Caliper placement re-birth

[BrianGar]

thread78-253307

Found this and thought of the caliper placement thread once again, interesting, thats all,
3/4 of the way down link, twin calipers nicely packed in there!

 

[potteryshard]

Upon reading the caliper placement thread, I was surprised to note that no one mentioned what I remember reading lo those many years ago when I was a teenager the main reason for placing calipers behnd the spindle: Although lessening of bearing forces is a bonus, the primary reason suggested by the suspension design book I vaguely recall is so that the caliper acts as a mass balance aft of the steering axis to reduce wheel shimmy. It is curious what goofy little factoids we remember...

 

[ivymike]

a mass balance aft of the steering axis

it's not clear to me what's meant by that. if you're suggesting it's added inertia, then why is it important whether it's in front or behind?

 

[evelrod]

Remembering back to when I first built a Lotus Cortina, 1966 or so...The stock Cortina GT had the Girling calipers mounted to the rear and the Lotus had them mounted to the front...Go figure.

I still mount our calipers to the front on both the Lotus Cortina and Mini Cooper race cars. On the Mini that's the only way they will fit and still have sufficient clearance for steering bits.

Rod

 

[potteryshard]

The friction force of the bearing translates to a drag acting toward the rear of the car... Any slop in the linkage is going to allow the tire/wheel assembly to rotate slightly around the steering axis which would in turn create a slight left or right horizontal component added to the overall drag at the spindle.

It seemed to me that the intertial vector of any excess mass in front of the steering axis would add to the disturbing force whereas the inertial of any excess mass aft of the steering axis would add to the restoring force.

Or so I reasoned out in analogy to the mass balance effect of aerodynamic control surfaces. I don't claim any of this is factual; this is something I recall reading in excess of 45 years ago... And, I don't claim to understand it now much less then.

 

[Tmoose]

Unlike most cars, Motorcycles with front forks pass their steering axis an inch or more behind the wheel center.
"Front" mounting a caliper with that steering geometry generally places it at a greater distance/larger radius from the pivot, compared to rear mounting. The result is greater inertia, and a somewhat lower natural frequency (resonance)for basic wobble type steering mis-behaviour phenomena. It seems unlikely to me that the change could be enough to move the resonant frequency above operating speed. But whether by fashion or for genuine function race bikes use rear mounting a lot.

http://www.khulsey.com/portfolio_stock_race_motorcycles.jpeg

 

[evelrod]

As I also ride bikes and have had a few "wobble generators", read that 60 and 70 era Japanese motorcycles...
Mounting the brakes, dual discs as example, to the rear makes sense if for no other reason than the wheel/caliper wt ratio. On most of the later Japanese bikes I have owned, the dual front discs were rear mounted...don't think I've ever seen it any other way. On the race cars, in my case, the Lotus Cortina (the Mini Cooper is a separate case as the brakes can only be mounted in the front because of steering issues), the overall mass of the wheel/tire make the placement of the rather light wt calipers... or at least the overall effect to steering/handling of the race car, rather moot.

Rod

 

[Tmoose]

"don't think I've ever seen it any other way."


These guys from new Hampshire mounted the caliper forward of the fork leg back in 1973.

http://www.vintagecycleprints.com/backissues/cycle/7304cyc.JPG

 

[evelrod]

Yes, Tmoose...I should have remembered that. I rode a Rokon in the early 70's. It was "different". It not only had the brake caliper mounted forward, it was also the first dirt bike I had ever seen WITH a disc brake.

Look closely at his left boot...no gear change. Wanna know why?

Rod

 

[dgallup]

Rod, don't let that belt get wet!

Those cast wheels didn't work out too well off road either.

 

[Tmoose]

"............don't let that belt get wet! Those cast wheels didn't work out too well off road either."

=================
For one brief shining moment the belt drive, cast wheels and caliper placement were not insurmountable impediments to good finishes at a significant off road event.

48th International Six Days Trial (1973)
Dalton, Massachusetts, USA

picture of ROKON team in Dalton woth a few cast wheels here -

http://www.speedtracktales.co.uk/mediac/400_0/media/DIR_68919/RokonTeam2.jpg

1973 ISDT results - American riders only
350cc class: 27 riders
6th Jack Penton(240) Penton Gold
12th Ron Bohn(229) Husqvarna Silver
13th Gary Snider(236) Rokon Silver
14th Carl Berggren(225) Lind-Qvarna Bronze
15th Jim Fogle(232) Rokon Bronze
17th Ron LaMastus(223) Rokon Bronze
18th Jim Simmons(244) Rokon Bronze

 

[thruthefence]

Aren't they still being manufactured? (Rokon)

 

[WhizzMan]

I'm guessing the automatic made the whole gear/clutch/chain (see, no lever on the handlebar) drive line NA. The title on the photo sort of gives it away as well.