andrethx Posted December 29, 2012 Report Share Posted December 29, 2012 i was thinking back to all of the discussions we've had about possible replacements for the ABS rotors on the later 505 turbos & v6 models. even though i don't have one of these models, i find it a very interesting topic. i was recently looking at christophe leverdier's 505 site, and one of the topics that he discusses is adapting rotors from other cars to fit the ABS 505s. what is particularly interesting to me is that he includes measurements from the 505 ABS rotors as well as the possible replacements. this got me thinking about the early 944 rotors, which seem to be the closest match commonly available to north american ABS 505 owners...rabin and others have mentioned that the 944 rotors should fit with a bit of machining. i looked up the dimensions of the 944 rotors and saw that they are quite a bit taller in the center portion that bolts to the hub -- 35.35 mm tall vs. 28 mm for the ABS 505 rotors. how could one compensate for this difference with machining? after one has drilled the correct mounting holes onto the 944 rotor and attached it to the ABS 505 hub, wouldn't the additional "tallness" of the 944 centers make the rotor sit "off-center" vis-a-vis the 505 calipers (i.e., 7mm farther inboard)? i'd appreciate the thoughts of the more technically astute forum members... andré Quote Link to comment Share on other sites More sharing options...
Bean Posted December 29, 2012 Report Share Posted December 29, 2012 I'm going to have to recheck my worn out rotors, but I thought for sure min thickness is 20mm. I tried finding new thickness but the best I could do was find another disk with 20mm min thickness and it was 22mm which seemed right. Maybe it's only 21mm new? As for machining - with the Porsche disk taller you should be able to use longer bolts and a metal bushing to space the caliper enough to compensate for the disk offset. If you were machining the hub to fit the 944 disks you could machine the face the disk mounts to by the offset amount so that the disk lined up into the caliper properly. You would also need to do the bolt pattern, and make sure the disk had a proper centre ring to sit on. I'll verify the disk numbers - but from memory I sold some rotors a while back that I measured at 20.53mm and they had 70k miles on them. Rabin Quote Link to comment Share on other sites More sharing options...
Bean Posted December 29, 2012 Report Share Posted December 29, 2012 Checked my old for sale post and the min thickness was 18mm - and I know I read this off the rotors after I bead blasted them. Still have no idea what full thickness was for sure - but they could be 20.5mm like the Porsche disks since they're supposed to share the same caliper as well. Rabin Quote Link to comment Share on other sites More sharing options...
andrethx Posted December 29, 2012 Author Report Share Posted December 29, 2012 I'm going to have to recheck my worn out rotors, but I thought for sure min thickness is 20mm. I tried finding new thickness but the best I could do was find another disk with 20mm min thickness and it was 22mm which seemed right. Maybe it's only 21mm new? please forgive my (possibly incorrect) terminology here -- you're correct, the "swept" part of the 944 rotor -- i.e., the outer part that goes between the brake pads -- is 20-22 mm. so if the rotor is centered between the brake pads, no adjustment to the caliper should be necessary (theoretically). but i'm talking about the thickness of the entire rotor here...the swept part as well as the center. what i'm saying is, the "brims" of the 944 "hat" and ABS 505 "hat" are identical, but overall the 944 "hat" is taller because it has a taller "crown." so mounting the 944 rotor on the ABS 505 hub, with no changes other than drilling the correct mounting holes, would mean that the rotor would not be centered between the brake pads... does that make sense? andré Quote Link to comment Share on other sites More sharing options...
Bean Posted December 29, 2012 Report Share Posted December 29, 2012 what i'm saying is, the "brims" of the 944 "hat" and ABS 505 "hat" are identical, but overall the 944 "hat" is taller because it has a taller "crown." so mounting the 944 rotor on the ABS 505 hub, with no changes other than drilling the correct mounting holes, would mean that the rotor would not be centered between the brake pads...does that make sense? andré Correct. That extra depth the 944 rotor has will move the rotor centreline further inside the wheel well (away from wheel spokes). Spacing the caliper mount by the correct amount would then move the caliper itself so that it realigned the caliper onto the disk. If you machined that same offset from the hub were the disk mounts when you machined in the 944 disk bolt pattern, the 944 disks could be mounted with no modifications. All depends as to me the cost difference of changing the hub to fit 944 disks would not be much more than having 944 disks modified to fit the hubs. One other idea is to machine the old rotors into rotor hats that you then could bolt similar sized disks to - or fit larger disks and relocate the caliper / fit larger caliper... But that's a very slippery slope. Rabin Quote Link to comment Share on other sites More sharing options...
JunktionFET Posted December 30, 2012 Report Share Posted December 30, 2012 There might be a problem with using a spacer to relocate the caliper; Seems like it would add a bit of flex that would ruin brake feel. While it may not be a big deal, especially on a street driven car, I'd wager it would not be the best for track use. Newer sportbikes have radially mounted front calipers. In other words, the caliper mounting bolts are in line with the wheel spokes ("radial") instead of perpendicular. They claim that this makes the caliper mounting much more rigid and gives the brakes better feel. I always thought this was unnecessary and it wouldn't make any difference... just marketing hype, you know? However after riding a bike with radial mounted calipers, I can say it makes a noticeable difference in the feedback and it even seems to affect how the initial bite hits. I suspect that the same holds true on cars--add flex to the caliper mounting, and you nuke the feedback in the pedal. Just something to keep in mind... Quote Link to comment Share on other sites More sharing options...
Bean Posted December 31, 2012 Report Share Posted December 31, 2012 Depends on the method you space it out I would think, and the material used for the spacing, but I would think any loss in feel would be undetectible. If anything I bet the old brake hoses have far more to contribute towards losing brake feel, and new or even braided lines would be a much better improvement. As an update to this thread - I checked into Coleman racing rotor disks and they use a 8 bolt 7" bolt circle pattern, and it looks like from the picture Andre linked - that there is just enough room on the stock rotors to turn them into hats, which might then be the easy way into a proper replacement. Much easier to make the rotor hats out of stock disks than to machine the hub to fit 944 disks IMHO if the idea works. Some single brake line Volvo 240 ABS 4-pot calipers to replace the stock set up and it would be a nice brake upgrade since the replacement disks could be ordered with much better venting - even gas slotted if you wanted. Rabin Quote Link to comment Share on other sites More sharing options...
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