Rear differential ratios

Two questions

1. What are the ratios of drive shaft to wheel revolutions of the various styles of Maxim and Seca? Specifically XJ650Maxim, XJ750Maxim, XJ750Seca?

I am wondering if switching to another differential will reduce RPMs at highway speeds.

2. Will a XJ650Maxim wheel fit into an XJ750Seca differential?

 

I don't have an answer, I'm a chain-drive guy. I do have a question though...why? A lot of relatively new XJ owners seem to have a problem running along at 5K or 6K they seem to feel the motor is "straining" or revving too hard. These things will run 7K rpms ALL DAY and love it. Four grand is barely spinning to an XJ. The motor is a LOT happier cruising at 6500rpm than at 4K, honest.

 

I guess its from all the years of driving a cage at 2200 rpm at 65 mph

 

For my bike 4500 RPM seems to have plenty of power.

 

Oh, the 550 has plenty of power at 4K also. It took me some getting used to, the Norton redlines at 6500, and likes to cruise at about 3800. I ride my 550 to work (in season,) 63 miles one way, which includes about 40 minutes (if I'm lucky) of constant high speed running, 75-80-85mph. It THRIVES on it. It's a LOT smoother at 80 than at 60. But it did take some getting used to spinning the motor like that.

 

I am still guilty of looking for another gear.

 

me too. But wouldn't SEVEN be overdoing it?

 

eh, another gear does seem like it would be nice.
my daily rider is a 600cc sport bike, and even it hangs lower in the revs at the same highway speeds than the maxim. plus if feels smoother, maybe it's just because it's newer and tighter engineered? either way, i've come to think of my maxim like an loose, worn in, rattling AK-47 that never lets me down.

 

I have seen on this forum before that if you put a 650 seca rear unit on a 750 maxim it will lower the rpms. But as Bigfitz said my 750 maxim loves 6500 and 7500 rpms. Does take some getting used to. My Harley likes 3000rpms=75mph.

 

If you put a Seca 18 inch rim on a Maxim that had a 16 inch rim, you get lower hiway RPM or a taller overall gear ratio.

Same thing with putting a Maxim engine in a Seca frame. There's a 1 tooth difference (I think ) in the primary or take-off from the crank.

Another trick that nobody has done is get the "middle gear" gears from a parted out 900, and install in a 650 - 750. They have to be shimmed, I think.
900 = 49/36
750 = 48/37

Do both mods for really tall gears.
Edit- - warning- - these numbers are not accurate, I'm working on getting the right gear numbers.

 

Ok, what is the "primary or takeoff from the crank'? and what is the "middle gerar"?

The Seca 750 has an 18" wheel and the Maxim has a 16".
If these are diameters or radii then the RPM difference for a fixed speed is 16/18 or 0.875. Thats a 12.5% difference is "tallness".

I would like to know more about the 1 tooth difference in the primary. Where in the specifications would I find that?

 

The 6 th (?) crank flywheel is geared (straight cut) to the clutch basket.
This also would make everyone on E-Bay wrong for saying a Seca 650 crank will swap into a Maxim 650 (without also swapping the clutch basket).

My first post was about gearing, and driveline swaps. All "differentials", or rear hypoid gears are 4.179:1 ~ more info in this thread -
http://www.xjbikes.com/Forums/viewtopic ... html#60079

The 12.5% difference - - you forgot to add tires !
The actual diameter of a real inflated tire can vary from the calculated diameter. Some people measure the radius minus the "squish" at the bottom. A Seca goes about 7 inches further every rear wheel revolution than a Maxim.

I did all this searching a year ago. Your answers lie deep in the Archives.

 

. . . And the Middle Gear is really 2 gears- - the 90* angle gears just ahead of the Bellows and above the "Middle Gear Drain Plug" that we are forbidden to touch.