My '59 Enfield /Indian 750cc Chief High Compression Build

[Koncretekid]

Upon further thought, I partially agree with you.  The effect of the chain pull causing the chassis to raise upward, and the wheel downward, only occurs in a change in attitude, such as a road racer slowing down for a corner and then accelerating out of the corner, or a drag racer on initial acceleration.  If the chassis moves upward, then there must be an initial upward acceleration with a resultant downward force. This might also occur when changing gears, as the the bike briefly squats, then experiences this effect again upon re-acceleration.  Once the chassis reaches its steady state position, no further force can be transmitted than that of its own weight. However, the opposite reaction, that of the swing arm starting at an upward angle to the pivot would try to raise the wheel on initial acceleration, which could reduce initial traction.  Otherwise, all the motorcycle manufacturers have got it wrong over the years. With gobs of horsepower, modern suspension and traction control, some bikes are now built with the swingarm pivot concentric with the drive sprocket to neutralize any such forces, I believe, to make the race bikes easier to control.

Do you agree with the above?

[Old Scrambler]

I like the thought process................and somewhat agree with all positive points............except; the primary reason to eliminate the distance between the drive-sprocket and the swing-arm pivot is to keep the chain on the sprockets under all conditions.  For our purposes, I look to reduce the swing-arm angle to almost neutral when at speed for best chain tension and the least amount of change in chassis movement.

[Interested Observer]

Chassis movement while under acceleration or deceleration is a combination of the internal driveline loads under discussion and the change of effective weight distribution (in effect, an external load) due to the acceleration.  (This accelerative load is an external effect, same as the acceleration of gravity is an external effect.)  In this case, wheel loads do change, but it is due to the acceleration-induced weight redistribution.

[bak189]

Interesting discussion regarding swing arm pivot and rear axle relationship......also drive and rear sprocket size play a big part in this.  It should be noted that the people who first became fully aware of this back in the 1960's were Ray Hensley (Trackmaster frames) and outstanding dirt track racer Niel Keen.  Understanding all these "problems" is what made the Trackmaster dirt track frames a winning combination to use.  Ray on his BSA A-65 engine went as far as changing the primary drive ratio in order to be able to get the  wanted ratio of drive and rear sprockets.  If you were using a Trackmaster frame you had the drive coming off the corner....it would really "hook up".   Nico Bakker (Bakker Framebouw, Netherlands) used this knowledge in the early 1980's on his outstanding road race chassis for some of the best riders of that time. All this knowledge was collected by seat and track time we did not have computers to do the work.....test on the track.

[Scottie J]

I got a little bit done to the chassis.  I welded 2 gussets under the frame for the upper shock mount, 1 that will slam the bike and one that well give it a "natural" stance.  The pic of me sitting on the frame is the with the shock mounted in the higher "natural" position.

JimL - The gears I have picked out are a 21t front and 38t rear.









Scottie J

[JimL]

(Sorry for the previous rambling discord. For the mercy of our sanity I dumped that last reply).

Is there any way you could run the chain slower?  I really think you'd go faster with less actual chain speed.  Since I dont know anything about your Enfield trans and countershaft sizing available, you can only do what you have, I imagine.

There were many successes over the years, with big countershaft sprockets, wire spoke wheels, and all the stuff we "modern builders" think was wrong.  You wont know until you try it, is the only certainty, and the only thing that matters at Bonneville is that the bike gets there!  You will wind up rebuilding, over the years, just like the rest of us, and thats OK.

JimL

[Scottie J]

By "running the chain slower"  do you mean run a smaller front sprocket and larger rear?  And you didn't have to delete that post, I found the info interesting and worth mentioning. 

Scottie J

[rd400f]

Hi
I don't know if this helps but it is an interesting read.

http://www.tootechracing.com/Engine%20torque%20-%20Suspension.htm

Richard

[JimL]

Smaller front and rear sprockets give lower chain speed at your top ground speed.  For example:  CBR1000RR comes with a 16t countershaft, early Busa was 17t, latest Busa is 18t.  I read somewhere that 16-17t is a good compromise for least chordal motion and reduced chain speed.

Those big 21t countershaft sprockets were probably a solution for shorter trans gearing and a rear sprocket that could not be made small enough due to wheel/brake design.  Back when a bike spent most of its life at less than 50 mph (on wet roads in that rainy island country), chain speed didnt matter much.  It will matter for your goals.

JimL

[Interested Observer]

Ref. reply 247: 

http://www.tootechracing.com/Engine%20torque%20-%20Suspension.htm

While this may be “an interesting read”, it is incomplete, and consequently, incorrect.  The author deals with some of the forces involved and balances them, but completely ignores the moments resulting from those forces, which also have to be balanced in a static analysis.
For instance, he claims that if the chain run is arranged to be horizontal, no chassis lift or squat will be produced.  However, as long as the chain run is above the swingarm pivot point (the normal arrangement) chain tension will produce a torque on the swingarm about that pivot point tending to squat the chassis.  Further, the related tractive force at ground level will produce a torque about that point in opposition, tending to raise the chassis.

[Old Scrambler]

So I've run through the gears and just topped out at the moment I enter the speed-trap..........for the next mile I'm maintaining speed at a steady rpm.........the course is smooth and the wind is nil..........can I assume the chain-torque is steady but at a reduced level from when I was accelerating?   If so, I would think the rear axle should be below the plane of the swing-arm pivot to maintain the chain tension and distance (number of links under tension) for steady speed and reduced risk of loosing traction. This would also maintain front-end geometry for best trail. 

[Briz]

If the rear axle is below the plane of the swing-arm pivot, this inevitably means that you're running a mile of unnecessary ground clearance and the bike has the frontal area of a barn door! Either that or the bike'll have a weird nose-down stance.
All this swingarm angle stuff is of very minor significance. Getting the bike low and out of the wind is a lot more important.

[Scottie J]

I'm partially screwed as far as gearing/sprockets goes.  Turns out the ONLY front sprocket I can find for this tranny is a 20 tooth and the smallest rear sprocket I can find is a 38 tooth which will only net me about 135mph.  I called the local bike shop to see what they had available for rear sprockets and the smallest they could order was a 38, but the girl then mentioned that they use a place in California that makes custom sprockets for about $60.  So I'm going to either have a 34T rear or a 35T rear sprocket made.  Based upon my gearing chart and the fact that I'd like to hit 150mph on The Salt (tho it's going to be a close game of HP vs Gearing) what size rear would you guys recommend going with?




Scottie J

[SaltPeter]

Hi Scottie

I am finding out, that it depends on the revs your engine makes it's Torque over.

Once you know this then you will have a better idea for your starting point and making your final decision.

My 2 stroke is a relatively Low Power/narrow Powerband combo so it is very very gearing sensitive.

There are two of us running Suzuki 250 RGV engines in different Classes down here in OZ, I'm APS the other is MPS and we share info. I had issues, but the MPS Bike ran well and went 131mph. A one tooth smaller Rear Sprocket was fitted and got an unexpected drop in Speed. It did not make sense based on our info leading up to the event  We did not have any Higher RPM Gearing.

After looking at the results of 2013 we found something we did not even think of, the calibration on the Tacho was reading about 600rpm more than actual RPM .

After much head scratching and Forum Trawling we stumbled across some info about Tach accuracy mmmmm...As we have very limited data Logging capabilities on these Bikes, but run the same Aftermarket Ignition, we did a comparison between the Zeeltronic Ignition Monitor Reading against the Tach and found the same variation on both Engines. This explained what was happening, given the Narrow Power range we are playing with, why the changes had the wrong result  . The MPS Bike was indeed Maxed out at 131 given the Conditions and the Tune.

So this year I am running the same Engine Tune as 2013 and the other Bike has had some changes made to the engine. But now we know the Tach is wrong, I will be using my re set Zeel based Shift Light as the Peak RPM guide during the Runs and I'll be making any gearing changes accordingly. The MPS is an unknown at this point.

I am doing Tuneups in incremental stages year by year. I have done the same Calcs that you have and then I saved up and got the options so I could juggle front and rear combos within the theoretical Top Speed/RPM range once I knew what the next level Tune gave me. I am lucky that I have a a number of Sprockets available but I did have to get one Rear Sprocket made to fully cover things.

I am also looking at long term/$$?? at changing the Primary Drive Gearing as well if I ever get the Mythical 80hp RGV Engine Tune 

So you can get it close and then go from there, but until you Run and find those Weird Dumb things well it is all part...

Pete 

[Koncretekid]

Scottie,
I believe I gave you some misleading information in my post #232 about your swing arm position. Thanks to Interested Observer, I have done some sketches and calculations that convince me that any gain you get from having the swing arm pivot higher than the rear axle will not be significant.  And as Briz says, lowering your bike to give you better aerodynamics will be more important.