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Click on these links for a "hot lap" of the TZ350 and 250 Website:
TZ Suspension Setup Guide.
Guide to TZ suspension setup.
Introduction:
While optimum suspension setup on our old racebikes is obviously quite limited by period technology, taking the time to properly set it within the confines of the few variables available can pay dividends at the track. Sure, you could go out and buy yourself a nice, new, special rear shock from one of the excellent suspension tuning companies still catering for 20 year old plus bikes, but some of us take our sport a little less seriously and choose to “make do” with the way our bikes came from the factory, servicing the shock or forks with an oil change and/or re-gas and utilizing the adjustments available on our stock bike, or at best, have the stock damping systems “re-valved”.
This article caters for this type of TZ owner and is by no means intended to
be the complete TZ suspension setup guide. It is just a collection of
thoughts, research and practical experience and is intended to hopefully get
your bike performing (handling) better than it was. Most of the principals can
be applied to almost any roadracing motorcycle, whatever the brand, model, or
age.
Picture: An early Bimota framed TZ350. ( Joris van de Wiele)
For a thorough, involved study of suspension, look through some of the other better suspension and handling publications available, check the “Reviews” link of the TZ350 and 250 Website for some suggested titles.
Anyway, here goes …..
First steps:
It’s a waste of time and effort trying to get a worn out bike to handle well. Make sure all the bearings and bushes etc. in the swingarm pivots, shock mounts, steering head etc. are in good condition, grease them and torque them up correctly. Also check the condition of your wheel bearings.
As far as suspension components go, check the fork and shock spring free lengths to make sure they are above the minimum recommended service limits. Thoroughly clean your forks internally and replace the oil with the recommended quantity (202cc for 250's and 350's) and viscosity (10W or 20W.) If your rear shock hasn’t been serviced in the last few years have it done by a reputable suspension shop. Most claim to be able to rebuild any shock, ask around your race club and find one who people have actually used and been pleased with.
Check the alignment of the bike’s wheels with string-lines and make sure the chain is running true on both sprockets.
Proper setup requires lap after lap of testing at the track. Ensure your tyre pressures are set as you would in a race situation. Here in Australia most of us usually run 28 to 30 psi front and 30 to 32 psi rear, though this can vary with weather and track conditions obviously.
The TZ350 and 250 Website
Spring in your step ( spring pre-load and sag.) :
Assuming your springs are in pretty good condition, the next thing to do is set the bike’s static sag at both ends.
The process covered below may seem a bit complicated, but it is the only way to factor out all the binding and stiction of the seals and bearings etc. properly. Simply sitting on the bike and having someone measure the amount it sags is the in-accurate way to do things.
To perform this task you will need:
The best thing to do here is convince two of your riding mates that they need to set their static sag correctly in the following manner, that way you can all help one another.
The TZ350 and 250 Website
The order of operation is:
Front End static sag:
Static sag = D1 - D2 + D3
2
This figure should be around the following
· 24mm for a 70kg rider
· 27mm for an 80kg rider
· 30mm for a 90 kg rider
By the way, these figures are not hard and fast rules, just a guide. You need to ride your bike at race pace and work out what works best for you individually. A fast 70kg rider might bottom the suspension more than an 80kg average rider and will therefore need more preload than his heavier rival. (Get the picture?)
It’s also important to monitor your forks travel by say having an O-ring around one fork tube when actually testing it at the track so you can tell when you are using almost all of the available travel..
Adjust your fork spring pre-load by either screwing the 3-way adjusters on top of each fork cap ( F and later models ) and / or installing preload spacers under the caps to bring the static sag to the figure you seek. Sometimes even coins can be used as spacers !! A simple pre-load adjuster can be made by tapping a 6 or 8mm bolt into the fork cap which can be screwed down onto a coin shaped piece of metal which in turn pushes down on the spring.
(Riders over 95kg should consider getting hold of some heavier fork springs.)
Picture: Tony Edwards' TZ350G
Rear End static sag:
Perform the same sequence of steps on the back end, with one mate balancing you and the bike from the front while the other does the pushing / pulling / measuring at the rear.
Once again do the same calculation.
We are looking for similar figures again for each rider weight. Preload on the rear of a TZ is adjustable.
General comments on springing:
It’s preferable to change the springs themselves rather than increase the pre-load because pre-loading can cause the suspension to be too stiff in the initial part of it’s travel and too soft later, which is exactly the opposite to what we require. If you need a fair amount of pre-load to get near to your optimum static sag you are probably better off going for a bit more than the recommended sag, ie. with less pre-load, to at least try and give the suspension’s damping action a better chance to work properly.
One thing to keep in mind is that by increasing the amount of oil in the forks a little, say 3 to 8mm in level, you can retain a plusher ride in the first part of the fork’s travel and yet still avoid bottoming out on the big bumps. This is due to the reduction in the amount of compressible air and it’s replacement with non-compressible oil.
You might set your static sag as per the guide above and ride very aggressively in the heat of a race and constantly bottom your suspension anyway. In this case you will probably need a bit more spring preload (this is probably the first thing to try anyway) but always remember, changes to springing will affect your damping settings. Monitoring travel keeps you informed of the amount of travel your suspenders are using up. A simple O-ring around a fork tube does this for you, at the rear end a “texta” or “marker pen” mark along the shock shaft might work (which will disappear as the shock travel is used up) if you can get a skinny enough texta to fit between the coils is a suggested monitor. Failing this, simply riding the bike and feeling when it’s just bottoming, then jacking up the pre-load slightly is another method that definately does work. Ideally the suspension should be just off bottoming out on the worst bump in the track at the highest speed.
The TZ350 and 250 Website
Slowing things down ( the mystical art of damping adjustment).
There should be no need to go right into the definitions of damping here since everyone should be familiar with the principals already. Suffice to say that compression damping controls the speed at which the suspension compresses, rebound damping the speed at which it returns.
Adjustment in these regards on an old TZ is very limited indeed, with just rebound damping being able to be altered on the rear (monoshock models only) and no adjustment at all on the forks. There are really only two ways to change the damping where adjustment is non-existent, altered internal “valving” (orifice numbers and sizes, shims, etc.) or a change in oil viscosity.
Obviously changing the oil is the cheapest method to try first, but remember that heavier oils do funny things when forced to go through little holes (heat up and thin out, cavitate, etc.), though with such short races in our sport these days this is probably less of an issue than it was in the 70’s. Another thing to keep in mind is that a change in oil can effect both compression and rebound damping at the same time. If you feel your bike needs a little more compression damping on the front end and a little less rebound, then changing the oil will not do the job for you. One could improve and the other be a little worse than before.
Whatever changes you make (especially ones made during testing) must be done one at a time and written down.
The easiest way to cover damping adjustment is to list possible damping problems and what the symptoms are of each:
Front End Damping:
Not enough compression damping:
· Front end dives badly under brakes, or bottoms out, gives a spongey ride.
· Steering control is lacking, front end has a vague feel to it.
Too much compression damping:
· Harsh ride, you feel the “jolt” of each bump through your wrists.
· Front tyre feels like it’s bouncing or “skipping” off the road surface after hitting a bump.
· The bike is difficult to turn into corners and feels like it’s riding a bit high also steering wide.
· Very little dive under brakes.
· Patter under deceleration/ braking.
Not enough rebound damping:
· Front end patter encountered mid corner. ( See also the “Front End Patter or Chatter” section. )
· Spongey feel to front end and a feeling that traction is low.
· Under aggressive riding the front end won’t settle down and the bikes chassis attitude suffers.
· Bike “pogos” too much through a chicane.
· Front end doesn’t recover after aggressively riding over a bumpy corner.
· Front end shoots up after braking.
Too much rebound damping:
· Forks feel like they are locking up and ride is stiff.
· Feels like the front tyre is not maintaining traction through gentler bumps and dips in the track.
· Front end seems to “pack down” over a series of bumps. It feels like the front end is tucking and skipping over bumps after the first one is encountered.
The cures:
To reduce the damping effect use a lighter oil and/or change internal fork valving.
To increase damping effect use a heavier oil and/or change internal fork valving.
Remember that while damping has virtually no effect on static sag, conversely, spring pre-load does effect damping action. So for this reason it’s important to sort the spring sag/ pre-load out before tackling damping.
Damping is a real “trial and error” process. You need to be able to lap the same track for a considerable amount of time, 2 or 3 laps at a time, with no major weather changes, at the same race pace, making single adjustments and recording them and the result each and every time. Whatever you do, don’t be tempted to alter two or more things at the one time as this will only serve to confuse the issue. Be patient and take notes.
The TZ350 and 250 Website
Front end “Patter” or “Chatter”.
Front end patter or chatter, or whatever name you like to call it is one of the most un-nerving experiences a motorcycle roadracer can have on the track. You hook into a fast sweeping corner, thinking you are doing everything right and it starts, feeling like there are hundreds of little ripples in the track surface, causing a rapid rise in your heart rate !! The only thing you can do out there to reduce it is slow down or, sometimes but certainly not all the time, speed up a bit, the latter requiring rather large “gonads” to attempt since you are probably already traveling as fast as you feel you can in the corner anyway.
But all is not lost, even on these 20+ year old bikes there are a few things you can try to hopefully eliminate it and get back to concentrating on going fast, everywhere.
Picture: Ferry Brouwer's replica OW15 125cc racer, code named the "FB15".
Below is a list of most of the current accepted causes of pattering and some suggested remedies to try.
Patter can be caused by any, or a combination of the following:
1. Out of round tyres
2. The tyre’s bouncing action and the fork tubes longitudinal bending (flex) acting in unison.
3. Too soft a spring or too little pre-load.
4. Too much oil in the forks causing the air contained to be over-compressed and in turn limiting the fork’s usual functions of damping and motion.
5. Fork twist in corners where the tube to bushing clearance is used up and causes the forks to effectively “bind”.
6. Ride position and the way the combined mass of the sprung weight and rider are distributed on the front and rear wheels, (called “chassis attitude”.)
7. Cavitation ( or aeration ) of damping oil.
Some suggested ways to alleviate the above problems:
1. Try a different front tyre.
2. Alter the frequency of the bouncing by changing the un-sprung mass of the front end. You could try a different profile front tyre, adding or removing a disk, changing from a spoked wheel to a “mag” wheel or vice-versa.
3. Buy heavier springs or increase pre-load.
4. Reduce the amount of oil in the forks a little at a time. Be careful with this one of you already have the recommended quantity of oil in the forks, you might find the forks tend to bottom out more.
5. Some people claim fork braces help stop this twisting action. A thicker diameter axle will do a similar thing.
6. Try raising or lowering your forks in the triple clamps, raising or lowering your handlebars, to change the weight bias of the combined rider and machine. Even altering footpeg position has a bearing on the way the front end behaves.
7. Have your forks re-valved to increase rebound damping or to allow the use of a lighter oil which has less of a tendency to alter through use than a heavier one.
8. Try a few adjustments to your rear shock, this can affect front end performance.
The TZ350 and 250 Website
Picture: Jean Jacques Desaulle at Lurcy Levis in France, 2001.
Rear End Damping:
Not enough compression damping:
· Difficulty getting the bike to tip into a corner and turn.
· Traction and control lost when shock bottoms too easily.
· Back end squats under acceleration and steers wide coming out of corners.( This is actually more likely to be caused by lack of rear ride height or spring resistance or preload).
Too much compression damping:
· Tyre overheats and loses traction due to the shock being too “stiff” and the tyre doing too much work.
· Harsh ride gets worse with increased speed.
· Back end kicks up when going over medium to large bumps.
Not enough rebound damping:
· Vague/soft feeling with increased speed, rear end wallows over bumps and traction is reduced.
· Shock returns too fast on exit of corner, traction suffers due to rear end chatter or pogo-ing.
· Back end jacks up too fast under brakes.
· Back end seems to bounce on bumps.
Too much rebound damping:
· Rear end packs down over a series of bumps, bike runs wide in corner (due to the rake being altered), steering becomes slow.
· On sudden deceleration rear end feels like it’s skipping or hopping on entering corner.
· Harsh ride, low traction, limited control.
· Rear tyre skips over bumps and heats up after a while as rear end packs in.
The TZ350 and 250 Website
Front end shake at speed:
This is an interesting one which can be divided into two sections. Listed are a few things to investigate, a starting point at least.
1. "Tank slapper" type handlebar shake.
Too "soft" setting of the steering damper, ie not damping enough.
Steering head bearing over-tightened.
Too much rebound damping at the rear end making the rear end hold the weight towards the rear for too long.
Rear end too soft, under-sprung or not enough pre-load which will lighten the front end too much, making it "nervous".
Badly worn swingarm bushes or bearings.
Fork tubes too far up into the top triple clamp making the bike too "quick" in the steering through altering the rake and trail excessively.
Insufficient preload in the fork springs.
Mis-aligned wheels.
Something on the bike is broken like a frame tube, or loose / worn like steering head bearings, axles, swingarm pivot / bushes.
Front and rear tyres not paired properly, ie. not compatable.
2. Vibration through the bars.
Front wheel not balanced properly.
Too stiff setting on the fork preload, too stiff springs.
Too much compression and/or rebound damping at the front end.
Damaged front rim, buckled or dented badly.
Out of round front tyre.
Conclusion:
So there you have it, a basic, practical overview of suspension setup on an old TZ. Obviously with limited scope for adjustment and alteration you aren’t likely to get it all spot-on correct in every situation, a compromise is always necessary.
You won’t get your old TZ handling like the latest 2002 model, but at least you now have a shot at getting it closer to handling perfection than your opponent who just leaves everything as it came from the factory.
Have fun !!
Some "rule of thumb" fork oil setup tips from Jamie Linxwiler:
Bottoming on the
front forks can be addressed by oil weight and by preload, but also can be
addressed by raising the amount of oil--remove springs, let forks bottom, and
measure. Should be around 6 or 6 1/2 inches to start; if it lacks bottoming
resistance, then gradually add oil 1/4 or at most 1/2 inch at a time but never
less than 5 1/2 inches, until it resists bottoming and if it is a serious
problem (ie a very rough track like Tanacross, paved motocross) then go directly
to 5 1/2 inches.
On the old bleed valve forks on the 70's TR /TD/TZ's, use 15 weight, or
depending on the brand that may make rebound too slow, 10 weight. Also go to
lighter oil if the track is bumpy, especially with fine-grained chop. In
general, use the fastest rebound you are comfortable with, too much rebound
causes many problems.
Thank you Jamie for some great ideas!
03/23/08 10:16 AM +1000
This website © Greg Bennett 2002.
