bjhines Posted February 15, 2006 Share Posted February 15, 2006 Don't use the SOLID DIFF MOUNTS!!!!! This is what 6 years occasional track use will do... I have a rear crossmember that is literally falling apart due to the installation of a solid front diff mount. There are at least a dozen cracks progressing throughout this crossmember... all 4 bolt holes have radial cracks leading out in all directions... Here is a shot in the car... http://album.hybridz.org/showphoto.php?photo=5912&cat=500 Here is a close up of one of the visible cracks with it mounted in the car... http://album.hybridz.org/showphoto.php?photo=5913&cat=500 This is one of a pair of similar cracks that can even be seen through a layer of gunk from the rear... there are far worse cracks visible with the unit removed... I get them posted tomorrow.. EDIT: Added pics... The bottom shows no signs of damage... making this very dangerous... Quote Link to comment Share on other sites More sharing options...
Mikelly Posted February 15, 2006 Share Posted February 15, 2006 I guess my question is could you imagine what it would do to a Urethane mount? Over time it would probably destroy that... I was at CreationZ (Guys who used to work at Datsun Dynamics) yesterday and Dave (one of the owners) was telling me how their stroker car was breaking mustache bars and front diff mounts, and completely trashing the differentials. I'm really wrestling with this right now since I've got all the hardparts in house to mount a Q45 in the stock location, using stock parts, except for a front mount I'll need to make. I just don't think it's worth it. Pretty sure I'm going to weld something up like Jamie Taylor did or maybe like tube80z (Carey) has done. Mike Quote Link to comment Share on other sites More sharing options...
Mayolives Posted February 15, 2006 Share Posted February 15, 2006 I little maintenance on that 25 year old piece of metal (cross member) might help prevent some of those cracks. Don't misunderstand me, I not being critical. Only stating the obvious. When rust sets in, the end result on the cross member is fatigue and then it's not ready for the abuse of hard driving. Quote Link to comment Share on other sites More sharing options...
blueovalz Posted February 15, 2006 Share Posted February 15, 2006 Did you solid mount the rear of the differential as well? My guess (god, I hope I'm right on this assumption) is that you did not. Blending solid and flex mounting on any piece of equipment will result in failure on the solid mounted portion. The flex mount (I assume on your mustache bar) will allow flexing, which causes the material around the front solid mount to flex. Metal will only flex so many cycles before fatique and failure. Solid mounting the rear would have prevented this small amount of flexing up front. Solid verses flex mounting...one or the other, but not both. This is why I solid mounted front AND back portions of the differential. I also solid mounted the motor, but then I was required to solid mount the tranny as well. Going on 15 years and counting with no cracks or breaks. Quote Link to comment Share on other sites More sharing options...
dj paul Posted February 15, 2006 Share Posted February 15, 2006 thats the only explanation that makes sense terry. hopefully this is the case because if not, all us solid diff guys are screwed. but, i cant think of why this was not thought of before it was done. of course you cant have mixed mounts, the torsion allowed by the bushing mounts, even if they were really stiff like polyurethane, WILL cause cracking as you see. even if you solid mounted the back, but left the front stock you will see this as well. as terry said, metal can only tolerate a certain amount of flexing cycles before this happens. you know when you cut a peice of metal, but theres just that little peice left, and you can twist it off by moving it back and forth? same thing. Quote Link to comment Share on other sites More sharing options...
preith Posted February 15, 2006 Share Posted February 15, 2006 Did you solid mount the rear of the differential as well? My guess (god' date=' I hope I'm right on this assumption) is that you did not. Blending solid and flex mounting on any piece of equipment will result in failure on the solid mounted portion.The flex mount (I assume on your mustache bar) will allow flexing, which causes the material around the front solid mount to flex. Metal will only flex so many cycles before fatique and failure. Solid mounting the rear would have prevented this small amount of flexing up front. Solid verses flex mounting...one or the other, but not both. This is why I solid mounted front AND back portions of the differential. I also solid mounted the motor, but then I was required to solid mount the tranny as well. Going on 15 years and counting with no cracks or breaks.[/quote'] That's exactly what I was going to say, my guess is the mustache bar was NOT solidly mounted. I just cut off the bushing mounts and welded on 2" x 1/4" thich angle iron. Quote Link to comment Share on other sites More sharing options...
tube80z Posted February 15, 2006 Share Posted February 15, 2006 The flex mount (I assume on your mustache bar) will allow flexing, which causes the material around the front solid mount to flex. Metal will only flex so many cycles before fatique and failure. Solid mounting the rear would have prevented this small amount of flexing up front. Solid verses flex mounting...one or the other, but not both. In the past year on my autoxer I have pulled the hole (washer and all) through the front crossmember with a solid mount. The mustache bar was solidly mounted as well as the engine and tranny in this car. At the time I figured it was old metal and that was it. In hindsight I think that it would make sense to reinforce this part or make a new one in a car that will see major abuse. I would also rethink the mustache bar/rear suspension mounting i fyou intend to solid mount the diff. I'd be looking at creating a cradle (dicussed in other threads) very much like you see done on rally cars. Cary Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 15, 2006 Share Posted February 15, 2006 My theory on this is that this happened because the mustache bar was still rubber or poly mounted. However, even if the mustache bar was mounted in aluminum, if the engine/tranny are soft mounted you're still going to have problems. If you mount the diff solid (both mounts) and the engine/tranny on rubber the engine is still going to want to twist over when you put torque to it, but it can't because when it trys the torque goes down the engine and trans through the driveshaft and into the diff mounts. So basically you're using the diff mount to keep the engine from rocking over under power. Had this conversation on another forum, a guy had this problem and I said next time you go out you're going to bust the driveshaft. Guess what happened? Either rigidly mount the whole thing or soft mount it all. I disagree with Mikelly that this kind of stress will still occur with a poly mount, because the problem is rigidly mounting just one component of the drivetrain, then expecting that one component to control the torque of the whole drivetrain. My $.02 Quote Link to comment Share on other sites More sharing options...
bjhines Posted February 15, 2006 Author Share Posted February 15, 2006 ok.... The car has Zero rust.... the failure is probably because the mustache bar is a spring...and...It is indeed mounted using poly bushings... I have 3 other Z cars... I have never seen this cracking before... It is unique to the car with the solid front diff mount... I am getting my kid to bed now...naptime is enough for me to get some better shots while she is sleeping... I am concerned that the short nose diff mounting bars will cause a cantilevered/ lever situation and do this to the unibody of the car.... Quote Link to comment Share on other sites More sharing options...
2126 Posted February 15, 2006 Share Posted February 15, 2006 All very interesting replies! I believe anytime you alter a part, of a system, that was originally designed to function one way and then you drasticly change it without altering the complete system, you are asking for early failures....it's just common engineering knowledge. Regarding jmortensen's reply about having a diff ridgidly mounted and the eng/trans soft mounted....the only thing that changes is that the energy the old soft mounted diff used to absorb is now focused else were, because the new soild mount does not absorb the same given energy. So, a soft mounted eng/trans and soild diff still torques, as it always did, but now the eng/trans/driveshft will see a more intensity force (no soft mount to absorb it). Quote Link to comment Share on other sites More sharing options...
zguy36 Posted February 15, 2006 Share Posted February 15, 2006 Either rigidly mount the whole thing or soft mount it all. I disagree with Mikelly that this kind of stress will still occur with a poly mount, because the problem is rigidly mounting just one component of the drivetrain, then expecting that one component to control the torque of the whole drivetrain. Are you forgetting about the u-joints? You can hard mount the differential without having any adverse effects on the engine. The engine can still torque over because the u-joints allow the movement of the engine. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 15, 2006 Share Posted February 15, 2006 All very interesting replies! I believe anytime you alter a part, of a system, that was originally designed to function one way and then you drasticly change it without altering the complete system, you are asking for early failures....it's just common engineering knowledge. Regarding jmortensen's reply about having a diff ridgidly mounted and the eng/trans soft mounted....the only thing that changes is that the energy the old soft mounted diff used to absorb is now focused else were, because the new soild mount does not absorb the same given energy. So, a soft mounted eng/trans and soild diff still torques, as it always did, but now the eng/trans/driveshft will see a more intensity force (no soft mount to absorb it). Not sure I'm reading this right, but what happens when you have rubber or poly mounts on the drivetrain is that the engine torques, the transmission moves, the differential moves. If you eliminate the possibility of one component to move, the others still will want to, and all of that torque gets focused on the one piece that can no longer move. I think we're agreeing... Are you forgetting about the u-joints? You can hard mount the differential without having any adverse effects on the engine. The engine can still torque over because the u-joints allow the movement of the engine. Think about what you're saying. You're saying that a U-joint should absorb a twisting motion. That's exactly what they should not do. They should transfer ALL twisting motion. If that's not the case, your U-joints need replacing. Quote Link to comment Share on other sites More sharing options...
johnc Posted February 15, 2006 Share Posted February 15, 2006 The Z front diff mount is designed to control for/aft and nose vertical movement. The moustache bar is designed to control torque. By installing just a solid front diff mount that mount now also controls torque, which as we can see, the diff front crossmember was not designed for. A proper solid diff mounting requires solid moustanche bar bushings and moustache bar reinforcement to make it stop acting as a for/aft spring. The ROD had that and never had a diff mounting problem in over two years of racing. Engine and trans were also solidly mounted. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 15, 2006 Share Posted February 15, 2006 Excellent point John. That single mount in the center isn't capable of handling anywhere near the same torque that the mustache bar can with it's 2' wide span and solid connection to the frame on either side. Quote Link to comment Share on other sites More sharing options...
bjhines Posted February 15, 2006 Author Share Posted February 15, 2006 I have to agree with 2126 abd Zguy36.... There is no more force applied to the engine tranny when mounting the diff solidly... Hence the telescoping AND universal jointed driveshaft... The engine/tranny and the Differential are DIVORCED... There may be a difference in "feel" because the deflection of the soft differential mount is gone... less "windup"... the drivtrain will feel more solid... but this would not add stress to the engine/tranny mounts... JohnC is correct and the solid mustache bar is a great idea... the differential front mount takes force in 2 planes...vertical...and fore and aft... fore and aft does not seem to be that big of a problem... it is the vertical force that tears things up... I am going back to a soft mount and I am adding a "strap" to hold it against the NEW crossmember under accelerating force... Quote Link to comment Share on other sites More sharing options...
shire240 Posted February 15, 2006 Share Posted February 15, 2006 Will having just poly mustache bar bushings with rubber engine, tranny, and front diff bushings cause stress to the rear crossmember? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted February 15, 2006 Share Posted February 15, 2006 Forget about straps. There are much better much easier ways: http://forums.hybridz.org/showthread.php?t=95128 http://www.classiczcars.com/forums/showthread.php?t=17417 Quote Link to comment Share on other sites More sharing options...
johnc Posted February 15, 2006 Share Posted February 15, 2006 While building up the ROD I was down at Bill Savage's shop and we did a little test. Setup: 1. Welded R180 diff. 2. Difff stib axles locked in place. 3. Solid moustache bar bushings. 4. No front diff mount. 5. Front of the diff located by three straps. We put a torque wrench on the front pinion nut and applied torque while watching the moustache bar. At about 125 ft. lbs. we could see the bar bend into a very slight "S" shape viewed from below. For that we deduced that even solid bushings on the moustache bar won't keep from imparting side loads into the front diff mount. Welding a gusset across the face of the moustache bar and retesting showed no flexing to the point where the pinion nut started to turn (around 240 ft. lbs.). Quote Link to comment Share on other sites More sharing options...
bjhines Posted February 15, 2006 Author Share Posted February 15, 2006 cliff's idea seems to be the best I have seen... 2 tabs and a bolt...I owuld sandwich some rubber between the bolt and the bottom of the crossmember to make sure it was always under tension and could not ding the crossmember too badly... Does anyone else think that welding a large area on a piece of spring steel/mustache bar is a bad idea....??? Quote Link to comment Share on other sites More sharing options...
Guest iskone Posted February 15, 2006 Share Posted February 15, 2006 I don't even have a moustache bar in my 240, it is replaced by steel plate and the front diff mount is square tubing. I show no signs of fatigue but as you can see that is some thick steel I think the diff would fail first. I think it was built some time ago. I know it was a track car but can't tell you how seriously it was used. My engine is mounted with rubber mounts, can't remember about the tranny but don't recall anything. Anyway it's an option for you take if you want everything solid in the diff. Quote Link to comment Share on other sites More sharing options...
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