Author Topic: Gearing for Bonneville  (Read 6704 times)

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Offline JustaRacer

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Re: Gearing for Bonneville
« Reply #15 on: December 16, 2012, 01:38:57 PM »
DA is air density?

Density Altitude.  It is the effective "thickness" of the air.  Stolen from aerospace.
My doctor told me to go out and kill people.
Well, sort of.  He told me to reduce the stress in my life.

Offline panic

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Re: Gearing for Bonneville
« Reply #16 on: December 16, 2012, 04:13:57 PM »
If your tune results in peaky power near 7K, consider using the close-ratio (24/22) high gear set to reduce the RPM drop on the 3-4.

Offline JimL

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Re: Gearing for Bonneville
« Reply #17 on: December 16, 2012, 09:26:49 PM »
That peaky power is a pesky problem.  My pushrod 650 (or 680) is prone to wheelspin in the first two gears because the power hits very hard as it gets "on the cam".  I started with 18/42 sprockets and am now using 18/38 which has helped the traction issues and added about 15mph from the same power.  If you can get closer ratios on the top two gears, it'll have to help.  I plan to go 17/34 next year with a slightly lower 5th gear.  This video might show the issue; youll hear the problem.  I have never been able to use full throttle in the first two gears, so learning how to get the most will be key to better speed downtrack...we just dont have a lot of acceleration at speed with these little pushrod engines.

http://www.youtube.com/watch?v=6ErXLJwSRcE&feature=youtube_gdata_player

Regards, JimL
« Last Edit: December 16, 2012, 09:31:42 PM by JimL »

Offline stay`tee

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Re: Gearing for Bonneville
« Reply #18 on: December 17, 2012, 01:03:22 AM »
if you havent done the laps at Bonneville its impossible to tune for it when ya some place else, :roll:,, my suggestion would be to get your bike on the dyno and get the best possible tune there, take RAD (Relative Air Density, (RAD guages are avliable from all good speed shops)  readings in conjunction with final dyno pulls,, keep notes,,, once at Bonnie, readjust your jetting to suit RAD on site, make small timing changes required for humidity, this will put you right in the "ball park", 8-), with consistant data for future events,  :-)

horsepower readings will give an indication of what the machine is capable of, set your gearing to take the best advantage of this,  :-),,,
« Last Edit: December 17, 2012, 03:02:16 AM by stay`tee »
First Australian to ride a motorcycle over 200mph at Bonneville,,,

Offline Truckedup

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Re: Gearing for Bonneville
« Reply #19 on: December 17, 2012, 05:51:13 AM »
 Ok,were got some 650 pushrod guy here...The rule book has the 2011 record runs...what is the 650cc MPG record? Still 123.9?
 My rear sprockets are 21-47 right now on a unit Triumph.With the current tire that's 125.9 at 7000 rpm.We have several rear wheels available so the sprocket sizes might be totally different in the end.And maybe rims with different diameter tires.Using the standard 4 speed.
 My engine should be a bit peaky but it has slightly less cam than some use and using a tight as possible quench in combustion chamber to get a faster cleaner burn to make power.
 My drag racing Triumph's was 40 years ago in a modified production class.Jetting changes were done if you ran did time trials in daytime and then serious racing at night when cooler.Sometimes I didn't mess with the carbs or timing and just launched harder and generally abused the equipment to go faster.I'm thinking that the harder launch won't work on the salt  :-D
 I believe that 750 Triumph twin flat track bikes tuned by Axtell made 71 reliable HP at 7000 rpm back in the early 1970's.A 650 about 63 HP.At low elevations of course.
Triumph 650 LTA MPG record holder  133.1 MPH...

Offline JimL

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Re: Gearing for Bonneville
« Reply #20 on: December 17, 2012, 08:44:42 AM »
Those HP numbers sound about right for that era.  My T120 flattrack engine was set up for 1/2 mile, running JOMO 15B cams, Chevy valves in shortened Sportster valve guides (larger brass seats), 13:1 pistons, GP carbs, 26" exhaust, and a short 4th gear.  Just for comparison, that was high 13s at 5000 foot altitude on a warm day.  That was identical to my built CB450 on the drag strip, which was the same bike that ran a best unstreamlined pass of 118.733 at Bonneville 1969 (but only had 116.654 average).  With the fairing, best mile was 124.65, but not until I was coached by the veterans to be a lot more aggressive off the line!

Here we are 40 years later and these bikes are still about the same speeds, EXCEPT with AirTechs much better streamlining giving much more speed and being able to pull 10,000 rpm with a super short stroke.    My current bike is difficult to launch because there is no way to run a flywheel with the chain drive conversion; which is a deja vu flashback to that CB450 (which couldnt run a flywheel, either!) and was very peaky.  With a flywheel, that CB450 only ran 107 unstreamlined at Bonneville, in 2007, but I was off on jetting also.

You are correct; the M-PG record is 123.9, but I'm guessing Roosevelt was riding the 8-valve Weslake-Triumph.  The current MPS-PG-650 record is about 133 1/2, and that bike had the AirTech Charlies Toy fairing on a nearly stock CX500 chassis with stock shaft drive and rear gear ratio.  Streamlining is a lot of fun, and cheaper than horsepower!  About the video...not my posting on utube.  The bike only gets past 160 at the end of the 3rd mile.  It has never run better than 159 mile, with either engine. :?

Sorry for the rambling post; trying to connect the dots between the years, and see what we've learned.
Hope this is useful to your planning and building, which is some of the most fun. :-)

Regards, JimL
« Last Edit: December 17, 2012, 08:58:05 AM by JimL »

Offline Truckedup

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Re: Gearing for Bonneville
« Reply #21 on: December 17, 2012, 09:47:12 AM »
 Yes,the 650 MPG record holder is an 8 valve.That doesn't mean a 4 valve can't go as fast,it means if he was off a bit and the 4 valve has the perfect day with perfect rider......We are looking at partial streamlining classes also. Elevation does make a difference,at closer to sea level a Triumph like yours will run high 12's.And on a cool night with an all out hole shot, closer to 12.5
Triumph 650 LTA MPG record holder  133.1 MPH...

Offline wobblywalrus

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Re: Gearing for Bonneville
« Reply #22 on: December 17, 2012, 11:50:27 PM »
This way of doing the gearing helps me when I am running a new or unknown setup.  The gearing is set tall for the first runs so the engine will not reach peak power rpm.  Then I check everything, especially the mixture, and the chassis setup.  I drop the gearing after everything is OK to a ratio where the engine is running like 55chevr sets his up to do.

This is something I learned from racing those old British bikes.  Mine were not especially reliable when wound out hard, and by gearing the bike tall, I gave myself a chance to gather some good data.

You will see me do this with the new big motor this year.  My first runs will be with gearing set for 160 mph which I know it will not pull.  Then I will look at the plugs and exhaust header color, listen to the running engine with the motor stethoscpope, and a few other things.  Then a bigger sprocket will go on for the high speed runs if everything is OK.   

   

Online SPARKY

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Re: Gearing for Bonneville
« Reply #23 on: December 18, 2012, 10:04:29 AM »
Guys,  Thanks for these insights---for those of us who try to DATA MINE this site has no peer!!!! 


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