rodgling
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| posted on 27/4/15 at 08:05 PM |
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Caliper flex
After more investigation looking for the cause of my soft brake pedal, this is what I found (firm but not silly pedal pressure applied):
https://www.youtube.com/watch?v=kgiLzdwBbag
https://www.youtube.com/watch?v=SXxliocseS0
With the pedal released, the pads are perfectly square to the disc, so this does appear to be 100% caliper flex, about 0.4 mm each side. Callipers are
Wilwood Powerlites.
Is this normal? Safe? I'm certainly not thrilled about the resulting soft pedal, perhaps an expensive and time-consuming step up to Alcon or AP
is the way forward.
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JAG
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| posted on 27/4/15 at 08:13 PM |
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This is not unusual but it is very undesirable.
It's the one thing that's good about the Girling M16 - they're very stiff even if they're heavy!
What size pistons are in those calipers?
Do you know exactly how much the caliper deflects?
Do you know what line pressure you are using?
In a modern road car 2.0 - 2.5 cc at 100 bar is a good benchmark.
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
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daniel mason
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| posted on 27/4/15 at 08:16 PM |
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Powerlites will flex. Even the dynalite race caliper on the radical flexed a bit under load! The hi-spec race caliper replacements were much better.
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JAG
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| posted on 27/4/15 at 08:20 PM |
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Is it safe?
It kinda depends upon the Stress in the caliper housing and the number of Stress cycles that you will put the caliper through while the car is being
used. It needs careful analysis and then a very good understanding of what we call the 'Duty Cycle'
If the fatigue life of your caliper is low enough, the Stresses are high, the caliper will fail while in use - if the stresses are low then it may
well survive for the entire time you own the car or the entire life of the car.
It's something that big caliper manufacturers and vehicle manufacturers test for by cycling the caliper at the 1.0 to 1.5g hydraulic pressure
and up to 1 million cycles.
There is no easy way to do this with Kitcars - you are at the whim of caliper manufacturers like Wilwood etc...
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
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RichieW
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| posted on 28/4/15 at 07:21 PM |
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That really is a bit of an eye opener. Something to remove from my wishlist.
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adithorp
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| posted on 28/4/15 at 07:41 PM |
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All a bit strange, given there's lots of cars with Wilwood Powerlights (mine included) that don't have spongy brakes. Not heard of any
fatigue failures either and mine has had 7yrs' hard labour.
Could it be excessive hydraulic pressure caused by too much pedal leverage or too smaal master cyl' bore?
Hi-Specs however are well known for distorting, dragging and leaking. I know of 2 friend who've had issues on the road and there were multiple
reports from RGB racers
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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britishtrident
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| posted on 28/4/15 at 07:41 PM |
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As an experiment it might be interesting to put a large G clamp on each calliper and see how much the pedal travel is reduced.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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rodgling
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| posted on 28/4/15 at 09:34 PM |
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Don't think it's excessive pedal force or m/c size - it's a 0.625 (calliper bore is 35mm) and it takes very little force to feel the
softness in the pedal. After a bit the pedal goes pretty solid when you apply roughly emergency-stop level braking effort. Maybe a larger m/c would
help, or alternatively smaller calliper bore?
Yeah I also wondered about attaching a large G-clamp. Might give it a go if I can find a way to fit something.
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britishtrident
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| posted on 29/4/15 at 06:37 AM |
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The hydraulic line pressures will be expected a lot lower than a tintop and the symptom of the pedal eventually becoming solid under pressure would
be more typical of a calliper alignment issue than excessive calliper flexing.
The other possibility is of course a seized piston or pad but if it is happening on both sides then this is less likely.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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coyoteboy
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| posted on 15/5/15 at 05:59 PM |
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I see flex like this on OEM calipers too, to some extent but not quite this much. That said I'd be surprised if they didn't flex like this
as a light caliper. Are they multi-part? Are the joining fasteners up to torque?
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motorcycle_mayhem
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| posted on 15/5/15 at 07:54 PM |
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The flexing of Powerlites is well known in the paddock. I do not know of any fatigue failures, however, but if anybody else has I wouldn't be
surprised. These are very cheap calipers, which seem to be quite adequate for use.
I watched the pair on my Westfield flex dramatically braking for the hairpin at Pembrey. I then went to 'Outlaw' calipers, which were
stiffer, but more expensive (of course). Last pair I used on the car were metallic blue Brembo's, unsure of model, but they were incredibly
stiff and light.... eye wateringly expensive though.
I used 1.38mm Powerlites by the way, with a 0.7" cylinder (Front).
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cloudy
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| posted on 15/5/15 at 08:27 PM |
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Are you using the wider powerlites for vented discs? If so I guess the wider bridge reduces it's stiffness
www.warnercars.com
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rodgling
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| posted on 15/5/15 at 11:22 PM |
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Yes, 1.38" pistons over vented discs. Now experimenting with a bigger master cylinder to reduce pedal travel - hopefully this won't result
in the inability to generate enough force to lock the brakes...
Ultimately the right answer would be some decent AP callipers I suppose but will keep trying to get a decent setup out of the powerlites first.
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rodgling
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| posted on 31/7/15 at 11:46 PM |
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Update: fitted a 0.875" master cylinder and also a pressure sensor.
Now the pedal feels superb, travel is not excessive at all and leg effort is about right, probably on the high side of what most people would want but
on a track car it's exactly what I would want.
Maximum braking effort at Snetterton used about 710 psi (49 bar) of line pressure (this gave a peak of 1.5g so clearly around the limit). Wilwood test
the callipers up to 1500 psi and their website reckons normal use is 900-1200 psi, so I am no longer worried by the flexing.
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