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one front rotor versus two

Posted: Thu Apr 08, 2004 1:34 pm
by hulagun
Somebody mentioned fork twist when braking using a single rotor. I have actually never noticed this myself, even when racing bikes with skinny fork legs and one big brake rotor. I admit to having dual brakes on all my bevels, but only because they all came to me that way. If you weigh a bevel wheel with dual disks they are ridiculously heavy. This penalizes steering response and suspension compliance. Dual rotors have their place in racing when caliper heat can cause fading... a problem at race speeds. On most street bikes, dual disks is really about looking the part of a racer.

If I wanted optimal braking power from a single disk bevel, I'd use a Ferodo pad designed for cast iron rotors. I'd find out what the optimum size modern Brembo master cylinder (or a Grimeca that looked vintage) was for my single caliper. I believe a single F08 would want a 12 or 13mm master cylinder. I'd rebuild said brake caliper with new parts, and run a single stainless steel braided line. That should give good results.

To cut down on fork flex I'd correctly assemble my fork tubes, triples, steering bearings and front wheel using proper torque settings. If necessary I'd drill a fork leg to run axle clamp bolts on both sides instead of just one. I'd live with whatever flex remained.

Not as much work as it sounds and I'd prefer it over the added weight of a dual disk setup. Now that I am thinking about it I am going to return my 750GT to a single disk!

Ivan in SF

2 rotors vs. 1

Posted: Mon Apr 12, 2004 12:50 pm
by Phil Porter
Ivan, if I ordered one of Steves fork spring/rear shock setups that he gets from Works Performance, should I mention that I am running dual disks on the front? Would a fork brace remedy the fork twisting? I don't think I have ever seen a fork brace advertised for a Bevel Duck. I have ridden with both the dual disks and single disk on the same bike, and the dual disk feels more controlable with less effort...just my experience, and I'm not a racer. Phil Porter

fork braces vs fork twist

Posted: Tue Apr 13, 2004 10:08 pm
by hulagun
I have had a couple fork braces on my bikes and found them more trouble than they are worth on a street bike. A good fork brace properly fitted is a good thing IN *specific circumstances*:

- racing the bike or otherwise stressing the frame and forks
( in other words, if you are racing AND the fork tubes are flexing and you have upgraded front rim width and/or are using super sticky tires).

- IF properly installed.

On the street a brace can tend to cause unwanted stiction and it definitely adds extra unsprung weight.

Again, if you have properly installed your forks, and modified your left side fork leg so it clamps the axle like the right side does already, you may find fork twist negligible under most conditions. You have to weigh the amount of fork twist perceived against the 8 lbs of extra unsprung weight a second disk adds. I have never found fork twist bad, at least not bad enough to make me say "gosh darn, I need to add an extra 8 lbs of weight to my front wheel".

suspension / fork brace / single or dual disk

Posted: Wed Apr 14, 2004 9:18 am
by BevHevSteve
Hi Phil,

It makes no difference if you have one or two disks up front with my suspension kit [shocks and fork springs]. You will simply get improved handling either way period.

I have a single disk on my [500cc] single racer and have literally stood the thing on it's nose coming down into Turn 5 at Willow Springs lapping a slow rider only to find Farnsworth and Kopecky on either side of me with nowhere for me to go.... I have never wished for more brakes on that bike, just more power!

I didn;t go rush out and buy a fork brace after that either. If the forks are properly tuned, steering bearings installed correctly, front axle installed so there is NO bind....... IOW if everything is properly aligned/adjusted etc.. Then there is no need for the added complexity of the fork brace which could cause more problems with increased stiction and added weight [albeit minute].

Adding a second disk on a street bike is great for looks and increased stopping power with less effort but really not needed IMNSHO. But if that is what you want then go for it. Remember, if your current brake system is truely maintained and functioning 100% that is half the battle, if it is not functioning, perhaps it is time for a master cylinder and caliper rebuild.

On my bevel, I wouldn't think about removing that 2nd disk, I like lots of brakes, and if I had a sport with one disk, I'd be hunting down a 2nd disk, caliper and master cylinder because that is the way I am. just remember that the caliper and disk added will increase weight in a place where you want to reduce weight so be prepared for that tradeoff.

:) YMMV and all that....

one front rotor versus two

Posted: Mon Apr 19, 2004 1:35 am
by Nico Georgeoglou
Hi Ivan,
My name is Nico and I'm writing you from sunny Athens, Greece. I'm involved with bevel twins since mid eighties and I'm riding both classic (900SS, MHR, 900SSD) and modern bikes (916) around, same like you. About 6 years ago I was restoring my 1973 leading axle electric starter 750 GT and I had decided to go for a twin disc convertion complete with Brembo pump/calipers. Everything was there including the second disc brake rotor being bought from Sassi (Italy) at the time. As I was at the shop where I was assemblying the bike, I was admiring the front 19" Borrani complete with tyre and two disc rotors ready to be installed on the bike. It flashed me to try to pick the wheel up to see how light or heavy it was. I was surprised by the fact that the wheel didn't respond to my first try of picking it up as if it was glued on the floor!!! It was that heavy!!! Immediately I discarded my plans for a twin disc brake convertion for the same reasons you are mentioning too. And anyway the GT leading axle Marzocchi fork does not have twisting problems, as it is a hefty 38 mm in diameter. Its problem lies on its shock absorbing properties which are not something to be proud of, in the sense that on bumpy roads when you are entering a curve tends to move lateraly. Imagine how this tendency would be enhanced if you add a second disc rotor/caliper. So I was riding merrily on my Brembo set up and single Scarab rotor, the car-like solid one, I'm sure you know, unless you have a later GT with spoked rotor.
Then I contacted Phil Hitchcock of Road-and-Race in Australia who informed me that he could rebuild my original Scarab master cuylinder/caliper to better that new condition. To make a long story short, I sent him over the parts for overhauling and upon their return I discarded the Brembo master cylinder and caliper for the Scarab ones. Needless to say that the overhauled Scarab system works better than the Brembo one, needs less power on the brake lever. Bike is ridden in this form till today.
The only compromise by having a single disc up front is that you need to brake more powerfully, but you needed even more so with the Brembo system.
So now that I'm restoring a similar 1973 750 Sport narrow frame, very close frame/engine numbers to my GT, I'm also using the same original Scarab system, once again sent for overhauling to Australia (though not to Phil Hitchcock this time).
So, I fully support your intention to remove the second disc from your GT. I have to add that country roads in Greece are not so perfect, so I much prefer to have a better working front suspension and apply the front brake a bit harder whenever needed.
Happy riding!!...
Nico Georgeoglou

Posted: Wed May 05, 2004 11:15 am
by Phil Porter
Thanks Steve,
I guess I'm a bit thick on the fine points of suspension tuning(read-I know nothing!). Why would you not want to know about something that adds weight to the front end, yet need to know the weight of the rider? Seems to me that it should all go into the equation. I really like your idea of a complete suspension package for our bikes. To those who did the trial and error, thanks! I am not sure what catagory I fall into as far riding style, but I am mostly interested in not getting my teeth jarred out on road seams, regardless of speed, and a ride that is comfortable for a long haul in the twisties, but not case draggin racer fast. Where does that put me on your stylin categories? Also, it would be most helpful to know just which Avons to use.

Phil Porter

one front rotor versus two

Posted: Thu May 06, 2004 1:29 am
by Nico Georgeoglou
Hi Phil,

On a moving vehicle you have two kind of weights: sprung and unsprung weights.
On a motorcycle, sprung weight is you the rider, engine, frame, fuel tank, while unsprung weight is wheels, brakes, mudguards (as long as they move with the wheels), lower (moving) part of front forks/rear shocks and approx. half of rear swing arm.
In a few words what moves up and down following the road surface is unsprung weight. This unsprung weight is your connection to the road surface and is trying hard all the time while you ride, to keep you and the rest of the vehicle as little moving/rocking and as comfortable as possible. The better unsprung weight follows the road surface and the longer its contact is with the tarmac, the more safe and relaxed the rider feels. But all this unsprung weight (tyres, wheels, brakes, etc.) has some mass and some... weight. As we all know from school the lighter a mass is, the less stamina it has, and the easier keeps changing directions when is obliged to do so by some external force applied to it. So if you consider the road surface as the external force, it is easy to comprehend that the lighter the unsprung weight is, the easier and happier will follow the road surface.
Of course the unsprung weight is not the only parameter for a good suspension: suspension geometry and rigidness of suspension components, spring rates, frame rigidity, caster and trail angles, fork oil viscocity, are all very important for your bike's handling. Therefore all manufacturers when designing a suspension system are trying to keep unsprung weight as low as possible for the reasons explained above. A good suspension system is a continuous compromise among the various parameters, but weight is where to begin with.
Bevel twin Dukes were famous for their road holding and handling, not because they had a particularly low unsprung weight, but mostly because their components were heftily manufactured and they were doing what they were designed to do, contrary to the contemporary Japanese bikes with their flimsy stanchions, which just couldn't cope with more arduous road conditions, twisting and flexing uncontrollably.
Sure enough if you add a weight of 5-6 kgs a second complete disc brake system might weigh, to your nicely handling single disc braked bevel twin, you would somehow deteriorate its handling characteristics, without gaining much in stopping power. The only advantage is that you would need less finger power on the brake lever, but it is debatable whether you would get more stopping power.
It is up to you to decide whether you would trade a good suspension for a lighter hand brake lever.

Finally the sprung weight, which much depends on the weight of the rider, is not so important to consider as the unsprung weight. I mean that if you are carrying a tankbag full of things weighing 6 kgs, this would not affect your riding as much as a 6 kgs increase of the front wheel weight would affect it.

Happy riding!!!