Top speed question

[DevilDesire]

Hello, peeps!

I have some weird experience with my top speed. I ride a 03' naked GS 500. The thing is that I hit about 160 kmph (100 mph) in 4th gear and then, when i shift to 5th the speed starts to fall down >.<.... Is there something I can do, because I've got 5th and 6th gear to come and no more juice?! I top out in 4th and that's it...  I weight about 126 kg (~278 lbs) and am a bit over 190 cm (6,2-6,3 ft). When I shift with no clutch there is less speed loss but still it goes down.... 

[yamahonkawazuki]

Well, does it run rough when you do this? Or does it fall on its face. I got my slightly modded gs up to 120 indicated, yes i know stupid. But it was a long distace test ride to a charity cycling event. Course i weigh 140lbs soaking wet was down 1 tooth in front as well. What is your ratios front and rear?

[DevilDesire]

No, it doesn't run rough, it just falls on it's face... I have no idea what my gear ratios are but I am pretty sure the bike is with the factory chain and therefore the OEM size sprockets.

PS: I just don't think it is anywhere near normal to top out the bike's speed and still have 2 gears to shift up....

[Atesz792]

You may have the 34 BHP version, restricted for certain markets. Top speed matches.
Or maybe it's just a clogged air filter. Or too much weight (really no offense here, but you're not tall enough for your weight... medically speaking).

[Big Rich]

Top speed & gearing is a funny thing...... and aerodynamics comes into play at high speeds too.

If you were to install a dustbin fairing (think Bonneville flats racing), your top speed would increase dramatically. And with smoother airflow, you could drop your gearing down (smaller sprocket in the back) for an even higher top speed. But without increasing aerodynamic flow, your only choice really is get a larger rear sprocket. Unfortunately if you go too large, your engine with reach max rpm before the bike reaches top speed.

What it all boils down to though.....unless you are doing any sort of racing, your top speed is pointless. And testing it is pretty dangerous.

[DevilDesire]

No offense taken, Atesz! I dumped the gym with heavyweight lifting a few years ago and body weight remained the same but I got loosened How can I check if it has been restricted and if it is how do I remove the restriction?

Big Rich, I don't like that much fairing, I'll be putting a medium sized aerodynamic windshield though. Won't larger rear sprocket decrease my overall acceleration?

PS: The air filter is clean, I checked it.

[Big Rich]

Nope. A couple more teeth on the rear sprocket will increase acceleration. But your given rpm for any speed will be higher as well (ride on the highway often and the higher rpm can drive you nuts).

[DevilDesire]

I have a chain and sprockets change on the next-to-do list so I guess it won't be pain to go 1-2 teeth larger rear sprocket.

[yamahonkawazuki]

Nope. A couple more teeth on the rear sprocket will increase acceleration. But your given rpm for any speed will be higher as well (ride on the highway often and the higher rpm can drive you nuts).

or dropping a tooth or two on front sprocket. Easier than changing rears. Carbs clean?, and devil idk where youre at, so the restriction thing might be relevant here too

[gsJack]

Top speed should be reached at max HP and for a stock GS500 max HP is at 8500 RPM which would be about 108 mph in 6th on a stock geared GS500 with stock size tires.  Published road tests over the years have varied between 100-110 mph tested top speed.  Here are all the tests I've found:

http://img.photobucket.com/albums/v443/jcp8832/GS500tests_zpsd738ea8a.jpg

My 02 is same as your 03 but I've never had it up near top speed but I had my 97 GS500E up to 110 mph indicated in 6th gear back in 2002.  I shifted from 4th to 5th and 5th to 6th when it felt right and topped out in 6th but was running out of straight about then, maybe it had a bit more.

I weighed about 240-245# then I think and had a big Plexi 2 shield on the bike.  I had 110/70 and 130/80 Metz radial tires on it then and a 15T front sprocket.  The smaller 15T front sprocket should offset the larger 130/80 rear tire to give near stock engine speeds in gears.

Have you counted the teeth on your sprockets?  Your larger 140/70 rear tire plus a 17 or 18T front sprocket could bring your max speed down to 4th or 5th gear range.

[DevilDesire]

Thanks a lot gsJack! I will take the front sprocket cover off and count the teeth tomorrow to check that but according to all the replies my top speed isn't that bad assuming my weight. Just that 4th gear topping... I will count the teeth tomorrow and on the next sprocket&chain change I'll go 15/39 teeth.

By the way, I'm doing 50 mph @4000 rpm in 6th gear if that is relevant.

[gsJack]

By the way, I'm doing 50 mph @4000 rpm in 6th gear if that is relevant.

Yes it is, stock GS turns about 4800 rpm at 60 mph  Looks like you probably have stock gearing.  Be good to check it anyway before ordering new.  Don't think a restricted bike should hit 100 mph in any gear.  As nearly as I can recall I shifted into 6th around 100 mph indicated and climbed to 110 in 6th, it was 12 years ago.

4800x50/60=4000 rpm at 50 mph approx

[twocool]

The physics are unavoidable and brutally honest......

As you go faster, the aerodynamic drag goes up with the square of the speed...

Twice the speed....4 times the drag.......
4 times the speed ... 16 times the drag....
etc.


But what is really crazy, is that the physics shows that the HP required to go faster is the cube of the speed..........

So to go twice as fast you need 8 times the HP

To go 4 times as fast you need 64 times the HP etc...

This is why drag race cars have 2000 or even 4000 HP!!!!!!!!

With about 40 something HP on a GS 500....you're just not going to go all that fast...around 100MPH that drag curve starts getting steep, and there's just no where near enough HP to go any faster.........


Cutting DRAG however, will make you go faster, it's far easier to cut drag than it is to add hp...

If you cut the drag in half, you will go significantly faster with the same HP........

Motorcycles have fairly bad coefficient of drag...even GS500 with fairings are very "dirty" aerodynamically....


I suppose you could design a fairing in a wind tunnel, combined with optimum rider position, and get quite a bit more speed out of a GS500...

But this is almost pointless, as the GS is not designed for that ....the GS does what it is supposed to do very nicely...

Somebody on this group took a GS to Bonneville to set records...I think they were somewhat disappointed in the speeds achieved...

Cookie

Top speed & gearing is a funny thing...... and aerodynamics comes into play at high speeds too.

If you were to install a dustbin fairing (think Bonneville flats racing), your top speed would increase dramatically. And with smoother airflow, you could drop your gearing down (smaller sprocket in the back) for an even higher top speed. But without increasing aerodynamic flow, your only choice really is get a larger rear sprocket. Unfortunately if you go too large, your engine with reach max rpm before the bike reaches top speed.

What it all boils down to though.....unless you are doing any sort of racing, your top speed is pointless. And testing it is pretty dangerous.

[gsJack]

All true and to the point twocool.  While typing the posts above I was wondering if my big Plexi 2 shield made my wind resistance with the big Plexi 2 you once referred as a barn door gave less wind resistance than DevilDesire without the shield?  Also whether he was sitting near erect or laying down on the tank would be significant.  By the way I put a smaller Spitfire shield on later that same month for a trip to the Smokies and have switched summers/winters since.

[BockinBboy]

I think wind resistance is your biggest factor here... the gearing and hp at 5th and 6th can't compete.  I don't think its that uncommon to top out your speed before reaching the highest gear - for most bikes, the highest gear is an 'overdrive' gear and that includes the GS500.  These guys have had great comments on the topic, so I'll just leave it at a +1 for what they have covered!



- Bboy

[yamahonkawazuki]

I know when i rode my 2nd gs between clinton tn and harrogate, i indicated 120 cluster is by nature off, had her pegged, again, stupidity on my part it was close to midnight. Which worked out in my favour,  cause of the colour of the bike. Saw cop car lights light up then turn off again. Anyhoo i digress, i had a cruiser style windshield on there which helped my comfort a bit. With the 1 tooth down in front even with gear i was at the barrier that twocool mentioned

[Janx101]

I get a higher road speed (tucked down on tank behind fairing) in 5th and close to redline than I get in 6th ever!...... wind resistance vs power available. ....
Need more speed? Add cylinders,cubes or fuel injection!

[DevilDesire]

   Thank you a lot guys! Your posts were highly educational for me!
   I don't really need that 110-120-130 mph top speed because I cruise at 60-90 mph, mostly. I just found it disturbing with the 4th gear topping, maybe because I don't really have a big experience with bikes but now I see it's something pretty much normal.
   Again, thank you! 


PS: I'll still be going 15/39 teeth tho, just for the try. 

[Big Rich]

I would recommend a larger rear sprocket instead of a smaller front. Reason is, a smaller front sprocket makes the drive chain work harder to get around the smaller diameter. A 41 tooth rear will get you close to the same gear ratio as a 15 tooth front, and you would only need a chain that has 2 more links than stock.