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Author: Subject: 'Wooden' brakes - experts please advise
johnH20

posted on 5/11/19 at 07:53 PM Reply With Quote
'Wooden' brakes - experts please advise

I have now been running my Cyclone for 4 years of occasional track days and have developed it largely to my liking. One thing still outstanding is brake feel. Car is 630 kg ( 230 F/ 400 R ), front calipers are Wilwood Dynalites ( original cast type) and rears are Escort Cosworth ( cast iron and heavy! ). I have done all the brake calcs and the system is balanced with front lock up just before rear. Pads are Mintex 1144 front ( nominal mu 0.45 ) and rear Ferodo DS 2500 ( nominal mu 0.50 ). Pedal effort is ok ( 0.55 pedal ratio ). Car stops fine but without any feedback. I find this disappointing. My benchmark car is my ( now departed ) Lotus Elise S2 ( non servo/ABS ). With standard brakes but the obligatory Pagid RS 42 pads I had great confidence braking late ( by the standards of the average Elise driver ) and trail braking into corners. This is lacking with Cyclone. What to do?
I have read before that Mintex 1144 have a reputation for feeling 'dead' but I seem to have limited options for the Dynalite calipers. Similarly I have read that the cast Dynalite calipers are 'soft' but my issue is not obviously sponginess. My pedal box appears to be rigid in the sense that I cannot detect by hand any deflection under load. What am I missing ? Answers on a post card. TIA.

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phelpsa

posted on 5/11/19 at 09:44 PM Reply With Quote
We run exactly the same arrangement on our westfield except with m1144s on the rear and find it to be excellent, although we rarely ever get any heat into them due to the short runs.

What sort of feedback are you anticipating? Brake systems dont really feedback, it tends to be felt through the steering. Or is it a lack of progression with brake force, ie its not responding to your inputs in the way you expect?

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johnH20

posted on 5/11/19 at 10:04 PM Reply With Quote
Hi phelpsa, Difficult to put into words. I guess it is basically about modulation ie response particularly on or approaching the limit and the ability to feel impending lock up. Much the same in principal to steering feel and the ability to sense the level of lateral grip, or the impending loss thereof. ( I should emphasise that I am no racing driver and not out to break lap records but I have developed a certain level of sensitivity to chassis responses as a result of my involvement in vehicle development- in my former life! )
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phelpsa

posted on 6/11/19 at 07:57 AM Reply With Quote
What tyres are you running? In terms of feedback they are the number on culprit. When we chuck old R888s on our car it becomes impossible to modulate the brakes. It could be helped with bigger mastercylinders potentially but we normally run supersoft slicks.

One thing that is a bit unusual is the mismatch of calipers despite the heavily rearward weight bias. On road tyres the brake bias should probably be nearly 50/50. I couldn't say that its definitely a problem, but it could go some way to explaining the difference.

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johnH20

posted on 6/11/19 at 05:00 PM Reply With Quote
Hi phelpsa, thanks for your input, it is appreciated. I had never thought of tyres but I do indeed have the 'old 888s' ( 185 F/ 205 R GG compound pressures generally 18 F/21 R when 'hot'. Although old they still get plenty sticky when hot, not sure of actual temps.
You are correct that my dynamic weight distribution under 1g braking is 50/50. The design problem I had was matching the front and rear piston sizes.
The Dynalites have A = 3870 mm.sq the Cosworth have A = 2904 mm.sq. However the Cosworth were the biggest road legal hand brake caliper I could find. I have compensated for this with differential m/cs and pad friction coefficients. My balance bar is 50/50 ( by eye - been fine tuned on the road ). The system is well balanced in operation with front lock up just before rear. ( My N/S front is the critical wheel due to asymetric distribution despite cross weighting as best I can.) It all works, it just doesn't feel like I want it to! Maybe the problem is the driver - not sensitive enough!

[Edited on 6/11/19 by johnH20]

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Schrodinger

posted on 6/11/19 at 07:26 PM Reply With Quote
I am no expert but could it be that you are over braked so that they become very binary i.e. on or off? so that you get no chance to feel any diference.





Keith

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johnH20

posted on 6/11/19 at 09:39 PM Reply With Quote
There might be something in that Keith. I have just been looking at the calculations I did some time ago on my benchmark Lotus Elise. The Elise has significantly higher max pedal force for 1g retardation ( 33 Kg vs 25 Kg ) than my Cyclone. On the other hand we can all modulate servo brakes on our daily drivers without a problem so there must be some thing else involved as well. I might try some bigger M/Cs as suggested above.
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jeffw

posted on 6/11/19 at 09:45 PM Reply With Quote
I would certainly be looking to run the same pads front and back. Hawk DC-30 or similar would be a good idea.
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phelpsa

posted on 7/11/19 at 08:50 PM Reply With Quote
25kg/g is slicks and wings sort of territory. Even 33kg/g sounds low (thats about where we are on slicks), although I guess thats the balance you have to make for a road car with no servo. For a track car on road tyres i'd be looking for 40-50.

If you do change master cylinders please report back on your findings!

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johnH20

posted on 8/11/19 at 04:16 PM Reply With Quote
That is very interesting phelpsa. So is the logic that better modulation is achieved at higher pedal loads? If so I am not sure how I would square that with a well speced servo system, road car or otherwise. I also track my modestly modified MX5 ( when it is cold or wet! ) and seem to receive better response/feedback from that than my Cyclone. ( Twice as heavy and front biased under braking of course - also on 888s or R1Rs ), Again appreciate your input - I have a lot to learn.
PS. last year I increased the pedal ratio from 4:1 as supplied to 5:1 which seemed to improve things. The Lotus I think is 6:1 . Willwood pedal boxes are also more than 5:1 as I recall. So maybe bigger M/Cs and bigger pedal ratio might be the right direction, at least for a non racing driver!

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phelpsa

posted on 8/11/19 at 07:44 PM Reply With Quote
Servos are very non-linear, depending on the setup they are often not contributing much, if anything, by the time full pedal pressure is reached, particularly on cars without ABS. Modern ABS equipped cars have dual stage servos and take brake force modulation completely out of the drivers control.

The theory I believe is that your leg muscles are most sensitive to forces in a range above that of your body weight, so it is quite difficult to modulate small forces at 1g deceleration.

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johnH20

posted on 9/11/19 at 05:29 PM Reply With Quote
Thanks for that insight.
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