Looking for suspension tuning advice

[irk_miller]

This is the culprit:



1975 RD350, 1985 Honda CB700SC rear shocks at the lowest preload, 1982 Honda CB650 front forks with air caps, no air pressure, stock springs, 15W fork oil, 17" wheels front and back.

Here's the issue:

I have about a 1/2" of chicken strips to give you an idea where the bike really feels maxed out (less than what this early pic indicates).  I don't know what you can glean from one pic, but it should be clear I am a bit squat in the rear.  This was intentional, as I expected the rear end to be stiffer than it turned out to be.  I expected to also feel more rear end grab in turns and on take off.  I get it on take off, not so sure about the turns.  There's more shock angle, which may have softened the rear end a bit, but I also have much more preload to add.  I like the spring rate and damping on the rear.  The front, however, is clearly too stiff.  Is it too long?  Is my feeling of unease at all related to the power to weight of the bike?  Is this a condition of uneven suspension front to back?  I absolutely want to put this bike on the track, but I am going to get killed on entry in turns.  It's fine going throttle on exit, but it's a combo of throwing weight to the outside (maybe pushing?) and the rear end not feeling like there's enough bite. So I end up way to cautious entering, and raising hell to recover on the way out.

[sav0r]

You haven't really described why you think it is going to kill you on entry. It's too slow to turn in? Too fast? Rear end stability isn't good under braking?

[irk_miller]

You haven't really described why you think it is going to kill you on entry. It's too slow to turn in? Too fast? Rear end stability isn't good under braking?

Sorry, left stuff out.  Rear end feels unstable.  There is a sense it may be pushing, like it's slow to turn.  I also wonder if my handlebars are too wide and that's factor there.  Regardless, there's absolutely a feeling of instability in the rear.  Almost like a tire low on air.

[bitzz]

Your swingarm has too much angle. Should be 7-10 degrees from horizontal. If you lift the rear 1" you lose 1 degree of head angle. Doesn't the chain drag on the swingarm?
You say it's OK accelerating out of a corner: When you apply the power, the top run of the chain will be pulled to it's shortest distance between the sprockets, which will make the bike "squat"... and take all that angle out of the swingarm.
And once you lower the rear end, yes those forks are WAY TOO long.

Trying to set up modified suspension by "chicken strips" is not gonna get you anywhere, especially with those tires... what size tires and rims are those? Do the edges even contact the ground when you lean it over?
I would put the RD shocks back on, and lift the forks about 2-3" in the triples and see what that does.
Lots of good information here:   https://motochassis.com/articles/
Then once you get stuff back in a place where it will work again, you need to set up your transplanted suspension components... lots of good information here: http://didier.clergue.free.fr/gsxr/livres/Livre_Race_Tech%27s_Motorcycle_Suspension_Bible.pdf

That frame works REALLY well in stock form, with stock tire sizes. Yamaha has won a LOT of races with that frame (the RD frame geometry is the same as the TZ frame, except the TZ has 2 degrees less steering head angle... that's not your issue). I have been racing TZs and RDs for years... on 100 front and 120 rear, which works well for me. (Something I wish I learned a LONG time ago: You want to run the SMALLEST tires that will do the job. Tires EAT horse power and smaller tires eat less HP... and turn in with less effort... and stop better... and are cheaper... and don't wear as fast so they last longer). I decided a long time ago that Yamaha's race department is probably smarter than me... and now I kinda follow THEIR lead.

[irk_miller]

Your swingarm has too much angle. Should be 7-10 degrees from horizontal. If you lift the rear 1" you lose 1 degree of head angle. Doesn't the chain drag on the swingarm?
You say it's OK accelerating out of a corner: When you apply the power, the top run of the chain will be pulled to it's shortest distance between the sprockets, which will make the bike "squat"... and take all that angle out of the swingarm.
And once you lower the rear end, yes those forks are WAY TOO long.

Trying to set up modified suspension by "chicken strips" is not gonna get you anywhere, especially with those tires... what size tires and rims are those? Do the edges even contact the ground when you lean it over?
I would put the RD shocks back on, and lift the forks about 2-3" in the triples and see what that does.
Lots of good information here:   https://motochassis.com/articles/
Then once you get stuff back in a place where it will work again, you need to set up your transplanted suspension components... lots of good information here: http://didier.clergue.free.fr/gsxr/livres/Livre_Race_Tech%27s_Motorcycle_Suspension_Bible.pdf

That frame works REALLY well in stock form, with stock tire sizes. Yamaha has won a LOT of races with that frame (the RD frame geometry is the same as the TZ frame, except the TZ has 2 degrees less steering head angle... that's not your issue). I have been racing TZs and RDs for years... on 100 front and 120 rear, which works well for me. (Something I wish I learned a LONG time ago: You want to run the SMALLEST tires that will do the job. Tires EAT horse power and smaller tires eat less HP... and turn in with less effort... and stop better... and are cheaper... and don't wear as fast so they last longer). I decided a long time ago that Yamaha's race department is probably smarter than me... and now I kinda follow THEIR lead.

Appreciate the time you spent with the response. 

The chicken strips reference was specifically made to give you the idea that I am describing a FEELING and not the actual ability of the bike.  I should've been more straight forward.  I question my ability to effectively communicate often.  LOL.  The reason for the thread is that I'm trying to find confidence in the suspension and an understanding why I FEEL what I FEEL, so I can make informed decisions going forward.  The bike doesn't handle poorly.  It mostly doesn't feel the way I think it should feel. Some of it's traits are confusing me (like the soft rear tire feeling), and I've had less success tuning them out. I had an easier time converting an airhead to monolever than tuning this bike. 

To respond to some of the specifics:  These are 17" wheels and 2.5" width rims front and back tires match.  My chain does not hit the swingarm, especially with rider sag.  My rake is 25 degrees.  One reason for choosing the CB650 front end is because of the offset triple, so the 27 degree head angle of the stock RD is converted to 25 degrees of rake.  The swingarm is 2" longer than and RD, so there's a factor there, but as I should've pointed out, this bike also sees the drag strip.  Not for points or money, just fun weekend races.  I believe it can be made to be decent enough to hold it's own for both, though.

[m in sc]

forks are too long, and TBH, 37 year old shocks are never a good idea. id push the forks thru and inch or 2 and get some new shocks for it. like, new.   

[irk_miller]

forks are too long, and TBH, 37 year old shocks are never a good idea. id push the forks thru and inch or 2 and get some new shocks for it. like, new.   

Fair enough, LOL.   Another canned response.  It's pretty clear I'm going about this all wrong.  Genuinely appreciate the effort.  I'll figure it out. 

[dugsgms]

 Probably a dumb question but have you measured sag? Sounds like the bike is diving too much under braking and giving the rear a uneasy/loose feeling and makes me wonder if the front/rear spring rates are way off relative to each other.

While I do believe you have some underlying geometry issues due to fork/shock lengths I wonder if incorrectly matched spring rates are giving the bike a Dr. Jekyll/Mr. Hyde effect when the weight transfers.

[m in sc]

it's not a 'canned' response. shock and fork oil don't work well when aged. I just replaced the shocks on my 2014 cb1100 because they had lost performance.  my opinion, no point in trying to set sag, damping or rebound tuning with half the suspension using worn components.  .02. remember, my hybrid was set up very similarly component wise. my rear arm is 4" longer than stock and it turns in plenty quick.

[bitzz]

If you want shocks that work, like you're racing for points, you need a modern custom made shock.
A modern shock has 6 way adjustment (hi speed and low speed compression/rebound, preload and shock length) and you NEED to be in the middle of travel in neutral... ya' don't get that with a shock and spring  off some 550lb. OEM application or Amazon. You can start with some OEM shock and adapt to your needs... but that's a lot of work and I'm lazy and the local suspension guy entertains my dumb ideas and gives me a good price on stuff... but if you avoid Ohlins like the plague that it is, a decent shock isn't that costly in the grand scale of things... and a decent shock is rebuild able, 'cuz you'll want to change the fork oil and shock oil a couple of times a year
I'll tell you RDs and steel framed TZs don't need or want big stiff forks. They just pass a bunch of forces to the steering head ... that the frame wasn't designed for and doesn't need. The biggest fork Yamaha used was a 35mm, and a 110 tire.
If you're racin' and you're allowed fork upgrades, just drop in a "cartridge" kit. Costs about the same as a shock.
If you don't get unlimited fork updates: run an emulator. You get the same adjustments as a cartridge, but you have to take the fork apart to do the adjusting. Ed Sorbo, of Lindemann Engineering swears by emulators. Jim Lindemann was making "emulator" forks before there was a Race Tech.   

... OP: If you want to go drag racing, you want the rear axle almost level with the out put shaft (so the swingarm is almost horizontal) and the front axle as close to the lower triple as you can get it...for weight transfer... but you'd know that if you read the links I posted.

[sav0r]

You haven't really described why you think it is going to kill you on entry. It's too slow to turn in? Too fast? Rear end stability isn't good under braking?

Sorry, left stuff out.  Rear end feels unstable.  There is a sense it may be pushing, like it's slow to turn.  I also wonder if my handlebars are too wide and that's factor there.  Regardless, there's absolutely a feeling of instability in the rear.  Almost like a tire low on air.

The bike should be measured and setup off the stand and weighted. So there isn't much point in looking at the photo and trying to judge. I use an angle finder on the forks, an iPhone works fine, Android likely too but I'm not as familiar with their apps. Then set sags with preload. From there set ride height via preload to achieve rake. That's a basic setup process.

Years ago I found some decent RD setup notes. No idea where those went. But I run my forks a bit high in the clamps and the rear shocks long by like 15mm. I like the front of my bikes a bit stiff. But I haven't raced a motorcycle since I was in my teens.

[m in sc]

i mean, just sitting there its obvious its squatting in the back, that you def can tell. and the distance from the slider to the tree shows how much longer that a stock setup would be. I get countering the longer forks with rake angle adjustments and 17" rims.

My bike is actually raised a bit overall as well, but the geometry works. took a while to get right TBH. 
I would think struts might be better for the dragstrip. But im not bike drag guy. 

just spitballing.   

[sav0r]

I don't think suspension is ever obvious. It's not loaded in any way. Whatever is holding the bike up probably shifts the weight bias 20%.

[m in sc]

line of the bottom of the tank is angled up. its higher in the front than the rear.  look at the distance of the stanchions free length from the top of the slider to the lower tree.

add a rider the rear is going to squat even more. pretty obvious to me.

[sav0r]

I just have a hard time believing any amount of arm chair engineering is going to help via a single photo. Some measurement needs to happen. There has to be a setup and set down routine.