Lost Time

Looks like we have an engineer in the house.

DamonB

twokrx7, I am in agreement with everything you said. But in using the same location as the engine hanger your still applying the load through that area of the casting on the UIM. In looking at that I still think that area is not properly reinforced for long term reliability; especially considering that it is cast aluminum.

I don't have my UIM off in front of me right now, but what I remember is just a raised pad with two (smallish) bolts for the engine hanger.

ajmacdon

This disscussion also got me interested. I agree with 2krx7 but came up with some different numbers. I used transmission output torque at an arbitrary max vehicle acceleration of 1.5 G (too high, I'm being conservative).

This accel is possible for an instant if you consider the inertial energy of the engine (and was easier than acctually figuring out this energy If you also assume that for this same first instant the motor/trans mounts are not doing anything but flexing the brace could carry this entire load (again, conservatively, for an instant).

with these conservativly high numbers you could see 950lb in the brace. This gives a max stress in the manifold tubes of 3000psi and a max stress in the mounting flange of 11,500psi. The aluminum used for this type of casting will has a fatuige limit of about 17,000psi so you will not break the manifold. With that stress the deflection at the gasket surface is around 0.0005 inches, the gasket can handle this just fine.

So, it seems to me that the engine will be fine but I am going to do a bit more work to figure out how the fender will handle the stress and how large a rod is really needed to be effective. I want to come up with a better estimate of the forces involved, my 950 lb figure is way to high but it demonstrates that the engine is not a concern.

Not sure how to go about the vibration problem, I will check with some knowlegable friends on that one.

I should really do some work pretty soon.

Alex

Lost Time

It's so nice having really smart people on here.

ajmacdon

Oops, forgot one important spot. The stress with 950 lb in the rod at the lift-eye mounting bolt holes is almost 12,000psi, still well below the 17,000 design limit.

Alex

Import Convert

Engineers kick ***! I'd be doing the same thing if I was any good at math.................

Lost Time

ajmacdon, since you seem to have a good handle on all of this, do you believe that using one of the dampened braces would relieve enough stress to cause no concerns?

DamonB

Alex, congrats on all that fine work! What is the fatique limit of the bolts and their mounting holes at the engine hanger?

Lost Time

It's in his last post, 17,000 psi.

ajmacdon

Like twokrx7 said I really think the limiting factor here is going to be the fender and the vibration and I don't have a good handle on either one just yet.

A damped brace would certainly help with the vibration problem but I'm not even sure there is a vibration porblem yet. The damping would have to be tuned to this specific task before you could figure out loads which is probably easier in testing than analytically.

I am hoping to to figure out a safe place to put a solid link because it is a LOT simpler to figure out and the motor mounts *should* still be able to take care of any damping needs.

However, as is often the case with engineering problems it might be faster to JFB a damped brace and be done

Alex

DamonB

I can't begin to say I have the knowledge to do a Finite Element Analysis on the UIM, but the point I am trying to make is that there is much more going on than merely the rupture point of the aluminum. To determine the point of catastrophic failure on a part, you can't just look at the properties of the material. You must look at how the load is applied and distributed throughout the material. The weak link in this figure isn't the UIM itself so to speak, it's the point of attachment between the brace and the UIM.

Lost Time

If that's the case, that's exactly where you would want it. Before you lose the UIM or LIM you would want the bracket to give way. Think of it as insurance. I take it by your question, that you are attempting to make the thing bulletproof. But I'm sure you know, nothing is ever bulletproof and if enough force is applied something has to give. I'd much rather it be the bracket than something a little more expensive.

Import Convert

Well, no, you can't make it invincible, but I'd rather not be concerned about it at all. Granted that the hangar would be the ideal place to break if it had to, but I'd rather not use that as a mounting point even if I was going to go the conventional way. I made a good aluminum bracket that bolts up in the same location that held the engine rock solid, but there really is a massive amount of pressure there. It just seems like a lot of constant stress placed on parts that were never designed to take it.

Nobody has really given any opinions on what they thought about my idea that I described in my original post. I'd be really interested in hearing what people have to say about that.

Lost Time

Yeah, sorry. We have kind of hijacked your thread. It sounds to me what you're proposing is very similar to PFS chaining their mounts. You might try contacting them to see what kind of wear they're seeing on their 3rotor.

ajmacdon

You are exactly correct, the stresses around the attachment point are the highest of anywhere on the engine. However, that stress, as stated in my "oops' message, is below 12000 psi.

No finite is needed with this problem. All of my assumptions to simplify it to a paper and pencil problem erred on the side of safety(they were the worst case loading scenario and the smallest dimesions on the parts) and it still shows it will not fail so in reality the stresses will be lower.

I think your idea about solidifying the existing mounts is great. This is the only way to be sure that everything is loaded as intended. However, I question if it will be enough to restrict the movement significantly. It seems as if the moment arm between the trans output and the motor mounts is significantly less than between the trans and the UIM although I haven't measured. It is definitely worth a try.

the_glass_man

iTrader: (2)

Isn't anything a little JB-Weld can't fix.

Rx-7$4$me

sorry I brought this back from the dead, but I think this thread has very good info

Rx-7$4$me

bump

rxrotary2_7

damn! i remember this thread... i think i will put it in the archives for the info

J.S.J

My take on the subject is to use the mounting points on the intake and the fender-well but use a bushing in-between. make it about a 1/4 inch in thickness on each the power and engine braking loads. The vibration will be a non issue then. By using a 1/4'' bushing it will let the engine move just a tick but not rigid enough to damage the moorings.

spurvo

Did anyone ever figure out the shock tower brace to alt bracket? Seems the best disctibution of load, which would mitigate the fender wall tear out some have seen, miticate the lateral stresses on the UIM/LIM superstructure that Damien was worried about, and of course give the benefits of the tower strut bar. Me wants...

J.S.J

I don't have a strut between the shock towers but I think it would be a good place to mount any type of support, providing its a good strong rigid unit. It seems to me starting near the driver side of the strut and moving toward the center of the area would be a natural place to start. I think the strut would bend easier if the placement wasn't thought out. I really should get one and check it out.

rotorbrain

Look at the pics of the cars posted in the SEform under Seve Kan tuneing in Knoxville .On page 3 or 4 .There are some pics of a proven torque brase !!!

rotorbrain

Sorry the pics are on page 5 not page 3 or 4

spurvo

He's speaking of the following:

https://www.rx7club.com/showthread.p...6&page=5&pp=15