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Forum réservé aux questions techniques du Flat 4
Greenfairy a écrit :j'allais écrire ça...carrera79 a écrit : coucou
merci pour la réponse,
j'espère que tu n'auras pas de soucis car la pose de frettes est très difficile, il faut surtout une bonne méthodologie afin de s'assurer qu'elles ne vont pas bouger au cours de leur vie.
il vaut mieux être sur de l'operateur car une frette qui decroche dans une
descente de col et ça peut être ton dernier voyage...
je crois que les specialistes procedent par chauffage du tambour, insertion
de le frette et retraction pa refroidissement.
pour info j'ai eu un problème de désinsertion de frettes sur des tambours,
neufs de surcroit, et à deux reprises il y a 10 ans environ, preuve que ça existe
j'ai encore des photos du problème qui a été résolu par la pose de vis de blocage, comme sur les tambours de bugatti.
http://www.google.fr/url?sa=t&rct=j&q=& ... meEaLjy58Q
Donc je vais suivre cette affaire avec le plus grand sérieux...... je serais heureux de confier ce job à une société Française!
Je vous laisse le soin de traduire:
It’s a Porsche for Goodness’ Sake
Beware of businesses that take shortcuts to re-iron line your drums. The Porsche factory lined their drums in two ways. The first was a cross-hatch pattern and a lip as seen below. They then did away with the crosshatch and used a smooth turned finish on the outside of the liner still with the tiny lip to stop the liner working its way out. The Machine Shop uses a wire to simulate this tiny lip.
crosshatch
Of course, the factory or factory supplier fitted the liner during the (very hot) forging process. While cooling the aluminum drum shrunk in size, securely gripping the liner. Now, when replacing a liner, the drum is machined out to a size and the liner is made to that same size plus an amount of interference fit. If there is too little interference, the liner will come loose at operating temperature. Remember that aluminum expands about 90% faster than iron. So, if you have too much interference the elastic limit of the aluminum will be exceeded. Some guy named Young had a bit to say on this. I perfected this amount of shrink by 1) calculation, and then 2) by some trial and quite a few errors on the racetrack.
The aluminum in your Porsche drums is a very closely grained structure, and has been heat treated to a hard state. Cast drums cannot offer this strength, and as such the shrink fit is not adequate, meaning those pretty Italian knock offs sold by NLA will fail when worked hard.
I tried to accommodate the tiny lip on my liners, but this meant that the drum had to be heated to a temperature (to expand enough to accept the cold iron liner) that annealed and softened the drum. The solution was the stainless wire. Half a groove is machined in the drum, and a half groove is machined in the liner. Once the drum has shrunk onto the liner, 3 foot of wire is inserted into these two half grooves, now a 1/16″ whole hole. The entry opening for this key wire is then closed off. By using this method, drums are heated to the threshold well below an annealing temperature.
One other shop uses glue and a stud to affix their liners. (I wonder if JG stands for “Just Glued”?) Why would one depart so radically from the factory’s tried and true method? This is a cheap, nasty, and frankly cringe-worthy fix that should be avoided at all costs. Because there is glue between the iron and aluminum, there is no heat transfer, something that a performance minded owner is looking for. Also, by pinning the liner at 12 and 6 o’clock, the drum is only ever round at room temperature, see sketch below. This out of round will make your car have a lumpy pedal, and could lock a wheel sending your pride and joy into a ditch.
JGmethod
The spline is no place to skimp, either. The factory fitted these steel spline inserts in the same manner as the crosshatched knurl used on the very early drums. I use a very course straight knurl which is cut onto the steel insert and into the aluminum drum. The insert is then heat shrunk and pressed into the drum with a 50 ton press. This method has proved itself on the race track. Other shops have installed splines with a couple of studs inserted between the spline and drum. This does not work. See example of a failed spline below.
To make matters worse, these inappropriately restored drums are being sold at a premium price by two well known and trusted vendors that I know of: Sierra Madre and NLA/Stoddard. Hot off the press! As of August 2015, Stoddard has dropped this suplier. I now supply to Stoddard. Just ask if the drum you are getting is warranted for life and for RACING. If the answer is no, then you better get my drums, which are.
Another issue I’ve come across is people having to grind the ends of their brake shoes to fit into aftermarket cylinder slots. If there is a fitment issue, the aftermarket cylinder slots should be ground…not the original shoes. Or buy real OE cylinders…they are worth the extra money.
As these cars because more rare and valuable, it pains me to see people take shortcuts and spend money miserly as if they were dealing with an older British car. It’s a Porsche for goodness’ sake! Get your work done properly!
-End Soapbox-
It’s a Porsche for Goodness’ Sake
Beware of businesses that take shortcuts to re-iron line your drums. The Porsche factory lined their drums in two ways. The first was a cross-hatch pattern and a lip as seen below. They then did away with the crosshatch and used a smooth turned finish on the outside of the liner still with the tiny lip to stop the liner working its way out. The Machine Shop uses a wire to simulate this tiny lip.
crosshatch
Of course, the factory or factory supplier fitted the liner during the (very hot) forging process. While cooling the aluminum drum shrunk in size, securely gripping the liner. Now, when replacing a liner, the drum is machined out to a size and the liner is made to that same size plus an amount of interference fit. If there is too little interference, the liner will come loose at operating temperature. Remember that aluminum expands about 90% faster than iron. So, if you have too much interference the elastic limit of the aluminum will be exceeded. Some guy named Young had a bit to say on this. I perfected this amount of shrink by 1) calculation, and then 2) by some trial and quite a few errors on the racetrack.
The aluminum in your Porsche drums is a very closely grained structure, and has been heat treated to a hard state. Cast drums cannot offer this strength, and as such the shrink fit is not adequate, meaning those pretty Italian knock offs sold by NLA will fail when worked hard.
I tried to accommodate the tiny lip on my liners, but this meant that the drum had to be heated to a temperature (to expand enough to accept the cold iron liner) that annealed and softened the drum. The solution was the stainless wire. Half a groove is machined in the drum, and a half groove is machined in the liner. Once the drum has shrunk onto the liner, 3 foot of wire is inserted into these two half grooves, now a 1/16″ whole hole. The entry opening for this key wire is then closed off. By using this method, drums are heated to the threshold well below an annealing temperature.
One other shop uses glue and a stud to affix their liners. (I wonder if JG stands for “Just Glued”?) Why would one depart so radically from the factory’s tried and true method? This is a cheap, nasty, and frankly cringe-worthy fix that should be avoided at all costs. Because there is glue between the iron and aluminum, there is no heat transfer, something that a performance minded owner is looking for. Also, by pinning the liner at 12 and 6 o’clock, the drum is only ever round at room temperature, see sketch below. This out of round will make your car have a lumpy pedal, and could lock a wheel sending your pride and joy into a ditch.
JGmethod
The spline is no place to skimp, either. The factory fitted these steel spline inserts in the same manner as the crosshatched knurl used on the very early drums. I use a very course straight knurl which is cut onto the steel insert and into the aluminum drum. The insert is then heat shrunk and pressed into the drum with a 50 ton press. This method has proved itself on the race track. Other shops have installed splines with a couple of studs inserted between the spline and drum. This does not work. See example of a failed spline below.
To make matters worse, these inappropriately restored drums are being sold at a premium price by two well known and trusted vendors that I know of: Sierra Madre and NLA/Stoddard. Hot off the press! As of August 2015, Stoddard has dropped this suplier. I now supply to Stoddard. Just ask if the drum you are getting is warranted for life and for RACING. If the answer is no, then you better get my drums, which are.
Another issue I’ve come across is people having to grind the ends of their brake shoes to fit into aftermarket cylinder slots. If there is a fitment issue, the aftermarket cylinder slots should be ground…not the original shoes. Or buy real OE cylinders…they are worth the extra money.
As these cars because more rare and valuable, it pains me to see people take shortcuts and spend money miserly as if they were dealing with an older British car. It’s a Porsche for goodness’ sake! Get your work done properly!
-End Soapbox-
seb356 a écrit : Bon!!! ;-)... Je vous tiendrais au courant mais je ne compte pas faire de circuit...
A la base du poste, je cherchais des conseils pour les cylindres de roue....
Si pas de cylindre de roue, pas de remontage et donc pas d essais ;-)
A propos des cylindres: ATE ou chinois! J'ai de bon retours sur les deux! Le seul défaut des chinois concerne l'insertion de la mâchoire dans la fente du piston; cette dernière est parfois un peu étroite et doit être retouchée.
Info très intéressante sur le remplacement de ces frettes. Suite aux mêmes réserves sur la tenue des frettes, j'ai remplacé mes tambours usés. L'adresse du gars m'intéresse en MP, stp.
Pour les cylindres de frein, lorsqu'ils sont encore en bon état, ate vend le kit de rénovation avec le remplacement des joints, ressort, vis de purge.
J'ai l'impression que les tiens ont été sablés ... pour enlever de l'oxydation? Si la surface de glissement a changé d'aspect, il ne faut pas prendre le risque de les réutiliser. En remplacement, j'ai pris des Ate. C'était il y a 7-8 ans. Ils sont toujours nickel.
Pour les cylindres de frein, lorsqu'ils sont encore en bon état, ate vend le kit de rénovation avec le remplacement des joints, ressort, vis de purge.
J'ai l'impression que les tiens ont été sablés ... pour enlever de l'oxydation? Si la surface de glissement a changé d'aspect, il ne faut pas prendre le risque de les réutiliser. En remplacement, j'ai pris des Ate. C'était il y a 7-8 ans. Ils sont toujours nickel.
Profil - seb356
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