M900 rear brake - quick question

[spuggy]

So, I've noticed some 900's have the rear caliper above the swing arm, and some (seems to be the older ones) have it below.

I know that some 900's have a floating caliper - and I'm pretty sure that my Dark (which keeps its caliper above the swing arm) is the cheaper fixed variant.

So - what do the floating calipers look like?

Are the ones that mount below the swing arm the floating calipers, or are the floating calipers different again and there's two types of fixed caliper mount?

If there's two types of fixed caliper mount, any advantage to mounting above/below the swing arm (other than moving the CoG lower)?

[Darren69]

The floating rear caliper is fixed to the rear of the crankcase by a tie rod.

As shown on this 900S

http://www.steinborn.org/jim/forSale/monster/rearsets-CF.jpg

[Darkness]

Most m900 have fixed callipers, with a slotted part of the calliper bracket fitting over a spigot on the inner face of the swing arm. The floating calliper has a similar bracket, but differs in that it is designed to turn on the rear axle. It is prevented from turning much by a tie rod to the engine.
The purpose of this is to take the braking moment out of the suspension, though depending on the precise geometry of the parallelogram, it is theoretically possible to make the rear squat or rise on braking.
The 888/851 racers seem to like them (Same rear suspension), but I don't think my riding would benefit from the "improvement", but it does look pretty trick provided it's not hidden behind an exhaust can. The upswept cans on Daren69's photo' show it off well.

[Darkness]

Callipers below the swing arm keeps the weight lower, which is good, but makes bleeding harder (Unless you unbolt it from the carrier and raise it up on a bungee for bleeding) and subjects the calliper to more flying crap off the road.

[Dukedesmo]

As above.

I remember reading something about the floating setup that despite the claimed advantages of keeping braking forces out of the suspension, when they (Ducati Corse) did extensive testing they could find no real benefit and none of the riders could actually feel any difference.

As said they were used on 851/888 race bikes but the setup was not used on any later models for this reason (also wouldn't fit on 916 models as the disc is 'inside' the wheel).

I've got the setup on my Monster and I reckon the main advantage is similar to that of a fully-floating disc in that it may cause less drag when not braking.

http://ducatiforum.co.uk/data/photos/l/7/7726-1491812895-c9e489c4ade0fe3e2f5b1d7e78388655.jpg

[350TSS]

A long time ago when I was racing a 350 TZ Yamsel a guy convinced me to put a floating rear disc on the bike. At the same time I had some magnesium hubs on spoked wheels (made from Kart wheels) which were a lot lighter than the original drum braked Yamaha TZ wheels I had fitted. The difference with the lighter wheels and floating rear disc was night and day. One of the best and safest tuning mods for any bike is lighter wheels and in my view the second best is a floating rear brake caliper.
With a fixed rear caliper if you lock the wheel you effectively lock the suspension because for the suspension to work the contact patch for the tyre has to move fore and aft, with it locked the suspension can only move by skidding the tyre. Before if you went in too hard on the rear brake it would lock and then hop and you would get a rapid fire erch! erch! erch! as the locked wheel skipped.
Afterwards you could actually use the back brake hard particularly useful if you go in too hot and have to come off the front brake in order to turn in and make the corner but still need to scrub off some speed.
The geometry of the parallelogram has to be exactly right otherwise the suspension has to fight the locking wheel although to a lesser extent than with a fixed caliper. In the picture above the geometry is not quite right, the pivot point for the spherical bearing at the end of the reaction rod should be in the middle of the disc on the diameter and the rod needs to be displaced from the wheel spindle by the same measurement as the distance between the swinging arm pivot and the location of the spherical bearing attached to the crankcase. The parallelogram has to be just that - a parallelogram

[Nasher]

Great explanation 350TSS, thanks.:thumbsup:

Nasher

[Nasher]

350TSS
From your explanation I can fully understand why it works so much better than a fixed caliper, and also why your comment that the parallelogram needs to be perfect to work at it’s best is important.

So I assume it would be advisable to adjust the length of the calliper tie rod when adjusting the chain.
I’m not sure each individual small movement of the rear wheel backwards would make a difference on it’s own, but the difference in position for a new and end of life chain could possibly be noticed.
An out of adjustment rod length by a long way could even set up odd suspension reactions under braking that are worse than a fixed calliper.

Nasher.

[Dukedesmo]

The geometry of the parallelogram has to be exactly right otherwise the suspension has to fight the locking wheel although to a lesser extent than with a fixed caliper. In the picture above the geometry is not quite right, the pivot point for the spherical bearing at the end of the reaction rod should be in the middle of the disc on the diameter and the rod needs to be displaced from the wheel spindle by the same measurement as the distance between the swinging arm pivot and the location of the spherical bearing attached to the crankcase. The parallelogram has to be just that - a parallelogram

Very interesting stuff 350TSS. Although I'm not quite sure I understand the correct geometry you're describing?

In the above picture what would I need to adjust or alter to get it right? because the only available adjustment is the rod length and then not by much...

[utopia]

One of the best and safest tuning mods for any bike is lighter wheels and in my view the second best is a floating rear brake caliper.



As good as that ?
Such high praise may cause me to reconsider the notion at some stage.
Up to now I have been of the opinion that the benefits were minimal and would probably be pointless on a road bike where serious braking into corners would (should ?) be rare.
For that reason I have preferred the simplicity and clean lines of the fixed caliper.
I will now revert to my normal position ... undecided but pondering.

Envious of the Yamsel.
Saw an ex John Cooper one for sale at the classic show a few years back for under £2k.
No idea what I would've done with it but I still regret letting it pass by.

[Darren69]

Very interesting stuff 350TSS. Although I'm not quite sure I understand the correct geometry you're describing?

In the above picture what would I need to adjust or alter to get it right? because the only available adjustment is the rod length and then not by much...

I think he means the tie rod should be parallel to the swing arm for best effect? I'm guessing the Ducati setup may be limited because of space to do that because of exhaust pipes and so is the best compromise they could come up with.

[Dukedesmo]

I think he means the tie rod should be parallel to the swing arm for best effect? I'm guessing the Ducati setup may be limited because of space to do that because of exhaust pipes and so is the best compromise they could come up with.

OK, that makes sense. As it is it's not parallel but looking at the way the caliper bracket is made - it's a two-piece bracket and where the two join there is a bolt that, if the tie rod were bolted to it, would make the tie rod more or less parallel to the swingarm, certainly more so than it is currently.

I think I need to look into whether this would work, would need a spacer as the hole (where the button-head screw is) is counterbored, not sure if it might move the rod out too far though?

[Darkness]

With a fixed rear caliper if you lock the wheel you effectively lock the suspension because for the suspension to work the contact patch for the tyre has to move fore and aft, with it locked the suspension can only move by skidding the tyre. Before if you went in too hard on the rear brake it would lock and then hop and you would get a rapid fire erch! erch! erch! as the locked wheel skipped.
Afterwards you could actually use the back brake hard particularly useful if you go in too hot and have to come off the front brake in order to turn in and make the corner but still need to scrub off some speed.

Hi 350TSS, I'm afraid you've lost me with that bit of your explanation?
If the wheel is locked, surely the suspension behaves as though there's just a sprung and damped trailing arm with a curved rubber pad on the bottom? Calliper mounting becomes irrelevant?

[utopia]

Does it help if you consider that the wheel is locked to the caliper in both cases (when the brake is applied), but the caliper isn't locked to the swingarm in the floating set-up ?

[Darkness]

Does it help if you consider that the wheel is locked to the caliper in both cases (when the brake is applied), but the caliper isn't locked to the swingarm in the floating set-up ?
No, because if the wheel isn't turning, the road can't tell any difference?

I've also been giving some thought to where the moment goes when the brake is applied (With, or without locking the wheel): considering moments about the axle of the back wheel, the rear braking torque is ultimately resisted by a weight transference to the front wheel.

A fixed calliper creates a bending moment in the swing arm, which pushes the swing arm pivot down (There can't be a bending moment at a pivot), with a balancing force in the rear spring and damper. The forces generated in the swing arm pivot and rear spring are a couple that cause dropping of the swing arm pivot and rotation of the engine.

I'm now thinking the floating calliper creates the same effect at the swing arm pivot, except the balancing force is taken by the tie rod rather than the rear shock and damper.

So I'm now more convinced that the only real benefits are that it looks good and takes some bending/twisting out of the swing arm.

[Dukedesmo]

Intrigued by all this I've been looking at/measuring the setup on my bike. To get a true parallelogram I would need to raise the mounting point on the caliper (or lower that on the crankcase), this could be done by moving it to the other bolt and this should work as it's the same size bolt (8mm) so strength not compromised but as mentioned it would need a small spacer to accommodate the counterbore in the bracket, slightly offsetting the alignment with the swingarm.

This would leave the other 2 sides of the parallelogram (the offset from the swingarm spindle vs that of the rear wheel spindle) wrong and to achieve this would require a significantly longer tie rod as the front mount is directly below the swingarm pivot whereas the rear is quite a way in front of the wheel spindle.

This, in turn would also throw the caliper quite a way back/around the disc which might look a bit odd?

A new front mounting bracket with a lower bolt hole position could also solve the parallelism but would then need to see if it causes clearance issues with swingarm brace and exhaust?

It's certainly interesting but I'm not sure whether it's worth all the effort, especially for someone who uses very little rear brake?

[Nasher]

This is all rather interesting, but I'd suggest that the average rider, including me, would need to be pushing an M900 a bit to feel the difference.

On the other hand jumping into this mod and getting the geometry wrong could cause some odd reactions at the back end under braking that could be dangerous.

Nasher.

[Dukedesmo]

On the other hand jumping into this mod and getting the geometry wrong could cause some odd reactions at the back end under braking that could be dangerous.



Possibly but, although mine is not OEM equipment, the kit that Ducati fitted to various models has more or less the same 'wrong' geometry, so I would say it's safe to assume that it's not dangerous. Unless of course there have been incidents due to it?

[Darren69]

If you think of the principle of Newtons 3rd law "every action has an equal and opposite reaction" and simply by moving the opposite reaction from the swingarm and rear suspension it reduces rear squat or brake jack allowing the swingarm and rear suspension to still move which is useful on uneven surfaces which might otherwise cause the wheel to lock. Its really only of benefit if you use hard rear braking especially on uneven surfaces or when deep into a corner where a wheel lock would cause a slide.

Some manufacturers have used a similar principles on the front of the bike to reduce brake dive (BMW and Yamaha) are 2 that spring to mind.

[Luddite]

This is a very interesting thread. While I don't pretend to understand the mysteries of frame design, someone who does is Tony Foale. Those of you of a certain vintage will probably recognise the name; he was a big advocate of "funny from ends" and designed some really ground breaking chassis in the eighties and nineties. Perhaps his most well know design was the QL (Quantum Leap) complete with joystick controls instead of handlebars.

Have a look at his website to learn more about a true genius. www.tonyfoale.com

Anyway, back on topic, he wrote the bible containing everything you could possibly want to know about steering geometry and more: "Motorcycle Handling and Chassis Design: The Art and Science". A fascinating book, both highly technical but also highly accessible too.

He's done research into the effects of fixed and floating calipers and the results of those tests can be seen here:

http://i873.photobucket.com/albums/ab292/vincebessant/Mobile%20Uploads/wp_ss_20170410_0001_zps7lp1rvj9.jpg (http://s873.photobucket.com/user/vincebessant/media/Mobile%20Uploads/wp_ss_20170410_0001_zps7lp1rvj9.jpg.html)

As you can see the differences are actually measurable and affect not only the rear suspension, but the front as well.

[Dirty]

And they just look good :)

https://s30.postimg.org/9f7tyuum9/image001.jpg

[350TSS]

Missed the recent responses to this thread, Luddite, thanks for posting the Tony Foale stuff I am definitely going to try and find his book.
If I read the two graphs correctly (and one needs to be careful about the source of the information because the tyre loading in newtons may not tell you that much about how it feels in practice for that you would need standardised road conditions, temperatures, tyre adhesion levels, brake pressure levels and bumpiness of the surface of the 4 plots) then the floating caliper provides better and smoother braking at the rear and better and faster and more even load transfer to the front and therefore better braking there also.

[350TSS]

Hi 350TSS, I'm afraid you've lost me with that bit of your explanation?
If the wheel is locked, surely the suspension behaves as though there's just a sprung and damped trailing arm with a curved rubber pad on the bottom? Calliper mounting becomes irrelevant?

The curved rubber pad as you describe it (tyre) is designed to grip the road and to resist fore and aft (and lateral) motion. at the point where the wheel is locked the wheel cannot rotate. At this point the suspension which because of the arc prescribed by the length of the swinging arm cannot move because it requires rotation of the wheel in order to do so.
The suspension, whilst not technically locked, in order to move up and down has to overcome the tyre grip with the road. Obviously a longer swinging arm reduces this effect because the change in contact patch for each 25mm of suspension movement is reduced.

[Luddite]

Missed the recent responses to this thread, Luddite, thanks for posting the Tony Foale stuff I am definitely going to try and find his book.

I think that the paper version of the book has been out of print for some time and second hand examples are advertised at silly prices (well into three figures). Your best bet might be to go for the digital version, which can be purchased from his website for €50.