Old motorcycle weight reduction and redesign using modern knowledge

[mojomoto]

I was working on my cb750 brakes last night. deep in thought and i wondered. why so thick? now my first guess is to keep them from warping witch is probably true. but in my case i added a disk and caliper. My motorcycle would experience possibly less heat build up than a single doing all the work. If i drilled the rotors too. Is that enough to keep the rotors from warping. Is the metal different than modern rotors? Is there anybody who has experimented with thinner rotors?
Ive been working on this week also....
aluminum caliper pistons (why so heavy there too?) will this work? They might need to be steel. expansion rates from heat.
hard lines from the master to the brake line block (ive done this before. works great.)
machining calipers to take regular brake line ends and not the goofy honda rubber to steel line ends.(did that last night)
Got rid of the fork springs (experimenting using compressed air) hope it works since it got rid of alot of weight.
Has anybody else come up with an idea to lighten the load so to speak?

 

[tobiism]

Old bikes with single disc brakes used heavier thicker discs. The bikes that had duals usually had discs about 2/3 to 1/2 the width of the singles. The KZ650 is a good example.
I wouldn't mess around with aluminum pistons in the brake calipers. There are much better places to save weight throughout the bike.
I'd be putting those springs back in the forks lickety-split. Don't even try to ride that thing without those springs! Those fork seals will never stand up to the amount of air pressure you'd need to try and replace the springs.
Seriously, thats just being dangerous.

 

[junkyard jon]

later dohc honda's had air forks. you can run the complete frontend, just swap the stem

 

[mojomoto]

later dohc honda's had air forks. you can run the complete frontend, just swap the stem

Junkyard Jon...Do you know the difference in seals from an old sprung slider to an air filled fork? Id rather keep the originals to keep it looking era correct. Id like to see how i can lighten and improve the old stock stuff. Besides i just got a complete billet tripple clamp from cyclexchange.net on my motorcyle and it looks sooooo sweet. Check it out. Id hate to have to put a lumpy stock cast set back on.

 

[mojomoto]

Old bikes with single disc brakes used heavier thicker discs. The bikes that had duals usually had discs about 2/3 to 1/2 the width of the singles. The KZ650 is a good example.
I wouldn't mess around with aluminum pistons in the brake calipers. There are much better places to save weight throughout the bike.
I'd be putting those springs back in the forks lickety-split. Don't even try to ride that thing without those springs! Those fork seals will never stand up to the amount of air pressure you'd need to try and replace the springs.
Seriously, thats just being dangerous.

Thanks for the info on the kz650 my brother has a few in his collection. Ill have to take a look at the rotors. It would be a major weight reduction.
I know the air instead of springs idea sounds stupid but....I did a little testing with my bicycle pump and pumped in the fork about 130+psi. The tube would barely budge with all my weight. The seals didnt leak or blow. Now i have about 65psi and mounted on the motorcycle. The front is now a bit on the stiff side. I could drop the pressure to 50 psi < to be stock softness. Even if i hit a pot hole and violently compressed the forks they would never exceed 100 psi. Thanks for your concern for my safety though. I may turn out be an example of natural selection. It wouldnt be a bad idea putting a couple of aluminum tubes in them to limit how far they would go if there was a dual blowout.
What ive learned about mass produced products is that they are ALL made to be nearly idiot proof. Overbuilt. No motorcycle company would sell a motorcycle without springs because anything that needs anything but minimal attention to be safe would never be put into production. Air shocks will need constatnt attention to be "safe".
There are some motorcycles with air assisted shocks but they have springs inside. To stiffen an air shock the air pressure has to affect the shock more than the spring inside it. Right? Ive never heard of these blowing. Leaking? Yes. I can feel already that you still think im crazy.
About the aluminum pistons...I will skip them for machining material out the steel ones. They are waaaaay too thick.

 

[tobiism]

Those older air forks were just air assisted, they still had springs, and their was no difference in the fork seals. Keep in mind though, that they only used a max of 7-8 p.s.i. if I remember correctly.
There is no way those seals will stand up to daily operation at road speed. Don't forget that you can't simulate with your arms the same force those forks will see hitting a pothole with all of the weight of the bike behind it.
Just a leak in those seals will bleed out all of the pressure, and thats all you need for disaster.
This is a bad idea.

Thanks for the info on the kz650 my brother has a few in his collection. Ill have to take a look at the rotors. It would be a major weight reduction.
I know the air instead of springs idea sounds stupid but....I did a little testing with my bicycle pump and pumped in the fork about 130+psi. The tube would barely budge with all my weight. The seals didnt leak or blow. Now i have about 65psi and mounted on the motorcycle. The front is now a bit on the stiff side. I could drop the pressure to 50 psi < to be stock softness. Even if i hit a pot hole and violently compressed the forks they would never exceed 100 psi. Thanks for your concern for my safety though. I may turn out be an example of natural selection. It wouldnt be a bad idea putting a couple of aluminum tubes in them to limit how far they would go if there was a dual blowout.
What ive learned about mass produced products is that they are ALL made to be nearly idiot proof. Overbuilt. No motorcycle company would sell a motorcycle without springs because anything that needs anything but minimal attention to be safe would never be put into production. Air shocks will need constatnt attention to be "safe".
There are some motorcycles with air assisted shocks but they have springs inside. To stiffen an air shock the air pressure has to affect the shock more than the spring inside it. Right? Ive never heard of these blowing. Leaking? Yes. I can feel already that you still think im crazy.
About the aluminum pistons...I will skip them for machining material out the steel ones. They are waaaaay too thick.

 

[mojomoto]

Sounds about right, 5~15 psi for some.
Air only needs 160+ and usually linked forks because volume is so low. It is a true progressive 'spring' though
I remember almost crashing a 76 Goldwing because I was watching the pressure gauge varying from 150 to 450 psi ( don't remember the conversion kit, S&W I think?)
Needed special seals becasue friction is so high with air only springing, 'stiction' is a major problem

PJ

Well now i know it has been and can be done. Right now I have my motorcycle on a front end stand and letting it sit for a week or so. Hate to find it on the ground some moring. Before i took out the springs i cheked out the compression on the stock springs. They were very cruzier mushy. Now, at 60psi, there is a definate different feel more like a progressive spring. like you said. My GS1150ES had (linked air assisted). Do you know of any air assisted 35mm fork with linking hardware from anything? Thanks

 

[Fishhead]

I was working on my cb750 brakes last night. deep in thought and i wondered. why so thick?

Brake technology has come along way since the 70's. What was state of the art back then is now obsolete (on new motorcycles).

hard lines from the master to the brake line block (ive done this before. works great.)

A hard line from the master to the splitter may be prone to crack and leak from vibration. Braided strainless steel brake lines would be a better choice (IMO) and also allow adjustability in the handlebars and controls.

Got rid of the fork springs (experimenting using compressed air) hope it works since it got rid of alot of weight.

Bad idea. Air forks caps were usually made to compliment the stock spring forks and to give it more adjustability. Deleteing the springs in favor of air only may work for a while, but a small leak would leave you sitting with no suspension travel at all.

Has anybody else come up with an idea to lighten the load so to speak?

I like the GL 1000 front ends and I put newer brake components on them. Using the newer technology of the discs and calipers lightens up the front end alot and gives fantastic braking at the same time.

a slide show of some of the brake kits I have made
http://s111.photobucket.com/albums/n130/dgfischer/Fishhead%20Big%20BRakes/?albumview=slideshow

A slide show of a 1981 CBX 330mm discs and AP racing 6 piston calipers being made up
http://s111.photobucket.com/albums/n130/dgfischer/Randaaks%20330mm/?albumview=slideshow

My website
http://www.fishheadbigbrakes.com/

Fishhead

 

[DrJ]

Wow, that's fantastic. I was wondering how easy it would be to install a modern caliper to an old front end.

 

[Swagger]

I changed at least 100 sets of fork sliders, calipers and rotors, even manufactureres can take weight loss too far.
Late model 41mm forks are about half the weight of stockers

PJ

Right on the head......
I picked up a fork/triple and brace set from a BMW K-bike for use on my 450/500 hybrid. They are 41mm Showas and weigh a lot less than the skinny flexy stock 35mm pogos. Not only that but the action itself is just plain smoother, more rigid by and order of magnitude and let's be honest here......they look badass!
You can pick them up pretty cheap, these same forks were used by Yamaha and Suzuki according to the cat a Racetech I talked to. It'll be a fairly simple matter to swap the steering stem and off you go. You'll difinitely want to upgrade the rear end to suit as well.....

 

[mojomoto]

Thanks for all the good ideas and concern.
Ive got a xs650 street tracker with fzr wheels and brakes already. A mixed modern/old.
Want to see how far i can take stock. My plan is to keep it looking like it was made in its era.
For braking i might go to thining (2/3 stock width) the rotors and drilling them or going back to a drilled single with racing brake pads.
The fork tubes... Ill keep the springs and probably put them back in. Just want to give the air thing a try.
The mail man just came! My Tommaselli clipons just arived! Back to the shop.
Keep thinking about what i can do to lighten the load.
thanks again

 

[mojomoto]

Progress on the cb750...

lightenend the front brake pistons. (Complete waste of time.... they are about half their weight now.) The stock ones are ungodly thick. Why? I dont know. Ive had pistons out of modern calipers and they are quite thin. So, im giving it a try. Ive got plenty of spare parts and if it doesnt work nobody else will have to wase their time trying it.
vent drilled the rear brake housing.
Aluminum axle spacers.
Aluminum rear brake adjuster.
Tarozzi rear sets. Machined to fit.
sprockets and o ring chain.
brake pads.
Still havent put the springs in the forks.....Holding pressure....(I know that doesnt mean anyting.)
When it warms up i will drill the rotors.
I might surface the rotors on the outside 1/3 thinner then machine the calipers to suit. Thats a maybe.
I took out the starter and blocked it off.
Took out the battery and box.
some good custom brake lines to come.

 

[boingk]

Sounds like a good project there mojomoto, I got the weight bug a while back with my Aprilia RS125 and pulled a few kilos off it just doing simple things like removing passenger pegs, replacing the battery with a smaller unit and fitting a different silencer.

Agreed with everyone else that the air forks are generally a bad idea unless the bike has been manufactured that way. I mean, losing weight is one thing but when it involves stripping a major design component then you really gotta think again. I do admire the creativity behind your build, though.

One thing that will definitely let you run lighter is replacing the battery with a capacitor. A 10,000 micro farad capacitor should get you out of trouble, ask at your local electronics store. This way you won't run into too much trouble with excess loading such as lights and indicators, and you're still saving a few kilos at least.

If you have cast wheels, spoked ones are much lighter. You may also want to remove your centre stand, as this is generally quite a well engineered (read "heavy") bit of equipment and is generally supurfluous to everyday riding. I'd still keep it in the garage for maintenance, however.

Cheers - boingk

 

[Wee Todd]

chuck in's somes tataniuns stuffs,,liters!!

 

[mojomoto]

Agreed with everyone else that the air forks are generally a bad idea unless the bike has been manufactured that way. I mean, losing weight is one thing but when it involves stripping a major design component then you really gotta think again. I do admire the creativity behind your build, though.

One thing that will definitely let you run lighter is replacing the battery with a capacitor. A 10,000 micro farad capacitor should get you out of trouble, ask at your local electronics store. This way you won't run into too much trouble with excess loading such as lights and indicators, and you're still saving a few kilos at least.

If you have cast wheels, spoked ones are much lighter. You may also want to remove your centre stand, as this is generally quite a well engineered (read "heavy") bit of equipment and is generally supurfluous to everyday riding. I'd still keep it in the garage for maintenance, however.

Cheers - boingk

Thanks for the incouragement.
Ive been getting alot negetive feelings about the air forks idea. I do appreciate the concern tho. I think the biggest thing to worry about is stiction if anything. Wearing out abnormaly quickly and leaking is VERY possible. The bottom seal is big and in a very thick groove. It is basicly a hydraulic seal just like in a hydraulic cylinder. The chance that seal will "pop" out are 0. The top seal is pretty much a dirt seal and is trapped in with a big c clip. Chance of that one popping out....if the bottom seal heats up from pressure/friction and starts to melt and start leaking... It is possible. The chance of catastrophic failure is low. If there is a leak i will notice the fork feeling mushy and getting worse. If it does leak to the point of annoyance ill put the springs back in.
I bought a capacitor from j&p. I dont know the rating. Ive heard that the bigger is better in this instance. What do you think.
I have no center stand or mounts anymore. No rear pegs or frame peg mounts. Ill be making a new kickstand using light steel tubing. Ive already got the spoked wheels. The last mod id like to make is putting on some aluminum rims and stainless spokes. Id really like to find a company that makes Titanium spokes. Ive found Ti bicycle spokes but nothing thick enough for a motorcycle. Ill put up a post and ask everyone if they have seen such an item. Its been cold in the shop and ive been sidetracked too. Got to making a machine bb gun. Its a total waste of time but alot of fun. Thanks again. If you think of anything else that isnt need that i can get rid of......

 

[boingk]

I bought a capacitor from j&p. I dont know the rating. Ive heard that the bigger is better in this instance. What do you think.

Definitely, although you can get to the point where buying a small battery (2Ah to 4Ah) is both cheaper and more effective. The only advice I've heard is that a 10,000 micro farad (written as 10uF, most likely on the capacitor body) capacitor will let the bike run off its charging system once its going and still have a slight buffer for higher current drain.

If you think of anything else that isnt need that i can get rid of......

Supurfluous engine covers and any bodywork that doesn't serve a purpose. This includes badges, sidecovers and tailpiece if any. The stock seat can be replaced with a fibreglass item and minimal padding, think half-inch stick-on neoprene. The stock exhaust system can be replaced with a lighter 4-into-1 system with an aluminium silencer. Airbox can be replaced with pods, the cylindrical foam-element type would probably be the lightest. I would keep stock instrumentation and lighting to avoid hassles with the law and/or other road users seeing you. If you wanted to cut down even more you could use a smaller fuel tank, but I personally would do it.

Other than that, I'd throw a set of mild cams and high comp pistons in the old girl. Perhaps a big-bore kit. Might not cut weight down a heap but will certainly improve power once you jet for them.

Cheers - boingk

 

[Rocan]

interestingly enough, boring the cylinders cuts the overall weight of the engine by a pound or so, depending on the type of engine and how much material is removed. the increased size of the piston does not make up for it either- cylinder lining is heavy, piston materials are not.

 

[boingk]

the increased size of the piston does not make up for it either

No, it wouldn't, especially if you get some performance items that'll most likely be lighter than stock.

Oh yeah...gear levers and be replaced with aluminum items, and the same for the handlebars if you haven't put on clipons or something already.. If your brake and clutch levers are steel then replace them with aluminum as well.

You could run a total loss electrical system and have the flywheel lightened, but thats getting into serious race territory and will really impact on your on-road usability. A moderate flywheel lightening would be good though, with more eager revving and better reponse to throttle inputs.

Remember though, that theres a limited return on doing all this to what is essentially a lumbering dinosaur of a bike. I think its great you're doing it (hell, I'm doing a ~600lb stock GS850!) but you really need to think about what you're doing and if you may need to get another project. I've got the 850 and a Kwaka 100, for example. Whizz around town on the 100 and save the 850 for longer trips.

Cheers - boingk

 

[sharperdill]

Gun-drill all the bolts. Cut excess length off the bolt, any threads hanging beyond the nut are not doing their job anyway. De-tab the frame. (You may have already done this?) Some of the remaining tabs can be thinned or have a radius ground on them Drill a fudge load of speed holes in the sub frame. (its only holding you and your tail piece up) Maybe go with an aluminium tank. Swap to Ti valves and Ti slides in your carbs.

Why did you go with an o-ring chain if you are trying to reduce weight?

 

[boingk]

I would NOT recommend gun-drilling all the bolts. They're the size they are for a reason - to hold the bike together! Shearing, torsional and tensional forces will be much more easily tolerated by a solid section bolt than a tubular one.

Likewise, I'd also not recommend drilling the subsection. Make another one or shorten it for sure, but don't drill holes in it. It may only have to support your weight but think of what happens when you go over a big bump - you start moving. More force is placed on the subframe, especially when it returns to the normal position and you still have downward momentum.

Just a few thoughts - boingk