Front Fork Dive

[leswaller]

I do not like the amount of fork dive in my R60/5, I know that many say that the forks go down like a poleaxed donkey and this is normal, but compared with my Velocette the dive is extreme.
I recently changed the fork oil (7.5w) in the hope it might improve things but it hasn't. The seals seem ok and the units don't bottom out.
Any suggestions as to how to reduce the feeling of dropping down a lift shaft please? Incidentally I do lead with the rear brake so as to reduce the balance shift.
Les

[nhmaf]

If you've got the original (or even a ~20 year old replacement set) of springs in your fork it is certainly possible that they're clapped out - though these earlier model bikes do have the squishy suspension of a 1970s era American car..

There are progressively wound spring kits available for the R65, R80, R100, etc. but they don't seem to have much of anything for the /5 range of bikes, except for the R75/5 they show a fork spring kit # 11-1101.  I'm not sure if it would fit your bike or not, I thought the forks were the same in the /5 series, but I don't know...

http://www.progressivesuspension.com/forkspringsmetric/index.html


http://www.progressivesuspension.com/prodSearchResults.aspx?yearID=1973&makeID=38&modelID=919

But Bob's BMW in Maryland does list some for 1970-1984 airheads - I'm not sure of the fitment, but perhaps you could inquire of their parts department:
http://store.bobsbmw.com/product/progressive-front-fork-springs-for-r-bikes-70-84

Perhaps if you called the Motobins store over in your region they may have some other suggestions on parts.  Sorry I can't provide  more assistance.

[montmil]

Try carving off a piece of Sch 40 PVC tubing -diameter to fit- and stuffing it on top of the fork springs, Start with about 0.50-inch length. Cheap experiment and you most certainly won't be the first to do this.

[nhmaf]

Monte's suggestion is a good (and most importantly, cheap!) option to try - I'd give that a thumbs-up too!
Adding an inch or less of preload to the springs may stiffen the fork up a bit if it is those first few turns that have gone especially saggy

[tvrla]

What's the sag? How far does it settle from no-load on the centerstand?

Going to Progressive (brand) springs is heading waaaay too far in the other direction. Much too stiff!

I believe the springs are all interchangeable from 1970 to 84. Springs for an R100RT or RS are stiffer and much better quality than Progressives. Cheaper too.

When talking suspension we need to know your weight and riding style. Stiffness of the springs (and front end) is a whole different matter than preload. Let's first determine what's going on with the stiffness. I suspect old sagged springs and possibly they're also too light for your weight.

[Barry]

Stiffer than stock springs are the last thing I'd look at although I'm a lightweight. As has been said stiffer spring should not be decided on without first doing proper sag measurements that take actual rider weight in to account.

Fork dive is inevitable with normal damper rod telescopic forks and on an R60 you have more travel than an R65 so they are going to dive some. In fact if you don't use nearly all of that travel under extreme provocation then the design intent of the forks is not being met. Having said that fork dive can be controlled to some degree by increasing damping and also by increasing the other progressive spring that you already have in your forks and that's the air spring. Increasing the oil level reduces the air volume above the damper piston which adds substantially to the spring rate when the forks are getting towards maximum compression. Even a 5 or 10 mm change in oil level makes a detectable difference.

All of this tends to reduce the maximum extent of fork dive. Initial fork dive can't really be controlled except at the expense of a very stiff  ride which may or may not acceptable. It's all about damping velocity. Fork dive induces low velocity damping while even small sharp edged bumps in the road induce high velocity damping and the two are mutually exclusive. You can't have a soft ride and little fork dive with ordinary damper rod forks. That's why modern bikes use cartridge forks which have a better compromise between high and low velocity damping. They effectively have relatively high rates of low velocity damping to control dive combined with a high pressure relief valve in the shim stack which open up when you hit one of those sharp edged bumps to reduce the high velocity damping that would otherwise be induced.

One option you can use on an air heads is a cartridge emulator although the reports of  success are mixed given it's not a cheap option. A google search will bring up a few cases where emulators have been fitted to airheads,.

[tvrla]

The other solution is to not use the brakes.

Not kidding.

Develop a riding style of anticipating conditions and curves and go easy on the brakes. These airheads do well with this style of riding - the suspension geometry stays steady, corners well and you'll find yourself going deceptively fast, but doesn't feel like it. These bikes don't do well with the motoGP style riding of zipping along the straights at very high speedss, hard braking before the turn and then getting on it coming out.

Steady throttle is the best approach. It's just the way these bikes want to be ridden.

[montmil]

One last comment from me, Les...

If you have a more modern bike in your stable, as I and many of us do, the comparison of 30+ year-old engineering -more years than that with your R60- and current MC technology is hard to resist. The semi-vintage Airheads and their pre-war designs -pick your favorite war- are not even close to what's available straight off today's dealer floors.

Outside of expending considerable amounts of your beer fund for snake oil fixes, dilithium crystals, sub-sonic turbulators or golden bullet valve enhancements, I'd suggest doing some simple and economical experimentation with fluid weights and the cheapo PVC plug.

And your front dive under braking, normal as it is, could be partly caused by rear suspension components damping and rebound functions. Mo' money, there.

Of course, you could just leave it be and cruise the R60 as is.

[smiley=2cents.gif] [smiley=beer.gif]

[Rob Valdez 79 R65]

The other solution is to not use the brakes.

Not kidding.

Develop a riding style of anticipating conditions and curves and go easy on the brakes. These airheads do well with this style of riding - the suspension geometry stays steady, corners well and you'll find yourself going deceptively fast, but doesn't feel like it. These bikes don't do well with the motoGP style riding of zipping along the straights at very high speedss, hard braking before the turn and then getting on it coming out.

Steady throttle is the best approach. It's just the way these bikes want to be ridden.

That is the best description of how to ride an airhead I have ever seen!

I like Dougie's (Ontario) signature line over on Boxerworks:
"A machine on which a gentleman can keep up a rather brisk pace."

[Bob_Roller]

It's been mentioned here a few times when this topic comes up, about emulators for the damper assembly inside the fork .

It's some sort of valve that prevents excessive movement of the forks, sorry, but that's all I remember about them !!!!!!!!!!!!!!!!!!!!!

OK, found a link to them .

http://www.motorcycleclassics.com/classic-motorcycle-repair/race-tech-suspension-bmw-6.aspx

[Barry]

I tried it explain in my post above what emulators are about.

The problem stems from the way damper rod forks react to different inputs. When we talk about damping velocity it's nothing to do with how fast the bike is going. It about how fast the forks are having to react to inputs from braking or bumps in the road.

Slow velocity movement of the forks like fork dive produces slow velocity of the the oil through the damper orifices so the compression damping effect is small and the forks feel compliant and soft.

High velocity movement of the forks that results from sharp edged bumps even very small ones produces high velocity of the oil through the damper orifices so the compression damping effect is very much higher and the forks tend to feel harsh.

Damping forces are progressive with increased fork velocity so damping characteristics are not linear with velocity but instead rise very rapidly. This makes the low vs high speed damping compromise worse so if you increase the slow speed compression damping to reduce fork dive the sharp edge bumps become even harsher until eventually you effectively have hydraulic lock.

Modern cartridge forks and emulators have a normal orifice which takes care of the low velocity compression damping but they also have a shim stack which opens up to flow more oil when the forks encounter something harsh that induces high velocity movement. It's like having a safety valve which increases the size of the damping orifice when needed. When you hear of modern forks having adjustable compression damping what they are doing is adjusting the pressure and rate at which this shim stack or safety valve blows off.

We didn't yet mention rebound damping so it's not a complete picture but the way the compression damping curve can be modified by using a shim stack is the key to how cartridge forks and emulators improve on damper rod forks.  

    

[tvrla]

Good explanation Barry!

I've put emulators in the R80G/S and love it. But I've got more tuning yet to do to have it perfect.

To add to Barry's explanation - our forks have to have a compromise between compression and rebound damping, because they're essentially tied together. Stiffen one, and the other suffers. Just like the problem with high and low speed damping.

The whole idea is to separate these different functions and make them independently adjustable.

The problem with rebound is it's normally not as stiff as it needs to be. Not enough rebound damping. But that's controlled by the weight of the oil. You can't run stiffer oil because compression damping will suffer.

BUT! with emulators, they can be tuned to behave properly with much stiffer oil, thus allowing you to tune the rebound! So the process involves getting the best weight oil for rebound, and then tuning the emulator to get the compression right.

I'd really like to install some emulators in these forks!

[Barry]

I agree about rebound damping not being stiff enough. The problem of not be able to adjust rebound damping independently of compression damping is what I believe causes some forks to clunk on full extension. There just isn't enough rebound damping.

When you have the springs out it's interesting to compare rebound damping with compression damping. Stroking the forks by hand will show very little resistance in compression but you'll really notice that rebound damping is much stronger on extension. Even so it wasn't enough on my forks particularly the hydraulic bump stop effect that takes place just before full extension when the damper rod holes fall below the level of the valve washer.  

There is one rather involved way of making an independent improvement to rebound damping and that's by changing the dimensions of the damper valve washer. BMW did it themselves when they made the later damper valve washers thicker which gave better rebound control. They also reduced leakage past the valve washer by tightening up the clearance between the washer and the damper rod.  That made the hydraulic bump stop more effective.  

Through several years of experiment I have basically reproduced both of those mods to make my early forks work very much better. And they are now clunk free which is just wonderful.  Clunking forks didn't make for a very satisfying riding experience. I sometimes wonder if that was the reason the bike had only done 6200 miles in the first 28 years.

[leswaller]

Thanks guys for some very helpful tips and a very informative discussion. I am out of the country until next week so can't give you any info on fork preload travel until then. If it is any help I weigh about 225lbs and as I am almost 70 yrs old I don't use a racing style of riding, preferring to cruise around nice and steadily. However as previously stated, whilst the general suspension ride is good, only a slight application of the front brakes causes the forks to drop like a bomb. As there are no obvious seal problems I am thinking it may just be my weight, but my '54 Velocette MAC does not react in the same way. I will probably try the PVC pipe trick but quite like the idea of the emulators if they are available in the UK

[tvrla]

The proper approach to any suspension issues is to first determine sag, which should be roughly 1/4 to 1/3 total travel. If the shock bottoms out, then stiffer springs are needed.

The PVC pipe trick is used to fix preload to achieve proper sag. It's not used to stiffen spring action.

So check sag first and let us know what you come up with.