XJ Tac Comparisons

[JonInDenver]

As some of you know, I destroyed a perfectly good XJ11 and created a shaft-drven XJ-powered trike from it. Now that this monstrosity is on the road, I wanted to see how my differential and tire-size variances stack-up aganst whole bikes.

At about 4700rpm in fifth gear, I'm running 55 mph. 5000 rpm gets me running comfortably at 60 mph.

Let me know your comparisons, as I'm trying to determine if I need lower or higher profile tires to even things up.

Thanks!

[JonInDenver]

PS:

I just ran the first tank of gas through it, managed between 20 and 25 mpg. I'll try to run it over to scales to weigh it tomorrow.

[beechfront]

mine isn't an xj, but roughly....


at 60 - 4,000
at 75 - 5,000
at 90 - 6,000

[Jerry]

My XJ agrees with Beechfront's numbers.

[John]

It seems that my 80SG, after the big bore, that my rpm's dropped by 3-400 at speed. 60 mph is ow 36-3700 vs 4k. 75 is now about
4700 vs 5k, so your engine speeds are a little high as compared to the stock bike.

[chevy45412001]

rpm

my xj is right with Beach and Jerry.

[brock]

i think mine are too. my '79xs hits 60 right at 4k rpm's. good to know.

[xExcessive 1120]

RPM and Tach Comparisons

IMHO, unless the laws of mechanics and Phisics have been suspended I cannot see how a Big-bore kit or change in displacement of the engine can affect RPMs at a given speed. Unless the gear ratios are changed directly by work on transmission or rear gear-set, or indirectly by change in rear wheel diameter, there can be no change in engine speed in a given gear at a constant road speed. There! I feel much better, Christopher. "The Gimp"

[John]

Chris, I didn't think anything would change either, but I have noticed a drop in RPM's since then. Now, mind you, the old engine was from an XJ, and I rebuilt the original engine from my bike. If there is a change in gearing between the XS and XJ, I don't know, but at 60 mph, my tach shows about 3700 now. Just my observation. I did replace the rear tire a while back, maybe that had something to do with it.

[xExcessive 1120]

RPM Change

AH! Different engine eh? If this includes the bottom end then perhaps there is a difference in the number of teeth on the primary chain gears. If that is the case (no pun intended) then this would explain a minor change in shaft speed and resultant effect on the final drive. Christopher "The Gimp"

[Hepcat]

What about the accuracy of the tach on these bikes. I remember reading something about the accuracy being somewhat in question due to the way it's relayed. It's not a mechanical reading...It sticks out in my mind that it was primarily an issue at low rpms. Could there be a difference between pickups?

[xExcessive 1120]

XS vs. XJ Speedometers

Your last post Jon, raises a question in what's left of my mind about a potential difference between XS and XJ speedometers. I know the XS is mechanical, driven off the front wheel. I am not familiar with the mechanics (electronics?) of the XJ, but I am most comfortable with a gear driven speedometer and as long as the cable is properly lubricated they seem to be very good even at low speed. I have to throw in a vote for a lubricant that I now swear by, especially for cables and that is Wurth HHS 2000. I got mine from a local mechanic and it was $13.00 a can and worth twice that. I first used it on a sticking seat adjuster/rail and it is amazing! It starts out as a light penetrating oil but when the carriers evaporate it leaves a long polymer grease that worked so well the first time I got in my old 2002 the seat went back and nearly put me in the trunk! I had a clutch on my '79 sidecar that was very stiff but after popping the upper end out I used the little tube and ran a bunch down the housing. When I checked the other end a few drops had run out, so it really penetrates. That clutch was and is just like butter. Cheers to all! Christopher. "The Gimp"

[John]

Chris, the XJ uses the same speedo drive as the XS.

[Hepcat]

The speedo is mechanical and gear driven on both bikes. But as far as I know, the tach is not. It takes readings from the A/C generator. I found an old post HERE where Jerry explains the difference in the charging systems of the XS and XJ. It would seem to me that a small change in voltage due maybe to resistance or dirty connections might mess with the tach...Is my logic ok there? I'm just saying that there's a lot more to the tach than a mechanical cable, it's electrical and I don't know what could potentially affect it.

[JonInDenver]

"I had a clutch on my '79 sidecar that was very stiff"

Did you have a leading ling front end for your hack?