advantage even if they carry a little more of their weight in the outer rim.The kicker is that it's all speculation bc even if I got weights for both wheels, to do a test to find the moment of inertia would be very difficult.

In conclusion, the wheel swap would prob not make a noticeable difference in the performance of my bike. Maybe if I was racing and was concerned with fractions of a second it would matter more. I think I am going to dig out those 5 spoke wheels and weigh them out of curiousity. Prob won't swap them right away but next time I need new rubber I'll throw them on just for fun. Thanks for the info and insight ✌

The Unitrack GPZ550 that had the 5 spoke wheels was actually lighter than the model before it with the 7 spoke wheels, I'll be interested to see the weights, I'm betting the 5 spoke wheels are lighter, and by a bit. Seriously, I wouldn't even consider any perceived performance loss, going on that theory, adding wheel weights to the rim for balance would rob you of similar performance, you'd need some very expensive equipment to measure that, and you'd never feel it anyway, Its not something you could measure on a dyno, you'd lose more performance riding from sea level up into the hills. Anyway, modify away and enjoy your bike, the 5 spoke wheels look great, they look even better in gold like the old Campagnolo and Morris race wheels.. Not dissing your engineer friend but in my experience, there are a lot of highly intelligent, anal retentive engineers, some of them just aren't real practical and just like to be the smartest guy in the room, its like working with some architects ....

750 R1 wrote: The OP wanted to know if the wheels are lighter , thats all , i'd go out on a limb and say they are , they have less mass than the 7 spoke wheels for a start,

Yes, the 5-spoke appears to have less total material and thus *appears* to be lighter.

750 R1 wrote: I have 3 different sets of 7 spoke wheels and they are heavy, the 5 spokes are on a smaller bike for a start, I'd be almost certain that Kawasaki put the 5 spoke wheels on the smaller bike because they are lighter, why put heavier wheels on a lighter bike..? It would make sense wouldn't it..? .

Yes, which is why it was surprising to hear the rumor that the 5-spoke wheels were heavier. However, the 550's all started out with 7-spoke wheels, and even after the 5-spoke wheels were available, the non-gpz 550's retained the 7-spoke wheels. And further, the later GT550's, which were basically shaft drive Kz550's, came out using the 7-spoke wheels for quite a few years after that.

750 R1 wrote: I'm not disputing what you said at all ...

In your initial post, you wrote :

Its has nothing to do with the weight of the rim, these wheels aren't rolling under their own power, the bike powers the wheels, you take weight off any part of the bike, doesn't matter where, and retain the same power, you've improved its power to weight ratio, which means the bike will be slightly faster given the lesser weight the engine has to move....

The word nothing implies it has zero effect. From that, I can only conclude you are arguing that the mass distribution of the wheels has no effect on acceleration and the only thing that matters is the overall weight of the bike, as a whole (as that is what is considered in power-to-weight ratios). Well, that would dispute my comment about the wheels and their differing mass distribution. This is the only point I see in contention.

750 R1 wrote: Here's something you left out, the rim portion, if they are the exact same width probably weigh very close to the same, now that there is 2 spokes less, that join at the rim as well, there is actually less mass at the rim of the 5 spoke wheels, all speculation as well but do you see my point...?

Of course. But I did not leave that out. I stated the rumored reason given for the wheels being heavier was due to the rim being heavier. I did not speculate on what aspect of the rim was different since I have no idea.

750 R1 wrote: You've neglected to respond to the fact that its still an advantage to use lighter wheels, regardless of where the weight is because of its effect on suspension, which would be far more beneficial than any almost impossible to measure rim weight difference and its effect on acceleration. Seriously mate, I can't believe why I have to explain why what you're saying is a non issue here, Again, no offense intended...

I did address the suspension issue. I wrote in regards to the weights of the two different wheels: "And if they are similar, then the unsprung weight issue will be minimal." I don't disagree with any of your assertions about the suspension benefits of a lighter wheel.

I don't know why you are compelled to "explain why what you're saying is a non issue here". I never said it was a major concern. I thought the summary in my second post would show that the difference would be small by virtue of the fact that the advantage can go to either wheel. That implies any advantage would be small Perhaps I did not emphasize that enough.

Since there are no numbers, it's all subjective anyway. A tiny advantage is a big deal to someone who races. While some people don't care if their moped does the quarter mile in 30 seconds. I don't know what the OP's desires are, so I mentioned it, since it is a factor that exists. Alloy Kz wheels are big, heavy wagon wheels (aesthetically, though, I like them). There is plenty of room for improving performance by going to a lighter wheel. Why not do it with actually knowing the factors involved?

If he cares about it, he can look into it further. Some like to further their own knowledge for curiosity sake. What's wrong with that? If they don't want a physics lesson, they are free to ignore it.