Richard's ZR750C1 Restoration Project

[Richard]

Took the bike for its first spin today, have a reasonable impression now but definitely need more miles. So far not seen any issue with fuel or engine. Steering is a bit hard on the arms, could be tire pressure or maybe in need for some decent new tires, time will tell

Sunset picture

@Freddy thanks for your advise mate, much appreciated.

And no they were not a gift, spent my sales bonus on them few years ago . I bought them from an official Ohlin dealer and they advised me this model. We agreed that they would take care of any adjustments when needed. Your remarks reminded me of this arangement. Now several years later I guess it's not for free anymore. Although I also have a pair of nice upside down ones for the front for which I will need their help as well as I don't have the knowledge nor the equipement to do this myself.

Anyway these Ohlins are fully adjustable so I don't think I would need another set of springs. I'm also reasonable light weight with only 72kg, no clue what that is in lbs or pound and stones...

[Freddy]

Took the bike for its first spin today, have a reasonable impression now but definitely need more miles. So far not seen any issue with fuel or engine. Steering is a bit hard on the arms, could be tire pressure or maybe in need for some decent new tires, time will tell

Anyway these Ohlins are fully adjustable so I don't think I would need another set of springs. I'm also reasonable light weight with only 72kg, no clue what that is in lbs or pound and stones...

At 72 Kg (76?? in gear) you may be ok with the springs. However I'd still recommending checking how compressed the top coils are with your weight on the bike. Only takes a second. No "adjustment", not even preload, will make any difference to how much that light section compresses with your weight on it. If that light section is fully or near very close to fully compressed with your weight on the bike, you don't need to be a suspension guru to figure it won't be contributing much, if anything, as you ride along on a less than perfect road surface. That's the very job of teh 'light section', to give a nice compliant feel over smaller imperfections, and if they are squashed right up they are doing nothing, its only the heavy part of the spring working.

Regarding the heavy feeling steering, don't overlook....
Excessively tight steering head bearings
Excessive rear 'rider sag' (rear sitting low making the bike have the geometry of a 'chopper'). Important regardless as a bike with excessive rider sag can be prone to headshakes under hard acceleration (inadequate weight on front wheel).
Excessive compression/rebound damping (front or rear). As I assume the front end is still original, it can only possible be the rear as a stock front has virtually no compression of rebound damping (not unless you stick something like 30W oil in, which creates another problem)

Damping is a massive variable. I can transform my bike from a very light steering virtually little bar pressure (just using 'body English' to pinch a Reg Primore term) to feeling like a tractor that has to be muscled around corners with just 2 clicks increase in rear slow speed compression damping. If you haven't changed the damping on the Ohlins from how they were supplied, consider ..... I haven't as yet EVER bought a quality shock from any maker that didn't come with way excessive compression and rebound damping for street use. No matter what instructions you gave them for so called 'build to order' shocks.

[Richard]

Freddy I will check the coils as you described it, maybe even take a picture with sitting backwards...
This needs to be done next week as I'm away during the weekend

The steering part is more complex, need to study this a bit more if this is related to front/rear suspension, but it sure needs to become solved. Will take some more time for sure but will share my progress

Thanks again Freddy!

[Freddy]

Freddy I will check the coils as you described it, maybe even take a picture with sitting backwards...
This needs to be done next week as I'm away during the weekend

The steering part is more complex, need to study this a bit more if this is related to front/rear suspension, but it sure needs to become solved. Will take some more time for sure but will share my progress

Thanks again Freddy!

If not already, certainly start by setting both front and rear 'rider sag'. A motorcycles intended steering geometry and suspension travel will never be correct if either is out. You want around 35mm 'rider sag' front, 30mm rear. Rider sag should be set on every motorcycle, its why they put the adjustments on the damn things.

A good article on the correct way to set rider sag if not familiar with it .... https://racetech.com/articles/SuspensionAndSprings.htm

[Richard]

So I did perform some measurements together with a neighbor:
Rear Sag: 32mm
Front Sag: 39mm

Next weekend I will spend some time adjusting the front sag together with my son if he's coming home.
The front has stickers about progressive springs being installed. Not sure if this makes matters worse when adjusting but we will see.

Next I did the backwards seated test and took a picture, the top of the spring still has some space being it not too much...

And finally I measured tire pressures, front 1.8 and rear 2.0 bar, brought them up to 2.0 and 2,2 bar. That did make a difference, not huge but steering is a somewhat easier now.
Hard to describe what I feel during steering, it's like when you start turning in, the front wants to steer-in further by itself and I have to keep it (with some force) in the direction that I want. Does that make sense?

[Freddy]

Did you measure (or at least roughly check) that you also have some 'free sag', both front and back. Free sag being the amount the weight of bike alone compresses the suspension down from fully extended. You need to have some 'free sag' both front and back with correctly adjusted 'rider sag'.

If there is more than 4mm free sag in the front (which there should be way more than that), reducing the front rider sag is just as simple as removing the bars, loosen the top triple clamp bolt, and remove the fork cap (careful on the last few thread so it doesn't spring out). If you do one side at a time no need to even jack the front up. Then just add 4mm worth of appropriate size flat washers, and reassemble. Repeat for the other side.

A word of warning .... do NOT use a torque wrench to pull the top triple clamp bolt up to the specification in the workshop manual. That is WAY too tight stretching the aluminium clamp, and eventually it WILL snap. Just do it up with a normal size socket rachet (8"?? long) till it feels 'neat'. The bottom all steel ones are fine to pull up to the specified torque.

The problem with the steering you are describing sounds to me like it is 'diving' into corners. A bike that does that generally doesn't hold a line very well once in the corners requiring constant corrections. Based on what you've described, my guess the problem is inadequate fork rake angle. It's likely being caused by some combination of ...
(a) a tad too much rider sag in the front forks
(b) those 'progressive' springs in the front forks. In all probability that are not true progressive springs, but rather 'dual rate' springs. Those rear shock springs are dual rate springs, two different constant sections. A true progressive spring has no two coils the same, changing consistently with every coil from one end to the other.
(c) I'll take a pure guess on excessive compression damping in the rear shocks, if you haven't adjusted them from how they were supplied.
(d) front fork oil that is as thin as water. Another guess based you are relying on a sticker that the springs are progressive. This indicates to me you haven't changed the front fork oil, as if you had you'd know if the springs were progressive (dual-rate) or not.

What's likely happening is the front is compressing way down under brakes as you enter the corner, or even in the corner due to centrifugal force, and the rear is staying up. Bike is cornering nose down, tail up.

What would I do? Lets take a step back to tyres. You haven't said how old the current one are, or type. Regardless piss em off and fit a pair of Continental Road Attack 3's. Run em 32psi front, 33/34psi rear (for your light weight). The Conti's are by FAR the best tyre available that fit a 750 IMO, by a country mile. You'll NEVER get a decent setup handling bike starting with old hard, and god forbid, bias ply tyres.

Next change the front fork oil. It should be done every year, maximum. Use 10W (I reluctant to recommend any particular oil wight because all have a problem). 15W will get a better result under most conditions but will exasperate a very dangerous problem, so I'm reluctant to every recommend it.

Next set the front rider sag just add 4mm of spacers (a stack of large flat washers)

Reduce the rear compression damping. In no picture can I figure what adjustments the Ohlins have on them. But regardless, how have you assessed rebound damping? You want it set pretty light (fast return) for road use. The reason for asking about rebound damping, is any adjustment you make to rebound damping on 99.9% of shock absorbers also affects compression damping. Oil flows both ways through the adjustable orifice. So as a simple test, just take the rebound adjuster on the bottom of the shock out say 3 clicks on each side and see what difference that makes.

If still a problem ... move to Plan B.

[Richard]

Tonight I did some measurements and checked the spring on one side.

The springs are indeed dual rate types as you expected. And yes I never changed the oil, actually I never touched anything in the front until this year. They are still available on the market for some 130 Euro.

I think the setup wasn't placed in the correct order, there is a ring which should be on top of the spring, instead it was placed at the top. Not really a big difference I guess but still...

Wrong order:

Right order:

I measured the free sag:
Front: 10mm
Rear:11mm

I don't have washers I can use so need to find some, perhaps tomorrow evening in the local hardware store.

About the oil, you mention 10W, would that be 10W15 or what do you recommend? I need to order that as well.

And finally about the tires, yes they are old and need replacement, I will check if my local motor mechanic can supply the ones you're advising.

So far for today

[Richard]

The progressive springs in use on the bike

[Freddy]

The washer was in the correct place, between the hard thin steel spacer and the soft aluminium fork cap. Ideal you want a flat washer on both ends, but if you've only got one .... on top of the spacer as it was originally.

Fork oil weights bare no resemblance to multi-grade motor oil. Forget what a 30 year old service manual may say. They come in a single weigh, starting at 2.5W and going through in 5 and 10 increments to 30W being about the heaviest you'll find. 10W to 30W is used in damper rod forks eg. your Zephyr.

The rear free sag for the amount of rider sag looks good. As you do still have some action in the light section of the springs I'd stick with then as is. BUT .... in the front it's exactly as I expected. Theoretically front free sag should be 60-70% of the rider sag (15-25% for the rear, there is a god reason for the difference). That should mean that for your 39mm front rider sag you should expect to have around 20-25mm free sag, you have only 11mm. The reason, those light section dual rate springs. And that is the problem with dual rate front springs. That light (closer wound section) that gives a nice comfortable ride isn't strong enough to properly support the bike under hard brakes and through corners. Downward force on a bike when cornering is 'lazy', it'll flow more into which ever end of the bike offers the least resistance i.e lighter weigh springs, compressing that end more and mucking up geometry. The reason a 'balanced' front/back suspension is so important.

What to do .... I'm assuming you don't want to spend money of the current front forks. So.... try reducing front rider sag to 30mm (to keep the front up as much as you can with the current setup). To do that cut a new spacer 10mm longer than the current one.

With the forks fully compressed stick in 110 mm 10W fork oil, measured from the top of the oil to the top of the fork tube. There is a specialist tool for setting fork oil height, not expensive, and is so much easier than a ruler etc.

Open out the rear rebound adjusters 3 click each side to allow the rear to sink lower easier (reason for adjusting rebound explained in previous post). You've now got the front up as high as possible, and the rear softened up a bit to hopefully sink lower ....... see how it all goes.

[Richard]

Thanks for all your good advice, it sure helps if someone with experience provides some guidance

I don't have the equipment to create a larger spacer myself, will have to order this elsewhere. In the mean time I did buy some large washers, hopefully not too large and I will start experimenting with those first to see what the diff will be. Will order some W10 tonight so should be good early next week to start testing.

About the location of the spring seat, according to this drawing it should be located as I showed earlier on top of the spring. I remember having seen this at other front suspensions as well, will not make a huge diff though.