A serious look into custom Middle Gear Ratio

If you all are familiar with my build thread, you know it's a little crazy. Well, it's possibly going to a whole other level. Having a 200 hp bike that can only go 130 MPH is unacceptable! With looking into swapping rear ends to even exploring reverse engineering the OU28 chain drive. I've come to the one conclusion, outside of being an engineer at Yamaha (in JAPAN), the only way to get over the low top end speed is to make a custom middle gear set.

I was able to calculate the speed & RPM in every gear. the website is http://www.gearingcommander.com the important data(XJ900) needed to input correctly is the Primary Gear Ratio (1.672), tire size (stock 120/90-18), and final drive. To calculate the final drive, you'll need to multiply the middle gear (1.055) x Transmission output (1.297) x Final drive (2.909). Interestingly, they call the middle gear in the service manual the transmission output.

As you can see, even with a tall bias tire on the rear, theoretical top speed is still under 140! With the bike being a 5 speed, and believe me when I say I've exhausted all roads to a 6 speed, I researched other bikes with similar hp and tq that my bike will have. The Suzuki GSX-R 1100 has estimated 140 crank hp and The Yamaha FZR 1000 has similar


Now the question is, what ratio is needed to replicate similar to this? The Middle gear ratio is 1 : 1.297. If you multiply the three gears together, it gives you a final drive ratio of 1 : 3.98. To get a ratio that is close to the GSXR 1100, a final drive ratio of 1 : 3.1 To achieve this we need to have a Middle gear ratio of 1 : 1.01

Here is the range with the custom ratio input



Now that looks a lot better! Now I'm messing around with theory here. I'm not sure exactly where it needs to be? I know I want it to be a fast as possible, but without sacrificing too much acceleration. The actual wheel hp will mostly likely fall around 175-180 hp (on max boost). My question is, will the bike be a dog until boost hits? If the max NA torque peaks at 5,000 rpm and boost comes in around 5,550 rpm (in theory), this should work? I will say I'm a newbie at this, but I'm trying hard.

Here are some pics of the gears in case to show possible clearance issues







I need to do some more measuring this weekend, but The middle gear needs to decrease in diameter and the transmission output gear needs to increase. A quick check was around 8.5mm diameter increase on the transmission driven gear, but I need to get a better measure when I have more time. With that increase diameter (+ 4.24mm radius), it should clear. There might have to be a little grind on the extra material that supports the transmission fork shaft. I see there is also an oil passage there as well, but that looks to be on the other side of which I would grind.

This won't be cheap, I'm praying it's not more than 4k. I wouldn't be surprised if it was though. The shop I'm at is getting a water jet in a couple of weeks, so I'll be able to cut nylon or delrin to mock up the gears before I go forward.

Let me know your thoughts guys? I'm playing around with gear ratios to hit the sweet spot, but it's hard to know what will work outside of number crunching? I have the power, but it's a turbo and it won't behave like a na bike.

 

I think you may find the same problem as a limiting factor in the transmission as you do trying to exceed/increase top end speed
redline . the gearing would likely have a "redline point" before they overheat and blow apart.

consider that the engineers at Yamaha took into account how fast everything spins when they established motor redline.
including middle gear and drive shaft and the bearings used,
I guess my point is you would have to beef up everything from the middlegear mod out to the rear wheel to take the extra stress

 

if you increase the middle some and look into the gearing downstream as well

 

Sounds like you have researched the issue pretty thoroughly, so you've probably already thought of this but...
I've often wondered if the final drive on the bikes with 16" rear wheels (Maxims) is the same gearing as on bikes with 18" rear wheels (Secas). If the gearing is different (such that it gives the same top speed when paired with the respective wheel size) then you could get a gearing lift by using a Maxim final drive unit with an 18" rear wheel. The Maxim X uses the same driveshaft as the Seca 900, but I don't know if the crown and pinion gears are the same or different.
Just a thought.

 

Why the middle gear, why not the final drive or the primary drive?

 

I looked into swapping the rear end out to exhaustion. The XJ series bikes have the lowest ratio of all Yamaha. To find the right rear end ratio, the equation is (1.055)(1.297)x=3.1 The answer for x = 2.27 (rounded). Our rear end is 2.909, which would be a 22% drop. From my research, I can find no compatible rear end close to that ratio. This would need to be a custom cut gear set too. I have looked over the disassemble / reassemble process of the rear end in the service manual. Each case is uniquely machined and have to be shimmed based on gear and case. Also, the wheel spline assembly is part of the rear gear. This also makes this potentially very expensive to machine. I feel this is a dead end. The middle gear, while still a complex machining process, is more of a drop in process. I can still use the bearings, clips, etc.

All XJ bikes share the same rear end. Most viragos as well, except for the later ones that share the Vmax rear. The Vmax/Venture rear is a much higher ratio.

I may be wrong, but the transmission ratio will not change? This is all after the transmission and all gears behind will spin faster. The load change is another matter and could be something that should be figured out. I'm no engineer, but changing the middle ratio by .297 should be too drastic? The middle gear set is pretty stout. I'll be digging into this a lot and keep updating as I find good info. If you guys have any insight on loads and gearing, chime in.

 

from what i see here, you haven't scratched the surface of this yet. gears are on shafts, the shafts don't move, the tolerance of the mesh is kind of important.
the gears change diameter by how many teeth they have. since the shafts don't move, you're going to have to pick gears by the tolerance of the mesh.
ex: say gear 1 has 40 teeth, gear 2 has 50 teeth and the shafts are X distance. if gear 1 changes to 43 teeth, gear 2 might need 48 teeth to keep X the same
maybe you can change the profile of the tooth to get the mesh right and maybe the new profile can be used for the power you need.
google "gear blanks" and look at what's involved

 

What about the primary drive gear between the crank and the clutch, I would have thought that easier to do than either the middle gear or rear end, although it means a change of gearbox and clutch speed/load

 

They are.

 

This is essentially unfeasible. The primary gear is machined onto the crank. You would have to make a billet crank as well as a custom clutch basket, no go.

I don't have a lot of time to elaborate because I'm headed out to watch the McGregor / Mayweather fight tonight, but I thought I'd share some pics of today's event. I measured the distance between the two gear sets and since equal distance is 1:1, i found 87mm between the two. I know, I know, it's much more complicated than that. I just wanted to a mock up and see if something in the ballpark is possible. I'll tell you, it took some thinking and grinding to get the bastard in. I'll elaborate more tomorrow. Cheers

 

http://khkgears.net/gear-knowledge/abcs-gears-b/basic-gear-terminology-calculation/

 

sorry im no help, but would like to see this succeed.

forgive my ignorance, would it be easier to swap the rear end,, to fit whatever tire you want and go chain final drive? play with sprocket ratios?

saying that I have no idea the modding people have done to these, so I don't know how feasible a conversion that would be

 

Thanks, but I've already been down this road.

Thanks Polock, I read this article and helped out a lot. I jumped on fusion 360 and they also had a gear generator plug-in. I did some measurements and found that the gears are:
20 degree pressure angle
2 mod gear width (a standard metric size)
roof fillet is 1.18

I found that the centerline between both gears are 85mm. Unfortunately, there isn't a 1:1 ratio in a 2 Mod gear size, only 1 Mod (maybe too weak). however, a close ratio to 1:1 is with 42 teeth (84mm)& 43 teeth(86mm) which equals out to 85mm apart! the 42t is 88mm in overall diameter, whew just fits. The 43t is 90mm, an over all reduction by 9.5mm.

This will give the middle gear ratio to 1:1.023
If we do the math, 1.055(primary) x 1.023(middle) x 2.909(rear end) = 3.13


I'll take 61 mph in 1st gear and a top speed of 166 MPH! That's a 28.5% increase in top end!

Here is a cad drawing I did on Fusion 360. It's a sweet piece of software, cloud based.



I still have a lot more numbers to crunch, but I think I've found the sweet spot. Let me know if you guys find any flaws with this? I'm know a few pro engineers that I'm going to bounce this off off.

 

So far the only flaw that I see is expense. Having said that, it's going to be a lot less expensive than the chain conversion that you started.

I never thought there'd be enough clearance to fit a large-enough gear to make enough difference. Excellent detective work sir.

 

Ok, now I understand, I haven't ever struck that with any of the bikes I've owned and rebuilt, it always been removable

 

Found these bad boys! The delivery is expected Friday. I'm going to test fit these over the weekend, exciting!

 

before you go too far, if this was a Briggs & Stratton mini bike i'd say go for it. but 200 hp and 166 mph, maybe you want to at least consider some of the things this guy is talking about. i know the people at yamaha did.

 

Right on! This will be an in depth research. These gears I bought are just off shelf stock plastic gears to test fit. I would never try to final install generic gears. This is more for clearance and functionality tests.

 

I sent our bike information to http://www.gearingcommander.com for gearing and speed. At the top there is a drop down menu and select brand model, then version. I only submitted the XJ900, but you can use that to see XJ650/750 figures. You need to change the primary ratio to 2.401. It's a pretty cool tool. You can change tire sizes and gear ratios and see your rpm level at any speed.

 

I've been working hard on Fusion 360 lately. I'm using this project to learn the software. Here is a running simulation of the modified middle gear ratio