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Discussion Starter #1
This is for a CB750k5.

I am interested to learna little about bike geometry. This winter my plan is to focus on the suspension and handling on my bike. Next winter I will focus on the engine and making a street worthy race engine, meaning I want to make the engine high performance but I will really only be on the street. Unfortunatley there is no racing around me.

Before I make my bike as fast as I can, i figure it should be able to handle the speed.

Right now the forks and shocks are stock and the tires are nothing special. it is only single disk in front and drum in back.

My plan is to rebuild the front forks with new springs and duel disk it. In the back I plan on fixing the swing arm play, new bearings (if needed), maybe disk the back break (any thoughts if this is worth it) and new shocks.

My question is, what is the best geometry for best handling. Should the rear be raised, lowered or kept the same? How about the front.

When I see a bike like the one below, it looks liek the rear has been raised by moving the shock mount forward.

[imghttp://i234.photobucket.com/albums/ee180/chrisadelman/CB750PostClassicRacer.jpg[/img]

This is my bike



I just put on new front and rear sprockets and a new chain (neither are in this photo).

I am going to clean up the frame in the rear where it sticks out under the seat. While I am at it, it would not be hard to move the shock mount.

Tell me what you all know about bike geometry, please.
 

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Discussion Starter #2
(this is a repost becasue my pics failed)

This is for a CB750k5.

I am interested to learna little about bike geometry. This winter my plan is to focus on the suspension and handling on my bike. Next winter I will focus on the engine and making a street worthy race engine, meaning I want to make the engine high performance but I will really only be on the street. Unfortunatley there is no racing around me.

Before I make my bike as fast as I can, i figure it should be able to handle the speed.

Right now the forks and shocks are stock and the tires are nothing special. it is only single disk in front and drum in back.

My plan is to rebuild the front forks with new springs and duel disk it. In the back I plan on fixing the swing arm play, new bearings (if needed), maybe disk the back break (any thoughts if this is worth it) and new shocks.

My question is, what is the best geometry for best handling. Should the rear be raised, lowered or kept the same? How about the front.

When I see a bike like the one below, it looks liek the rear has been raised by moving the shock mount forward.



This is my bike



I just put on new front and rear sprockets and a new chain (neither are in this photo).

I am going to clean up the frame in the rear where it sticks out under the seat. While I am at it, it would not be hard to move the shock mount.

Tell me what you all know about bike geometry, please.
 

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Mut -
Smart move in working on handling before any power increases. The picture above looks to me like they have a different swing arm on the unit and it may be a little longer than the stock swingarm. It also looks like a square box design. Aside from the shock/fork combo for adding to handling there is also frame flex that's both good and bad. In other words some frame flex can be beneficial to handling. I've read articles where cb frames have been improved with welded supports on the frame. (I'm sure Geeto will have additional info on this subject).

One of my goals is to do something similar on my 71 build for this winter. On area that should also be considered is weight reduction. Aside from buying a custom frame (feather bed or rickman frame) reducing unsprong weight should be part of your consideration. As an example, instead of installing a dual disk up front you may consider using a better and lighter brake setup on the cb. The stock disk (even drilled out) is still pretty heavy where as say may be a goldwing or another sport bike front end may be the ticket to look at.

Laying the rear shocks down at an angle is an old racer trick to get more distance the rear wheel can travel before bottoming out the rear suspension. Again using a modern swing arm from a sport bike with a single shock setup may be another way to go. It would reduce overall weight, provide less flex in the swing arm and allow better tuning of the shock setup.

I don't want to ramble too much on this subject. I would like to add one more important point - your dealing with early 70's technology and there is only so much you can improve on the cb unless your willing to open up the wallet for some serious cash layout. I'm sure you can greatly improve over the stocker with little cash out. Keep us posted
 

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Discussion Starter #4
Bmartin,

Thanks for the input.

Heer is my thought train. I want a really nice handling bike. I like my CB and enjoy working on it. Cash is not unlimited but I have a couple choices. Leave the CB close to how it is and go drop 11K on a CBR1000RR or equivilant.

I don't want to drop 5k into my CB and not have it perform very well. I can only imagine that a properly built CB still will have more potential and performance than I can handle or knwo how to handle (rigth now).

At some point I will need to determine if I can make my bike handle and perform or if I sell it and go modern. I just really like the idea of having a 33 year old bike that kicks ass.

If I drop lots of cash in the CB, I will own the bike forever.
 

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I don't think moving the shock mount is necessary, changing the angle of the shocks makes them behave differently and will require serious R&D.

There is a great writeup of small cb750 handeling improvements on the SOHC4 board in the FAQ secion (under the Hondaman FAQ). The basics are: a ribbed front tire (like avon speedmaster), new timken steering head bearings, new bronze swingarm bushings, etc. That is a good place to start since they are small things and you are not spending big money.

In the 1970s the old trick was to use shocks off a DOHC cb750F or 900F. This was done primarily because the new for the time shocks were leaps and bounds better than the SOHC stockers, and fairly cheap. Now they are just old worn out used shocks. The DOHC shocks are an inch longer than the SOHC and they do improve the handeling, but remember a cb750 is a semi unstable bike to begin with.

I was reading that article in this month's Classic Bike about Peter William's monocoque Norton racer and he made an interesting point. He said something to the effect that racers like unstable bikes because they are easier to transition while street riders like stable bikes because 100mph tankslappers scare the crap out of most riders.

The basics of motorcycle geometry are that the less rake and trail you have the sharper the turn in but the less stable the bike is in a straight line. The more rake and trail you have the more stable the bike is. With raising the rear of the cb (or lowering the front), you decrease the rake and the trial which causes the bike to turn in better and be less stable. Ideally you want to minimize the rake and mazimize the trail and that will supposedly give you the best of both worlds, handeling and stability (this is why a lot of race bikes in the 60s-80s used leading axle forks). I'll let someone else delve into the specifics of rake and trail but this should get you rolling.

honestly, a stock cb with quality suspension components is going to be better than most vintage bikes, and as far as I know most of the vintage cb750 racebikes I see don't monkey with the frame too much, it really is a good start if you spend some time and money on it.

Honda won daytona in 1972 on a stock design frame. It was a cromoly frame and they could have changed the design but the CR frame is pretty much the same design as a mild steel stock streetbike frame, so much so that the cr pats work on both a cb and a cr.
 

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Discussion Starter #6
Geets,
Thanks for pointing me to the article, I will read it.

For back modern rear shocks, whats you first choice. I have looked at progressive's website but there are so many I have no clue.

Lets talk fron end for a minute. I can grab a 86 gsxr with a blown motor for $200. the front end is complete and works. What about puting that front end on my bike. I know it is not a bolt on, but I have some help that could make it happen. I have read that modern suspension is not sturdy enough for the 500lb CB's.

Or should I just work with what I have?
 

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I agree with Geeto for low cost improvements (timken steering head bearings, ribbed tires and swing arm brase) - I've done all these changes to my cb and they were low cost and worked great. I also have koni shocks on the rear and they work pretty well also. How ever if I'm really on it in a corner, I can still feel the rear swing arm flexing a little. I can cure that be adding my dresden swing arm on the bike (winter project). BTW continental tire also sells a rib tire - which is my tire of choice. I use a 3.50X19 size - little bigger than stock and fills the fender on the front.

As to the front end - I have friend that have adapted R1 front ends on his CB's - (salvage yard deals) for little money. That seems to work well but I have not driven a cb with that kind of front end. I would say the best improvements you can make is to reduce over all and unsprung weight. Lighter wheels, brakes and such will reduce unsprung weight. Reducing over all weight can come from new type of tank, seat, fenders and such. All these improvements can be done for little money. I agree tho - you cannot beat a new sport bike with a 40 year old technology but it's fun trying...
 

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"In the 1970s the old trick was to use shocks off a DOHC cb750F or 900F. This was done primarily because the new for the time shocks were leaps and bounds better than the SOHC stockers, and fairly cheap. Now they are just old worn out used shocks. The DOHC shocks are an inch longer than the SOHC and they do improve the handeling, but remember a cb750 is a semi unstable bike to begin with. "



I was just going to say that . CB900F rear shocks ( if you can find them ) bolt right on and improve handling a lot .
This guy I think used components from a CBR600 or 900 .
 

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quote:Originally posted by Geeto67

changing the angle of the shocks makes them behave differently and will require serious R&D.

although I agree in terms of absolutes and academics the first part is true.... I find the "serious R&D" bit nearly disturbing. Just basic math and for all practical purposes so much a non-issue that I'd say very few know the actual spring rates on any of their bikes, much less actual hydraulic bias of the dampers.

There is a great writeup of small cb750 handeling improvements on the SOHC4 board in the FAQ secion (under the Hondaman FAQ). The basics are: a ribbed front tire (like avon speedmaster), new timken steering head bearings, new bronze swingarm bushings, etc. That is a good place to start since they are small things and you are not spending big money.


a ribbed tire??? you have got to be kidding??

snip< a cb750 is a semi unstable bike to begin with.

I pretty much agree with this too and I'm of the opinion the fork tubes and swing-arm never were even close to appropriately sized for the bikes' weights.

I was reading that article in this month's Classic Bike about Peter William's monocoque Norton racer and he made an interesting point. He said something to the effect that racers like unstable bikes because they are easier to transition while street riders like stable bikes because 100mph tankslappers scare the crap out of most riders.

sounds like that guy has lost his mind or something


The basics of motorcycle geometry are that the less rake and trail you have the sharper the turn in but the less stable the bike is in a straight line. The more rake and trail you have the more stable the bike is. With raising the rear of the cb (or lowering the front), you decrease the rake and the trial which causes the bike to turn in better and be more stable. Ideally you want to minimize the rake and mazimize the trail and that will supposedly give you the best of both worlds, handeling and stability (this is why a lot of race bikes in the 60s-80s used leading axle forks). I'll let someone else delve into the specifics of rake and trail but this should get you rolling.


you should carefully proof read what is bolded above..... the two statements are in direct conflict... and have a 50% or better chance of misleading a newb
 

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I don't know the forks but those are cbr600 f2 brakes/wheels etc. The swingarm is a JMC unit (looks like). Those are progressive shocks, not cb900f shocks - the cb900f shocks have an external resivour on them.

I would stay away from the sportbike suspension stuff for a couple of reasons:

1) requires a lot of machine work. If you don't have access to a machine shop this can get expensive.

2) requires extensive frame bracing. Massive forks plus retna detaching brakes will bend your frame like spagetti. With my h1 going from 35mm forks to 38mm introduced all sorts for metal groaning flex into the chassis. Same with the swinging arm.

3) lots of math. You are usually going from a 19" or 18" rim to a 16", plus the rake on the sbk triple trees is going to be different than that of the stocker trees, well you get the idea...lots of paper planning.
 

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sorry hack I was thinking less...not more but I wrote more instead....I fixed it just for you. I don't usually proof read.

ribbed tire - without at the very least a center rain groove, the cb750 will be more prone to a tankslapper. For some reason the bike loves ribbed front tires. I had avon venoms on the front without a center groove and the bike would shake its head at anything. I run metzler lazertechs, they still shimmy a little bit on deceleration (not noticable as long as you have both hands on the bars) but they do not shake as bad as those avons. An Avon speedmaster seems to eliminate all traces of the shimmy but it is not an ideal cornering tire.
 

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i like your intentions here, but i think you'd be MUCH happier with say a mid 90's sport bike. they are dirt cheap. i have an RD400 that i restored and i have a set of 03 GSXR forks that i aquired. my machinist friend is trying to get me to put them on the RD and i am balking on it (yes, turning down free machine work) because i know that if i put modern stuff up front on it, it will upset the balance of the bike. Even if i got the frame braced, updated the rear with SOMETHING better, i'm still probably looking at a $600 swingarm and what do i have? I have a 32 year old motorcycle that still can't keep up with any of my friends modern sport bikes once the throttles get turned back on. If you have another bike to ride and just want to guiney pig...i say go for it, i'm sure it will look awesome, but I personally think that it will be a lot more trouble than its worth to you. like geet said, if you put great brakes and forks on that bike, something is going to give when you pour on the pucker...they just weren't engineered to handle those forces. in my opinion, put some good tires on it, keep the maintenance up and just enjoy the fact that you have a reliable and cool old school bike. if you want great performance though, better look towards your local dealership. Good luck!
 

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quote:Originally posted by Geeto67


I would stay away from the sportbike suspension stuff for a couple of reasons:


2) requires extensive frame bracing. Massive forks plus retna detaching brakes will bend your frame like spagetti. With my h1 going from 35mm forks to 38mm introduced all sorts for metal groaning flex into the chassis. Same with the swinging arm.
I'll have to disagree with the general quote that the better forks are worse for the frame. The Kawa frames were the worse of the era and needed bracing even in stock form and were only exaggerated with performance up grades. Frame bracing is usually desired when doing performance up grades anyhow.

Some modern USD forks just look out of place on an old bike, especially when the rest of the bike looks stock or close to stock. I like the GL 1000 front forks as it takes the fork tubes from 35mm to 37mm and strengthens it up conciderably, a fork brace also helps alot. It is also very easy to get the GL forks to work on the CB 750.

I like to put newer, lighter brake components on the GL 1000 forks. Not only do I get better braking, but the loss of extra unsprung weight helps handling alot. Replacing the steel rims with aluminum rims will help shed some unsprung weight also.


The 6 pistons on this front end will probably be replaced with 4 piston calipers. I will lose more than 1 lb of unsprung weight just by going from the 6 pistons to the 4 piston. I never weighed the stock components(discs and calipers), but this front lost alot of unsprung weight with the newer components. There is a definite, noticable difference in handling and braking on this CB 750.

On some Yam XS 650's I have done, the newer braking system(complete-Master Cyl,Disc,Caliper,line,etc)weighs less than the OEM stock disc alone.

Do a search on Tony Foale for motorcycle geometry stuff.

IMO, Working on the chassis FIRST,then build the engine, is the better way to build a bike.
DG



Click for more brake kits I have done
http://www.geocities.com/fischervintagecycle/Fischheads.html
 

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I am going to agree for the most part with Mr Fischer. Even if you double disc the front end with stock discs, they were not that good to start with. I would go ahead and get the GSXR front end and put it on. They were a conventional fork in 86 and either a 38 or 41 mm tube so they wouldn't look that out of place. The 86 also had 18" wheels front and rear. It shouldn't be too difficult to get the triple clamps onto your frame. Either mix and match bearings and spacers, swap steering stems or have the GSXR stem machined and shortened. It has been discussed here before and it isn't too hard. I have done several. On the rear, I would put the GSXR swingarm on the CB and have some lower shock mounts welded to the aluminum swingarm. You may need to work on the chain offset some to make it all line up and one expensive part may be an offset front sprocket. I know they are available because dragracers needed them back in the day. If you know a good machine shop they could probably make up an offset adapter so you could run stock sprockets. I would use Progressive Suspension shocks, Works Performance, or if you have the money, Ohlins. Then you can run the GSXR rear wheel and brake set up. As far as the handling goes, put a steering damper on it. They need them stock and they need them even more if you raise the rear and lower the front. Then you can run whatever tires you want. I would put a wider front tire on to make the handling a little heavier. I just like it that way. With the 86 Gsxr wheels and suspension I think you could run a 150 rear and a 110 front.
There is some racing to be had by you, too. AHRMA runs Miller and Sandia, WERA runs Miller and I think MRA has some vintage classes.


Should be a great project.

Ken
 

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thicker forks on some frames can act as a more rigid lever putting more stress on the frame than originally intended in hard cornering. I will agree that on the kawi it only mad a bad problem worse, and that cb750s don't really suffer from this as long as you keep the fork diameter reasonable (37mm is fine....42mm I might look at some bracing) but don't just assume you can slap modern sport bike forks on an old bike and not have to investigate the frame.
 

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Little things first. This knowledge is based on upgrading a CB400f to CB750 suspension to vintage race. I grafted on a complete CB750 front end to get the larger forks and bigger brakes. The brake (single) can be greatly improved with pads from Vintage Brake (Ferodos). The rotor can be skimmed in thickness to save weight and the triple clamps can be replaced with one from the Honda CBX, it is much stiffer than the stock triple clamp and has double bolts on the clamping... That made a huge difference. Then I upgraded the rear to a box section Dresda swingarm (which isn't all that expensive) and High quality shocks...

That's where I'd start for relatively few $$$... Then I'd reduce the weight of the wheels by going to alloy rims... But that's more $$

Peace
karl
 

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The 86 GSXR is really only the ext generation of suspension and wheels from the CB750. It is still conventional forks and 18" wheels. I don't think it would overwhelm the CB frame except the swingarm. For $200, that would be a very cheap way to go. Plus yoou get the alloy swingarm and good wheels. I don't want to disagree with Karl, but I have raced a CB400f, CB550f and a CB750K and while the single disk on the 400 could be made to work well enough, we had to double disc the 550 to get adequate braking and the 750 is too heavy for even a doubling of the single piston calipers if you have a better choice.

Just my opinion, but I would use the GSXR stuff and get a pretty cool bike out of it for a good price.

Ken
 

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I've read that (and similar about modern sticky tires on vintage bikes) typed by several but my experience has been a weak frame is a weak frame....... and simply putting a better (lighter, stiffer, better damped, etc) pair of forks (or shocks or swingarm) on one, or even modern tires with lots of grip....

won't make a weak frame any worse but will indeed improve things by eliminating at least some weaknesses

sure... to take full advantage of the superior components you may have to do some bracing/gusseting but even if you don't.... and ride within the limits of the weakest component

I've never found such mods to result in a poorer overall machine

flat out....... the 35mm 0.185"-ish walled fork tubes on the 750's are just barely suitable for a 250 pound enduro.... and the same era 37's are only marginally stronger. Some of the tiny enduro/dirt bikes, these days, have tubes that are bigger, much lighter, and much stronger




now I gotta say........ putting zipper bike stoppers only on most any pre-mid-80's tubes and lower legs.......

the smallish, very heavy walled, low grade steel, and weak fork tubes.... well for sure it will stop better but it has to be mathematically certain the seals, tubes, and bushings/lower legs will quickly have their weaknesses exposed

heck, just running a cb with 10 over fork tubes results in the stock dimensioned forks wearing out shockingly fast (and lots of exploded fork seals) with just the stock brake

and if you tentatively crunch and compare the numbers....... six piston calipers and 320mm discs can generate huge forces by comparison

but for sure they are lighter than many less powerful brakes

although those bolt-up adapters do sorta look a bit overweight

looks to me like you could easily shed another pound by simply welding some mount bungs onto those lower legs and getting rid of the industrial looking T6 adapters


quote:Originally posted by Geeto67

thicker forks on some frames can act as a more rigid lever putting more stress on the frame than originally intended in hard cornering. I will agree that on the kawi it only mad a bad problem worse, and that cb750s don't really suffer from this as long as you keep the fork diameter reasonable (37mm is fine....42mm I might look at some bracing) but don't just assume you can slap modern sport bike forks on an old bike and not have to investigate the frame.

quote:Originally posted by Geeto67

thicker forks on some frames can act as a more rigid lever putting more stress on the frame than originally intended in hard cornering. I will agree that on the kawi it only mad a bad problem worse, and that cb750s don't really suffer from this as long as you keep the fork diameter reasonable (37mm is fine....42mm I might look at some bracing) but don't just assume you can slap modern sport bike forks on an old bike and not have to investigate the frame.
 
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