new gearing possible or not

XJ stock final drive= 4.18:1



The Seca 750 turns 4500 RPM at 60MPH in 5th gear.

SO:


The 3.03:1 swap that is suggested in your second post yields 3261 RPM @ 60MPH on 5th. (might be worth while)

The 2.72:1 swap yields 2928 RPM @ 60MPH in 5th. (I'd not do that one)

[Edited to remove brain-farts]

 

On the website it says the vmax final ratio with a 33/9 is 2.72 ratio. I think both gears could be swaped, surrounding parts are the same. What's is the rpm at 60 for that ratio?

 

Check my edit. I posted before proof reading

 

Are you using a website calculator for these figures? If so, can I get the link?

 

Edited my reply to eliminate a brain fart. I had the final drive ratio of the Seca wrong.

 

I'm using the math that I learned in high school.
On of the problems with swapping final drive gears is that they are helical; you have to swap them as a matched set if you want the final drive to last. Other than that you will just have to decide on how much acceleration and top speed you are willing to give up in order to lower RPMs at highway speeds.

 

I know the math for the rear end. Where it confuses me is on the calculation of the middle gear? All the website calculators are for cars. After I finish this project, I was thinking of putting a turbocharger on it and seeing what I can get out of it. Not really worth it if top speed is around 135 @ redline. (Xj900)

 

The article that you linked to is avoiding messing with the middle gear by only swapping the final drive gears (the author isn't as clear on this as he could be). The math works the same no matter what vehicle you are using. I skipped a lot of unnecessary steps by using published figures for the overall drive ratios and just did the math to find the RPM change for each overall drive ratio. the middle gear calc works the same as it does for any other gear set. Drive teeth divided by driven teeth gives the gear ratio. Here's a semi-decent guide on how to figure the overall ratio when dealing with more than two gears in a set http://www.wikihow.com/Determine-Gear-Ratio

 

what is our transmission gear ration? is that middle gear? or something else? Trying to see if these gear swaps can get to 150+ mph at redline. All the online top speed calcs ask for trans ratio.

 

You'll also need to figure for horsepower and aerodynamic drag (if you want to mess with a fairing) if you want to see how a gear swap will effect top speed. I'd hold off on gear swaps until you have the bike finished and can get it on the dyno. Then you'll have a baseline to work from.

The transmission has several ratios, one for the primary drive; one for each "speed" (1st-5th); then there is a ratio between each "speed" and the middle gear; and another ratio at the final drive (there is also a ratio between the final drive and the road, but good luck finding a different diameter wheel). The Factory shop manual and the Haynes have all of those ratios listed. What you are interested in is the overall ratio in 5th gear, so you need to figure for the meshed ratios of the primary drive, 5th gear, middle gear, and the final drive. It'll be a lot of math, but you should be able to find a combination that gets you close to what you need. I'd make a matrix of all the possible swaps so you can have a graphic that could be used for narrowing down your choices to the ones that will be useful.

[rant] The problem with using online calculators is that we really need to understand the math that is being done. When you know how the math works it's easy to see when the given solution is outside of the range of expected values (ie. needing to edit out my brain fart after I saw my mistake). If you don't understand the math that is being done you're just punching in numbers and hoping the algorithm being used is the correct one. [/rant]

 

I know, I just wanted to know the theoretical potential limits.

 

http://motorcycleperformanceanalyzer.com/

 

Thanks Polock for the link! how would I input the data for "primary" & "secondary" transmitions? They are asking for sprockets.

 

Select a gear drive bike form the list on the left of the page. I chose a BMW R1200GS. The transmission gear ratios can't be edited, so you may have to hunt to find a bike with the same 5th gear ratio. The BMW I chose is remarkably close though.

 

The theoretical and potential limits are the same; the reason the "app" is asking for sprockets is because chain-driven bikes are easily re-geared by changing sprockets.

Which brings us to both the theoretical and potential limits of the idea: You have a 30-year old shaft-driven Yamaha. How were you planning to "re-gear" it anyway? I don't think Yamaha ever offered any "optional" gearing, and the aftermarket isn't really producing all that much in the way of such things for long-obsolete bikes.

 

Fitz, He'll be swapping gars from other Yamaha models. The final drive gears are the same across several models, except for tooth count. The middle gears should also be the same other then tooth count. Yamaha (like all good manufacturers) is very good about parts interchangeability. For an example that I'm personally familiar with: the final drive gears for a 1985 Riva 180 will swap into a 2013 Zuma 125, the only difference being the tooth count.

 

could you do just bolt on, like the whole swingarm, diff and wheel, or just diff and wheel.
myself, i'd do the middle gears rather than mess with the ring and pinion

 

If I were changing the final drive I'd just swap the whole unit. Pull the wheel, five bolts later and you have the final drive swapped. Setting up crown and pinion gears can be a real PITA if you don't do it very often, so just swapping the whole final drive gearset is easier. Swapping the middle gear is much more involved; you have to make sure that the bevel gears are shimmed correctly, and that requires splitting the case (if I'm looking at the manual correctly). I'd want to split the case anyway because swapping one half of a matched set of bevel gears leads to big trouble later on.