smart51
|
| posted on 31/7/11 at 08:48 PM |
|
|
Brake pedal feel, is this just the way it is?
I replaced the brake hoses on my Suzuki Cappuccino this weekend to improve brake pedal feel. I'm a little disappointed with the result. I did
switch from Halfords brake fluid to Silkolene. It's hard to describe in temperate language how much better the silkolene is. It bled so
easily. There were 3 tiny trapped air bubbles in one calliper and none in the others. I used half as much fluid because I didn't need to pump
through so much.
Anyway, the pedal is quite soft under gentle braking and become firmer as you approach tyres locking up. Push beyond this (when stationary) and the
pedal gets firmer but still moves quite a bit. It feels like pushing against a spring. The car brakes cleanly at all pedal pressures and round
corners in both directions, so I believe it to be bled correctly. The kit car had a little travel on the return spring before a needing firm pressure
to apply the brakes. There was little travel and once the brakes were on, the pedal was solid and barely moved any further when pressed very hard.
Is what I'm seeing on the production car just a symptom of a long pedal design that takes little foot force to apply the brakes?
|
|
|
|
|
norfolkluego
|
| posted on 31/7/11 at 09:15 PM |
|
|
Guessing here, do they build some 'feel' into the pedal to cater for the servo
|
|
|
britishtrident
|
| posted on 1/8/11 at 06:20 AM |
|
|
Stuck caliper or misaligned caliper --- classic symptoms.
In the real world changing hoses will never make a detectable difference to pedal feel unless something was wrong to start with, like wise
changing fluid from anything other than bog standard universal DOT4
|
|
|
smart51
|
| posted on 1/8/11 at 09:01 AM |
|
|
Misaligned calliper? I've never heard of this. What is it?
|
|
|
adithorp
|
| posted on 1/8/11 at 09:09 AM |
|
|
...or the simple answer... There's still air in the system?
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
|
|
|
daviep
|
| posted on 1/8/11 at 09:35 AM |
|
|
quote:
Originally posted by smart51
Misaligned calliper? I've never heard of this. What is it?
If the caliper mount is bent or in some other way not positioned over the disc in the correct position then the caliper mount and disc will flex as
the brake pedal is pressed.
Best example is when single pot caliper slides sieze and the caliper doesn't move to take up the wear on the outer pad, you get a longer pedal
and if you have an assistant push the pedal you can see the disc bending until it comes in to contact with the pad again.
Davie
“A truly great library contains something in it to offend everyone.”
|
|
|
Neville Jones
|
| posted on 1/8/11 at 11:13 AM |
|
|
quote:
Originally posted by britishtrident
In the real world changing hoses will never make a detectable difference to pedal feel unless something was wrong to start with, like wise
changing fluid from anything other than bog standard universal DOT4
Oh dear dear dear, you're at it again. Obviously you've never changed from rubber hoses that stretch and balloon, to braided teflon as in
Goodridge and similar.
Chalk and cheese. Why do you think racecars use braided/teflon, if by any chance the rubber would be better?
Cheers,
Nev.
Where's my avatar gone? When it's needed!
|
|
|
JAG
|
| posted on 1/8/11 at 11:28 AM |
|
|
quote:
Oh dear dear dear, you're at it again. Obviously you've never changed from rubber hoses that stretch and balloon, to braided teflon as in
Goodridge and similar.
Chalk and cheese. Why do you think racecars use braided/teflon, if by any chance the rubber would be better?
I'm afraid it is YOU who are wrong. Modern rubber hoses are as good if not slightly better than the braided steel/stainless steel hoses that
everyone seems to recommend. I would suggest you start being a little more circumspect with your replies 
As for the OP's original problem it could be any of the above.
Did you check for misalignment and sticking of calipers before embarking on a hose change?
It sounds like you did a thorough bleed - although I think the Cappuccino has rear calipers? with a built in handbrake mechanism? If so then these
type of rear calipers are notorious for holding trapped air.
It may just be a function of the master cylinder Vs caliper bore sizes and it is quite dependant upon how the original system was designed by Suzuki.
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
|
|
|
smart51
|
| posted on 1/8/11 at 12:00 PM |
|
|
Thanks Justin,
Yes, the cappuccino has rear callipers with handbrakes. The bleed nipples are on the top of the calliper, which should aid bleeding. Are you
supposed to bleed with the handbrake on or off? I pumped it several times and then bled with it on. I figured that with the pedal depressed during
bleeding, the brakes would be on anyway.
I haven't checked the rear brakes for 2 years, since I rebuilt the callipers with new slides and seals. I've never checked the front
callipers, or any of the alignments. I suppose bending a brake disc to suit a partially working calliper would feel like a spring.
|
|
|
JAG
|
| posted on 1/8/11 at 01:22 PM |
|
|
Hi Colin,
Yeah I guess misalignment could behave like a spring - but I'd have to say it seems quite unlikely to me that it's either misalignment or
a sticking slider. Quite easy to check and worthwhile I guess before undertaking anything bigger/more complex.
If it were me I'd focus my attention on re-bleeding those rear calipers.
The nipple at the top is helpful but the biggest issue is air trapped in and around the internal handbrake mechanism. Inside your calipers there are
cam's, springs and a cone clutch arrangement. It all helps to hold the air bubbles and stop them making their way upto the bleed nipple.
In my previous role we have had to re-assemble this kind of caliper under the surface of a bath of brake fluid to ensure that all the parts were
properly filled without air bubbles. In service we used to supply some VM's calipers pre-filled with brake fluid 
Best tip I ever found for getting a good bleed was to bleed as normal, handbrake position makes no difference. Close the bleed nipple and then pump
the rear caliper piston all the way out (without it popping out of the seals of course). Then open the bleed nipple and wind the piston back in. This
should release some more air from the internals. Then re-bleed as normal. You need to do this on each rear caliper.
If you still have a poor pedal feel and think it's still bleed related then you have to repeat the process. I'm afraid that's not an
easy fix but, as far as I'm aware it's the only thing that works other than extended bleeding for hours with no guarantees!
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
|
|
|
Neville Jones
|
| posted on 1/8/11 at 03:33 PM |
|
|
quote:
Originally posted by JAG
quote:
Oh dear dear dear, you're at it again. Obviously you've never changed from rubber hoses that stretch and balloon, to braided teflon as in
Goodridge and similar.
Chalk and cheese. Why do you think racecars use braided/teflon, if by any chance the rubber would be better?
I'm afraid it is YOU who are wrong. Modern rubber hoses are as good if not slightly better than the braided steel/stainless steel hoses that
everyone seems to recommend. I would suggest you start being a little more circumspect with your replies
.
Try a back to back trial with pressure gauges and good measuring tools, before you start thinking modern rubber hoses are as good as braided teflon
ss.
When you've done that, then you might want to revise your reply.
As I pointed out, if the modern rubber hose was as good as the Goodridge type, then racecars would most certainly be using it! There is good reason
for using Goodridge/aeroquip, and it aint ease of use.
Cheers,
Nev.
|
|
|
smart51
|
| posted on 1/8/11 at 04:31 PM |
|
|
Neville, Justin used to design braking systems for a living. I'd take his word over almost anyones.
|
|
|
Neville Jones
|
| posted on 1/8/11 at 06:10 PM |
|
|
quote:
Originally posted by smart51
Neville, Justin used to design braking systems for a living. I'd take his word over almost anyones.
And I still do, but not for road cars.
If Justin is as experienced and knowledgeable as you make out, then he would know that the difference in expansion of current rubber auto industry
hoses and those used in the motorsport arena, are measureable and quantifiable.
You'd have to be deaf dumb and blind(no offense intended to anyone falling into those categories) to think otherwise.
If Justin would like to make the measurements and post them, then I will do the same, and we'll see if our numbers match.
I was asked by a well known braking manufacturer today to install a brake pressure gauge in a car to assess some out of the ordinary behaviour.
I'd be more than willing to do a quick like for like test if the situation calls for it.
Pump 1200psi into a teflon hose, then a rubber item. You'll feel and see the difference.I've done this test in the past, happy to do it
again. Modern hoses can have kevlar or other high modulus fibres, but still won't match the racing hoses.
If you've changed from rubber to teflon/braided, you may have moved the 'buffer' in the system from the (rubber) hoses to the
calipers, which can flex across the pistons. A lot more than some may think, particularly if ali calipers.
Then again, air could just as much be the culprit.
If you want some fun, try bleeding four pot calipers, with the fluid inlet between the pistons, and bleed nipples top and bottom. Vacuum, or open top
and bottom nipples, and push fluid up through the bores with an external pressure bottle.
Cheers,
Nev.
[Edited on 1/8/11 by Neville Jones]
|
|
|
Doug68
|
| posted on 2/8/11 at 05:48 AM |
|
|
Ok on a 2002 WRX I swapped the rubber hoses for Goodridge ones, it did make a difference to the feel but it was not the huge difference I would have
expected from past experience on motorbikes.
So I would expect on a servo assist car you are never going to get the same direct feel as on a non-servo assist system such as in a 7 or on a
motorbike.
Doug. 1TG
Sports Car Builders WA
|
|
|
MikeRJ
|
| posted on 2/8/11 at 07:19 AM |
|
|
Aeroquip hoses do make a big difference on bikes, but then you are applying pressure with your fingers which are a lot more sensitive than a foot with
a shore wrapped around it, and the brake lines tend to be quite long compared to car lines.
Never been very impressed with them on cars, they just don't seem to have enough flexibility to deal with the amount of articulation they get
(especially as they tend to be quite short) and it can be difficult to stop them rubbing on things as they twist up under steering lock. I
wouldn't even consider them for a road going tin top, but for a kit car the ability to get custom lengths made with whatever end fittings you
require is extremely useful.
|
|
|
Neville Jones
|
| posted on 2/8/11 at 09:57 AM |
|
|
On the caliper flex situation...
Consider this. With the average single piston being say 50mm, and the m/cyl being 22mm, then the area ratio is ~5+. If the caliper flexes 1mm,then the
m/c moves 5mm.
If the pedal ratio is 4:1, then that 5+mm at the m/c translates to 20+mm at the pedal end. Multiply this by the number of pistons in the system, and
the flex be 0.1mm, or much more as it can be and often is.
This can be,and is felt as 'spring' at the end of the effective pedal movement. The 'sponginess' is the caliper metal
operating in the elastic range of the metal. It will happen with all calipers, regardless of material.
And as ripley said, 'believe it or not'.
( As an r&d exercise for a customer more than 20 years ago, I made a carbon fibre caliper body. Even that flexed, in spite of being made from
high modulus carbon and claved to within a micron of its life!)
I know this flex to be a reality, and not easy to deal with. Most road cars will never have to contend with the consequences, as they rarely operate
at sufficient line pressures for it to be discernable or a concern. Racecars on the other hand, have the ability to 'life' parts so that
failure is not a consideration.
Cheers,
Nev.
[Edited on 2/8/11 by Neville Jones]
|
|
|
Frosty
|
| posted on 2/8/11 at 11:08 AM |
|
|
This thread has gone off topic a bit, but it's worth pointing out at this stage that a race cars usage of braided brake hoses is far less about
feel, and far more about temperature handling.
Braided hoses tend to handle a lot more temperature - if rubber hoses were used on race cars, the heat would make their longevity very
unpredictable.
I can't say I've ever noticed a pedal feel improvement just from braided hoses alone, but then I have never fitted them without changing
something else at the same time too, so it's hard to tell really.
Windscreen Wipers
Need windscreen wipers for your car? Order from a friendly kit car owner.
Don't pay a garage to do them. You built a car for God's sake - I'm sure you can fit your own wipers! 
|
|
|
whitestu
|
| posted on 2/8/11 at 02:09 PM |
|
|
Totally irrelevent to this topic, but I was always impressed with the high pressure systems on the Citroen GS that didn't use any rubber hoses -
even the flexible ones to the rear wheels were metal.
Stu
|
|
|
Dick Axtell
|
| posted on 2/8/11 at 03:59 PM |
|
|
quote:
Originally posted by JAG
In service we used to supply some VM's calipers pre-filled with brake fluid.....
Then they ruined all the prep by emptying out all the pre-fill fluid! And then they complained about pedal feel (& excessive travel)!
[Edited on 2/8/11 by Dick Axtell]
Work-in-Progress: Changed to Zetec + T9. Still trying!!
|
|
|
