Looking for suspension tuning advice

[1976RD400C]

My opinion is the whole bike has to come down, especially the front. Set the preload to have plenty of sag. Then have someone hold it up on one side and you jounce down on the other side foot peg and watch what happens. Do both wheels travel the same amount and rebound at the same rate? Watch how the forks on Moto GP bikes dive under full braking, the springs can't be too stiff. A stock bike configuration handles really well where it doesn't spook the rider.

[m in sc]

agreed^

The most important tool anyone has is their eyes to start with. Its pretty blatant to me. also, 40 year old used parts are going to be worn out unless its nos.

I gave no numbers, measurements, or data. just advice as a solid starting point before trying to get it setup. these issues need to be addressed before starting a legit setup process.   

a call to a good shock company to get a set of shocks set up for this is a good start, like when i had works set up the ones for my hybrid with the fzr arm and fz front end.  not an off the shelf part. 

http://worxshocks.com/

[sav0r (CL MotoTech)]

A Moro gp frame can handle pretty vertical forks for a reason. There is a lot of setup difference there.

I agree by looks the forks are long. In fact I run a 2" or stick out on my RD. I just don't see the point in guessing around.

[Hawaii-Mike]

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.

I agree, it squats which would slow the steering.  That would result in poor handling.

[irk_miller]

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.

This is honestly what makes me frustrated by this thread right now.  LOL

Not to make excuses, but better shocks are coming.  I'm doing this on a teacher's salary and the bike literally started as a box of parts.  I focused the bulk of my budget on trying to build a  60+ hp engine and electronics.  The shocks are good enough to at least get some good riding in to sort the rest out.  They have less than 5k miles on them.  They do not leak and they passed compression tests. As anyone knows, good shocks don't happen for less than $500. That takes time for me.  There is no reason I can't make this setup work decently or at least get a better sense of where my fail points are.

CB650 forks are the same length as RD350 forks, but with thicker tubes.  The difference is, a CB650 has an offset triple, so my fork angle changed from the stock RD 27 degrees head angle to the current 25 degree fork angle.  Not sure that's enough difference to cause so much consternation over the length, but it has to contribute some.   I dropped the forks through the triples one inch and that does improve things.  I have zero doubt I need softer springs in the forks.  I have always admitted that.  The bike weighs 279 lbs, wet with a full tank of gas.  I think that might be a little lighter than a CB650.

I also think having handlebars with a smaller angle, and wider, makes a difference here.  I used to race mountain bikes, and I always cut a couple of inches off the ends of my bars to quicken my steering and make me fit better between trees.

The body work is not stock.  Using body work angles from a picture may not be totally accurate. 

The bike is not as squat as everyone is making it out to be.  In photography, there's something called parallax that manipulates visual angles.  I had hoped my descriptions would be enough to help people lend clues, so I can get feedback on what causes certain conditions.  In hindsight, I completely regret submitting a picture at all.  That opened me up to the canned responses and typical forum stuff.

The Radian swingarm is 2" longer.  These shocks are 14.5", so I hoped it would balance out.  I am maxed on rear end height, though, based on my swingarm angle.  The chain does not hit the swingarm, but another inch of rear shock and it probably will. 

As I said before, the bike doesn't handle poorly, it just has certain conditions I am not yet comfortable with, so I haven't been totally confident going full lean.  That's not saying it can't.

[oxford]

I'm no suspension guy but I would try and check some "trail"'numbers and see where your at.  Offset trees aren't exactly changing "rake" angles.

[m in sc]

agreed.
^

i understand about the angle of the pic, so maybe i stand corrected here. what is the angle of the lower frame rail when on a level surface? then the swingarm? I mean, theres so much geometry changed it all adds up pretty quick. the trail is a good point, i fought that on mine as well a  bit. also a reason i stayed with 18" rims.


I still think until you address the rear shocks its all guesswork. also,  you say the front is too stiff. can you remove some preload? from what the original post says..

it squats in the back.
the back end doesn't inspire confidence in corners (shock angle and rate seem to be the issue here)
the front is too stiff.(because the forks are designed for a much heavier bike). my fzr front end, i played with spacers and fork oil
     weight a lot, and the spacers inside set the preload/sag where i liked it.

again, our bikes arent -that- far apart on setup approach, but i think th eback needs to come up. however, this may affect the turn in.

I run a scotts damper on my bike. its hard to see, but its under the lower tree. with it off, at high speed it can get a bit too quick on the front end..
.02




 

[sav0r (CL MotoTech)]

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.

This is honestly what makes me frustrated by this thread right now.  LOL

Not to make excuses, but better shocks are coming.  I'm doing this on a teacher's salary and the bike literally started as a box of parts.  I focused the bulk of my budget on trying to build a  60+ hp engine and electronics.  The shocks are good enough to at least get some good riding in to sort the rest out.  They have less than 5k miles on them.  They do not leak and they passed compression tests. As anyone knows, good shocks don't happen for less than $500. That takes time for me.  There is no reason I can't make this setup work decently or at least get a better sense of where my fail points are.

CB650 forks are the same length as RD350 forks, but with thicker tubes.  The difference is, a CB650 has an offset triple, so my fork angle changed from the stock RD 27 degrees head angle to the current 25 degree fork angle.  Not sure that's enough difference to cause so much consternation over the length, but it has to contribute some.   I dropped the forks through the triples one inch and that does improve things.  I have zero doubt I need softer springs in the forks.  I have always admitted that.  The bike weighs 279 lbs, wet with a full tank of gas.  I think that might be a little lighter than a CB650.

I also think having handlebars with a smaller angle, and wider, makes a difference here.  I used to race mountain bikes, and I always cut a couple of inches off the ends of my bars to quicken my steering and make me fit better between trees.

The body work is not stock.  Using body work angles from a picture may not be totally accurate. 

The bike is not as squat as everyone is making it out to be.  In photography, there's something called parallax that manipulates visual angles.  I had hoped my descriptions would be enough to help people lend clues, so I can get feedback on what causes certain conditions.  In hindsight, I completely regret submitting a picture at all.  That opened me up to the canned responses and typical forum stuff.

The Radian swingarm is 2" longer.  These shocks are 14.5", so I hoped it would balance out.  I am maxed on rear end height, though, based on my swingarm angle.  The chain does not hit the swingarm, but another inch of rear shock and it probably will. 

As I said before, the bike doesn't handle poorly, it just has certain conditions I am not yet comfortable with, so I haven't been totally confident going full lean.  That's not saying it can't.

Measure the trail, probably not a bad thing to do. I run 25 degrees rake on my street RD, but have a bit of extra trail due to the FZR forks. I find it more than stable, if not a bit too slow on turn in. No damper needed, and never a head shake or wobble unless I am starting to lift the front wheel. I've only managed a little over 100mph on the bike, so certainly not race worthy. So take that for what it is.

For rear dampers, really you just need enough rebound damping. People make this so complicated. Yes, some adjustment is good, yes high speed and low speed rebound adjustments are snazzy, as are high speed and low speed compression. But once you get into that level you are looking at blowoffs and all sorts of other shock options and talking about $5k (or more) a piece dampers. If the damper can control the spring you more often than not are okay. Especially so at the rear of the bike. What people don't realize is that any type of orifice adjustment dampers are basically garbage, the exception being low speed rebound via the shaft needle. Orifice adjustments are just a cheap way to make more money. Generally, they barely change anything, and orifices are the wrong way to change damping. You need additional deflective disk adjustment if you want to do it right and that gets expensive real fast. Sorry for the rant.

My RD is about the same weight, I run 73lbs rear springs on the rear. I find they are just about right. Squats on throttle but not excessively and is compliant and well balanced throughout.

Anyways, develop a nice setup and set down routine. I like to use scale pads, but that's not necessary, I just have them from the race car stuff so I use them. Once you start making changes do them one at a time, assess the change, then measure the bike after the ride. You can use that set down as the setup routine for the next change. Document everything. It is tedious, but with a nice record of adjustment you will start to decipher how those setup changes will come together. It will take time. There probably won't be a single magic bullet, it will take a combination of things to get you comfortable. And of course on a race bike the devil is in the details.

[m in sc]

i agree. my shocks arent adjustable and work fine. but they were also built for that exact setup.  can be had for well under 1k.

[oxford]

As long as you can get some good numbers for what you want for sag, just checking it will tell you if spring rates are off or not.  It won't tell you what you need for springs but it will tell you if it's off rather than saying it "feels too stiff".  Get the spring rates in the correct range and it's tuning from there.

Like trail, this is something else that doesn't cost anything and really is needed info for suspension setup.  Without starting to get some sort of actual numbers as a base line it really is just guessing.

[teazer]

Lots of interesting input.  I would start with some critical measurements including swingarm droop, rake and trail and then sag at both ends. They won't necessarily give you great insight but will give you a baseline.  One test I do on suspension after sag is set is to do the bounce test that someone suggested,.  Press down firmly at the rear of the tank or front of the seat and see if both ends drop and rise at more or less the same rate.   That will help you to see if one end is sprung or damped differently to the other.  that will also reveal if teh spring rates are too soft or too hard.  Pre-load changes sag but not spring rate, so you can get sag right and still be off on spring rate.  The bounce test helps to identify that.

But back to the symptoms.  You said it's unsettled as you get off the brakes and flick it in to initiate the turn. Wide bars will accentuate any movement or flex in the frame or suspension and will make it feel less stable, so your idea about narrower bars is spot on there.  That frame will flex, quite a bit compared to any modern bike as you probably realize.

Do you trail brake right up to the apex or do you jump on the brakes and let off fast?  Rider input and smoothness undoubtedly influence corner entry feel.  The fat front tire may also be an issue if the sidewalls are flexing or if it is not hot enough to generate sufficient grip but they usually manifest themselves in front end washout.  In my limited experience racing a TZ, heavy front tires are more of a problem mid corner and exiting as the weight can cause chatter, but your issue seems to be corner entry.

Too much forward pitch (dive) under brakes may also be an issue which is where those measurements come in.  Try it as-is and then change one variable at a time and see how it changes feeling. And keep a log of the changes and results.

RD frames and TZ frames don't really handle that well compared to almost anything today and do repay smoothness .

Emulators are a great idea if you don't already have them too.

Tony Foale's books are great but leave me wanting to add in extra frame tubes all over the place and all that will do is to move the forces to a different part of the bike that also isn't designed to handle them.

For the drag strip you want as little chassis movement as possible and the front end as low as possible - all to reduce the tendency to stand up and be noticed coming off the line.  You could get a set of RD forks and shorten them and add air caps to basically make them short and solid and get a spare pair of shorter rear shocks with minimal movement and stiff springs. You will feel every grain of sand you ride over on the strip but ET and RPM will improve.

Another random thought is that under heavy braking, the back end comes up and it's possible that it needs a stiffer setting on a steering damper to calm the back end down.

[irk_miller]

I'm no suspension guy but I would try and check some "trail"'numbers and see where your at.  Offset trees aren't exactly changing "rake" angles.

25 degrees is measured rake.  I will follow up with trail numbers. 


Thanks so much for this feedback, fellas. It's exactly what I hoped you guys would bring when I opened the post. 

Teazer:  I am absolutely trail braking late in the turns.  Early on, I started off slamming both front and back brakes in the straight, because the bike felt like it didn't want to slow down and manage the turn, so I would feel the need to add more brake.  As I have gotten used to the bike, I am pulling far less front brake on entry, or I am adding a little front brake initially, then transitioning to back brake.  I'm completely off the brake by the apex and going full throttle from there on.

On the drag race tip, I have a retired Fazer drag bike I used to race on dirt.  I shortened the forks on that bike and dropped them several inches through the triples.  I might've had 2" of suspension travel left, if even.  I think I added 8 inches of swingarm on that bike, though.  I was thinking of simply dropping the forks through the triples just in times to run the strip with the RD.  It's absolutely wanting to lift off the ground.  The bike wants to wheelie in four gears.  That's an easy solution to swap between street riding and putting on the strip some weekends, I would think.

Savor:  Your advice has always been great.  We're killing it with our Electrathon cars we build in my high school engineering classes.  There's are several decisions on those cars that have come from your feedback.   

It's pretty clear I am going to waste a bunch of time if I don't get the emulators and springs sorted in the front end.  I am comfortable in my geometry, but with that I am not claiming it's totally right.  It just doesn't feel like geometry issues to me. 

[sav0r (CL MotoTech)]

As per Teazer, you set your ride height to match your rake and your swingarm droop. Two different ways of saying the same thing. The language of setup is nuanced I guess.

In racecar terms the bounce is usually called a "bounce and roll." This is because the inherent stiction in all the moving bits needs a bit of movement to get it to settle to where it naturally should be. Especially so the tires, but the tires don't behave the same on a motorcycle when loaded. Penske did a cool study on how to increase low amplitude high frequency sensitivity of suspension components using a shaker rig, they found using WD40 on all bearings to be the best solution. lol. For a motorcycle I have always found just sitting on it is good enough, it bounces well enough. It's a two person job to do a proper setup or set down. You need the rider, then you need somebody to consistently do the measuring and recording. There's just no way around that in my opinion. You can half ass it by yourself, but it is best done by two people. Ask my wife, she loves it... There are tricks to do it alone, zip ties on shock shafts, etc.. If you can do it consistently I suppose it works.

Anyways, when you start at zero you really have to dig away and be methodical. It's not that much fun to be honest. Check out my race car thread. We took a car that won literal races and have spent years picking away at it just trying to finish a 40 minute race on a street course for no prize at all. We aren't trying all that hard, but it takes time and effort and money.

The things we do for our love of machines...

[teazer]

The trick for me with road race braking it to progressively apply the front brake.  It happens fast, but squeeze the lever to load the front end and compress the forks and get the tire to bite and then apply more brake to get it slowed down. My son tried the brake grab trick and the front end locked up and spat him and our TZ350E down the track. Too much too soon as they say.

Smooth is the word. And the same with letting the brake off as side loads increase and the front end lightens.  Too much residual brake and it overcomes the available friction and down you go.  Let it settle unless you are Ok with a loose set up and the bike moving around.

I rarely use the back brake because it just causes lock ups as I enter a corner and I don't need the help to fall.  I can do that on my own. Back brakes do serve a purpose to settle the back end and especially in the wet or when it tries to wheelie, but it's hard to moderate braking action with stiff race boots. At least for me it is.

A long long time ago, a certain Stanley Michael Bailey Hailwood rode a Manx Norton in a vintage race at Winton in northern Victoria.  It was a handicap race and he started much later than everyone else.  Other riders were riding as hard as they could and it showed.  Mike the Bike was clearly just out for an afternoon ride on an old bike.  Everything he did on the bike was smooth and seemed effortless as he rode around everyone to win the race.  I can't ride Like the late SMBH but I can try to be as smooth as possible.

[irk_miller]

Oxford, I have been misspeaking on rake. I was calling fork angle "rake", and calling the rake "head angle".

Here are the numbers: rake is 27 degrees, which is standard RD, so nothing changes.  Fork angle is 25 degrees because of the -2 degree raked triple.  The offset is 1.25. 

Here's the kicker.  Trail is 5.75", which is straight up Harley territory.  Thinking I should be closer to 4"?  It's stable as hell cruising down the road, that's for sure. Literature seems to vary at where to max out trail.  I am inclined to hold fort on the rake and trail and go with narrower bars, but these forks are about to be an investment with springs and emulators to do it.