How light can an XS be?

[Capt_Zoom]

Capt_Zoom's comment about power/weight ratio reminded me of an old formula published 30 years ago.

For you 1/4 mile dragheads, these were supposed to give you an idea of expected trap speed/time:

ET = 5.825 * cube_root_of( HP / Weight )

MPH = 234 * cube_root_of( Weight / HP )

On a lightweight bike, going on a diet is cheaper than modding the engine...

I vaguely recall seeing these before...had forgotten about them. But yeah. Its how much people will plunk down for a few extra HP when losing 20lbs would make the bike faster. Even on big bikes. On my 700lb 113 ci stroker raider I have friend putting in 120ci kits just to get an extra 5 HP. Simply losing a few lbs would have done the same for their quarter mile.

 

[Capt_Zoom]

http://www.stealth316.com/2-calc-hp-et-mph.htm

I'm using the Patrick Hale's Formulas which gave the best numbers for the raider: Amazing the difference weight makes on a bike.

My 700lb raider with my 200lbs
ET = 12.116 sec 1/4 mile
MPH = 112.50

My wifes 365lb xs650 bobber with her weight
ET = 12.034 sec 1/4 mile

MPH = 113.26

Time to start sup'ing up my raider. Can't have my wife's bike faster than mine.

 

[Boons]

http://www.stealth316.com/2-calc-hp-et-mph.htm

I'm using the Patrick Hale's Formulas which gave the best numbers for the raider: Amazing the difference weight makes on a bike.

My 700lb raider with my 200lbs
ET = 12.116 sec 1/4 mile
MPH = 112.50

My wifes 365lb xs650 bobber with her weight
ET = 12.034 sec 1/4 mile

MPH = 113.26

Time to start sup'ing up my raider. Can't have my wife's bike faster than mine.

What all has been done to xs650s motor?

 

[Capt_Zoom]

Hers has open Intake, pipes, jets, canadian needles, pamco, iridium spark plugs. Probably going to rebuild the motor in a month or so. Lighter cam chain, and if I need pistons I'll go with better/lighter pistons as well.

Interesting. Shedding 85lbs and having a light weight rider changes a lot.

According to wiki:
Cycle World tested the XS650 in March 1979. It ran the standing-start quarter mile 13.86 seconds with a terminal speed of 96.05 mph

 

[TwoManyXS1Bs]

Hey Zoom, thanx for the stealth316 link, fascinating, I like the LRT 'best fit', for us street folks...

 

[arcticXS]

TwoManyXS1B, There seems to be an error in your formulae, as more HP will give a higher ET, and less terminal speed.....Maybe HP and weight should be swapped around in both cases?

Entering the 80 ish HP and 660 lbs weight (incl for my Ducati ST2 in the corrcted ET formula, with Weight/HP, it actually gives close to the 12 seconds ET that magazines have published

 

[TwoManyXS1Bs]

There seems to be an error in your formulae, as more HP will give a higher ET, and less terminal speed.....Maybe HP and weight should be swapped around in both cases?

Whoops! Thanx, articXS, edited/fixed that post.


I had a brainfart rate guage installed a while back, the needle keeps edging upward.

 

[weaselbeak]

My Virago with me on it is over 700 lbs. My 115 lb grandson on his SR250 single doesn't make385 total. I can barely get away from him. Lightweight rules!

 

[TwoManyXS1Bs]

A somewhat uninformed reporter was interviewing Burt Rutan about his ingenious aircraft designs, and the discussion turned to weight issues. She asked him how he determined if something was too heavy to use in an airplane. He replied "I throw it into the air. If it comes back down, it's too heavy"...

 

[650performance]

As a point of reference for those seeking to reduce their machine's weight, a typical XS650/750 flat track bike with an aftermarket (Champion, Shell, C&J, etc.) frame will come in right around 290 - 300. That's with rear disc only. A drum is a bit heavier. My AHRMA race bike with a single disc at each end is 305 lbs. wet but no gas.

 

[gggGary]

650performance; Nope that won't work, we HAVE to see pictures!

There are pics somewhere on the web of an Aussie who went nuts lightening the engine drilled holes in the holes...

here's one example of that theme

View attachment 2287

 

[osteoderm]

Richard Pollock of Mule Motorcycles has some good tips on his website, just look around there. As far as I remember, he stated that the FZR600 (or was it FZ600) fork was the lightest he had come across as a reasonably cheap option. I beleive this had 38 mm tubes, so will require different or modified triple trees also. He may also have mentioned using bored out TT 500 triples with this fork on an XS

I read the Mule's "recipe" for this and ran with it! '76 TT500 triples, '92 FZR600 fork legs. Some research suggests that similar-vintage FZR400 forks would also work. Putting the front end together, I'd say that the single biggest weight drop was with the lower triple; the TT500 is aluminium and gracefully sculpted, while the stock XS lower triple is an iron lump. Overall, the total weight difference wasn't as great as I thought it might be, but the thoroughly modern performance improvement certainly was!

While my material cost on the swap, brakes included, was under $400, the machining to make it all come together is non-trivial, and would likely double (or more) the all-up price unless you had the ability to work yourself.

 

[Capt_Zoom]

So about how many lbs would an aluminum frame and maybe swingarm save a person. I'm considering this for my next build.

 

[TwoManyXS1Bs]

So about how many lbs would an aluminum frame and maybe swingarm save a person. I'm considering this for my next build.

An interesting challenge. Assuming steel at 490 lbs/CuFt and aluminum at 169 lbs/CuFt, you could do a 'what if' scenario. Ex: suppost that to maintain structural integrity, you need to use twice the thickness of aluminum to replace steel, or 338 lbs of aluminum to replace 490 lbs of steel, approximately a ratio of 0.7. Then a 50 lb frame could become 35 lbs. Of course, proper engineering and reduction of 'over-engineered' heavy steel areas could yield more savings. But, this is just one way to approach this challenge.

Your mileage may vary...

 

[gggGary]

Chromoly is a more practical "home shop fabrication" frame material. You would need some advanced skills to do it "safely".

 

[TwoManyXS1Bs]

And, if you have a 'thick' wallet (can't sit down without falling to the side), you could go 'exotic', titanium and carbon fiber...

 

[arcticXS]

Steel frames can be very light. A Norton Commando frame must weigh way less than half of an XS frame, and those were made in the 60's......Ducati frames are also very light, actually lighter than some alloy beam frames. And steel quality does not matter that much, since the Young's modulus (elasticity or stiffness) of all common steels are more or less the same. Cromoly and other high strength steels are just that, stronger, but not significantly stiffer in elestic deformation.

 

[TwoManyXS1Bs]

Weren't there some XS builds where the frame is reduced and the engine used as a stressed member?

 

[arcticXS]

Weren't there some XS builds where the frame is reduced and the engine used as a stressed member?

Mule Motorcycles, several of his builds use the engine as stressed member, no cradle

However, I do believe the stock XS frame, even with no cradle, will be really heavy. I would really like a frame like the old RD 350s, then there would be lots more space around the carbs, the frame would look better, and most likely would be stiffer and lighter.

 

[Capt_Zoom]

Even more than the weight reduction i'd like to build another bratstyle bobber but replace the original swingarm mount rails. I'd like to figure out a way to do it that looks cleaner and less bukly. Maybe more along the lines of the SR500.