Mechanical Engineers? Modifying JTR Mounts for More Offset/Math Calc.

[johnc]

I would prefer to mount the engine to the crossmember. The load paths of frame mounting don't really excite me.

 

Why? The front frame rails are strongest at the TC rod buckets, which handle greater loads then the torque an engine can produce.

[RTz]

Why? The front frame rails are strongest at the TC rod buckets, which handle greater loads then the torque an engine can produce.

 

When the engine is mounted to the frame rails, the frame now sees a localized load point in every direction (lateral, vertical, longitudinal). When crossmember mounted, there are no lateral loads (from the engine) and the remaining loads are dispersed broadly through the upper and lower frame rails and inner fender liners.

 

Additionally, moving the mount 'pedestal's' back 5.5" isn't close enough to the TC bucket area to have any relevance, or so it seems to me.

 

Of lesser importance, an unnamed member has done some compliance testing of the bucket area, and has found, with 'normal' lateral chassis loading, there is quite a bit of flex in the TC bucket. While it may be the strongest part of the front frame rails, it still flex's more than I'd like to see.

 

I'm aware that a number of our members have done 'frame mounts'. Its simply my preference to avoid it if its reasonable.

[BRAAP]

Regarding engine loads and load paths this raises a question. (Ron, I’m borrowing from your post which I think I’m talking about the same thing?)

 

 

With the engine mounted to the cross member, all vertical loads, engine weight, etc, are transmitted through the cross member, into the lower frame rail, through the fender apron/strut tower to the upper rail/strut tower where the weight of the car and engine rest on the upper spring perch.

 

Lateral loads, such as in a corner, with the engine mounted on the cross member, the “lateral weight”, or lateral load, of the engine is now transferring directly through the cross member directly to the lower control arms, i.e. the chassis/frame rails aren't seeing any real lateral loads from the engine, (except from the tranny mount of course).

 

Now if the engine was mounted directly to the frame rails, the vertical load path is slightly altered as the engine weight is now being induced to the lower frame rail further rearward. To a MUCH larger degree is the fact that now the lower frame rail is also seeing the lateral loads of the engine through the lower frame rail to the cross member then on into the lower control arm, i.e. asking the lower frame rails to take on a LOT more forces laterally that it never used to see with the engine mounted to the cross member. Is that an issue for an autocross/street car regarding chassis flex and controlling load paths through the chassis? I’m not sure, and I guess it would depend on how serious the approach is to such a track car and also how sensitive the driver is to such things and how much comprise he/she is willing to live with.

Granted, this scenario did not take into account any chassis strengthening mods and is all theory.

[heavy85]

Regarding engine loads and load paths this raises a question. (Ron, I’m borrowing from your post which I think I’m talking about the same thing?)

 

 

With the engine mounted to the cross member, all vertical loads, engine weight, etc, are transmitted through the cross member, into the lower frame rail, through the fender apron/strut tower to the upper rail/strut tower where the weight of the car and engine rest on the upper spring perch.

 

Lateral loads, such as in a corner, with the engine mounted on the cross member, the “lateral weightâ€, or lateral load, of the engine is now transferring directly through the cross member directly to the lower control arms, i.e. the chassis/frame rails aren't seeing any real lateral loads from the engine, (except from the tranny mount of course).

 

Now if the engine was mounted directly to the frame rails, the vertical load path is slightly altered as the engine weight is now being induced to the lower frame rail further rearward. To a MUCH larger degree is the fact that now the lower frame rail is also seeing the lateral loads of the engine through the lower frame rail to the cross member then on into the lower control arm, i.e. asking the lower frame rails to take on a LOT more forces laterally that it never used to see with the engine mounted to the cross member. Is that an issue for an autocross/street car regarding chassis flex and controlling load paths through the chassis? I’m not sure, and I guess it would depend on how serious the approach is to such a track car and also how sensitive the driver is to such things and how much comprise he/she is willing to live with.

Granted, this scenario did not take into account any chassis strengthening mods and is all theory.

 

That's exactly why I did this to mount the LS1. Trying to connecting the mass to the tires as directly as possible ...

 

 

Cameron

[Michael]

We don’t need FEA for a back-of-the envelope analysis. The JTR-type brackets can be thought of as beams (roughly speaking), with the engine as a point load on one end of the beam. The moment at the root of the beam goes as the distance to the point-load squared; 5.5†vs. 3.5†cantilever --> ~ 2.5X more moment. Meanwhile, if you double the depth of the beam then the moment of inertia (strength) goes up by a factor of 8, but that’s not what we are doing here; here we are doubling the thickness of the beam, and that is a linear increase in strength.

 

However, as Grumpy pointed out, 1/4" steel plate is already thick enough, since it is much stronger than the crossmember’s mounting pads.

 

In my Z the engine has 6.25†setback (altered firewall). The engine-mounts go to footers welded to pads welded to the frame rails. This is almost atop where the tension/compression rods protrude, as johnc suggested. And the aforementioned pads spread the load to avoid stress concentrations.

[johnc]

Of lesser importance, an unnamed member has done some compliance testing of the bucket area, and has found, with 'normal' lateral chassis loading, there is quite a bit of flex in the TC bucket. While it may be the strongest part of the front frame rails, it still flex's more than I'd like to see.

 

There is quite a bit of flex in the ENTIRE S30 chassis. I probably haven't said this clearly and directly, but the S30 chassis, even with an 8 or 10 point roll cage, subframe connectors, etc. is a flexible chassis when compared with vehicles designed 20 years later. This has to be taken into account and allowed for when designing mounts.

 

A good stiffness comparison is the strut front suspensions of the E36 BMW and the 240Z. Typical racing front spring rates for the E36 are 750 to 900 lb. in. depending on tire selection. That's for a 2,500 lb. full race car. For the 240Z typical front spring rates are 350 to 400 lb. in. and tha'ts for a 2,200 lb. full race car. Would I want to run 500 to 600 lb. in.springs on the front of a 240Z? Absolutely. Can I? Not in a million years because it will tear the chassis apart.

 

The crossmember/front LCA locating point deisgn on the 240Z is primarily focused on ease of assembly (engine and front suspension were installed from the bottom) and any load path advantage is secondary. Any engine mounting solution will see flex at the frame rails. On the Rusty Old Datsun I saw cracking in the front crossmember where it bolted to the front frame rails. I ended up welding the crossmember to the frame rails which solved the problem.

 

Its important to keep the S30's chassis flexibility in mind when. Stiffening one area only reduces flex in that area with the loads moving to a more flexible part of the chassis.

[johnc]

When crossmember mounted, there are no lateral loads (from the engine) and the remaining loads are dispersed broadly through the upper and lower frame rails and inner fender liners.

 

Any loads into the front frame rails (from the bottom, sides, and/or top) get transferred to the inner fenders, strut towers, upper frame horns, core support, firewall, etc. Its a unibody chassis.

 

Lateral loads from the engine also get transferred to the frame rails regardless of how the engine mount or crossmember is attached to the frame rail. The lateral loads don't just disappear because the engine is attached to an underslung crossmember. If there's 400 lbs. of lateral load from the engine you'll (theoretically) see 200 lbs. of shear, tension, and/or compression at one engine mount/crossmember end and 200 lbs. of shear, tension, and/or compression the other.

 

EDIT: Clarification/simplification...

 

Engine mounts welded to the frame rails and crossmembers bolted/welded to the frame rails both transfer loads from the engine into the frame rails.

[bjhines]

I was also concerned with the loads on the framerails.

 

I chose the later model crossmember with the extra gussets from the factory.

 

I also boxed the engine mount standoffs.

 

I added 14g 1"x3" framerails from the TC buckets all the way back to the rear cross frame behind the rear seats.

 

 

 

 

 

 

 

I clad the framerails in 14g and plugwelded them. I also wrapped the clading around and under the crossmember mounting area.

 

 

 

 

Then I gusseted the TC buckets and I tied the inner fender/tower apron to the top of the crossmember with 16g triangle gussets.

 

Then I added plenty of cage tubing for further reinforcement of the front end.

 

 

I also reinforced the firewall near the framerails for good measure.

[deja]

Holy reinforcement batman! The front end isn't going to move that's for sure.

[bjhines]

I autocrossed that chassis for 10 years with various modifications to improve grip and quicken response.

 

The car is RUST FREE!! and always has been, But my autocrossing years had begun to crack it up. The bottom side of the framerails were falling apart. I could see puckers with cracks around dozens of spot welds. The front sway bar had literally ripped the bottom of the framerails off in that area. The crossmember mounting area had puckered into the bottom of the framerails. The car had a few hard "offs" in it's life but nothing serious. The abusive driving with stiff suspension parts and hard bushings proved more than the Z-car can take. It is more than fixed now.

[RTz]

Hi John,

 

I'm not trying to say the loads disappear... just trying to say they get re-arranged.

 

With a crossmember mounted engine, in a lateral load, the load goes from the contact patch, through the spindle, strut housing, BJ, LCA, and into the crossmember, which is directly connected to the engine. The lower frame rail sees no direct lateral loading. Yes, a 'small' amount is feed into the TC bucket via the TC rod, and a smaller amount yet throughout he trans mount. Its dispersed. Obviously, there is a direct load going through the upper spring perch and into the upper rail which shares the load with lower frame rail via the strut tower itself and the inner apron. Again, I see the loads being dispersed, and having no direct impact to the lower rail, except that of being pulled down.

 

 

 

Edit: My only point is that frame mounting will produce highly focused loads on the rail (instead of the crossmeber), whereas crossmember mounting disperses the loads over a broad area of the unibody

[Pop N Wood]

Edit: My only point is that frame mounting will produce highly focused loads on the rail (instead of the crossmeber), whereas crossmember mounting disperses the loads over a broad area of the unibody

 

Attaching the motor to the car that far in front of the motor's center of gravity will also put increased static weight on the transmission mount.

[dr_hunt]

Well, I made mine. Took the stock chevy motor mount bracket (block) cut out just the portion that bolts to the block (triangle shape), welded a 1-5/8" mild steel tube at about a 30 degree angle or so to the stock nissan mount pad with my mig. also I have fabricated front motor plate to both heads that go to frame.

 

I think I'd be more concerned with the quality of weld than the strength of the matierial or thickness IMHO. Watching my dad tig weld airplane frames when I was a kid made a believer out of me, especially when someone has to fly the damn thing later!

 

Also with the resulting weight percentage being almost 50/50 with the jtr mounts and driver included, what's the point.

 

Increased load on the trans mount, gimme a break. What do you think is going on when you launch a car using a trans brake?!!!!! I doubt any static weight on a trans mount can compare to the rotational force exerted during launch. Don't split hairs, for asphault, rigid frames seem to give more predictable resultant forces IMO!

[RTz]

Another possible way to look at it... Because there is a direct link from the contact patch to the crossmember, when you turn the wheel the crossmember (carrying the engine) moves laterally. The only reason the chassis follows is because of the four crossmember bolts, and to a lesser degree, the TC rods and upper strut mount.

[RTz]

Also with the resulting weight percentage being almost 50/50 with the jtr mounts and driver included' date=' what's the point.

[/quote']

 

Some people aren't convinced 50/50 is 'ideal'... Porsche, Ferrari, myself...

[Pop N Wood]

Increased load on the trans mount, gimme a break. What do you think is going on when you launch a car using a trans brake?!!!!! I doubt any static weight on a trans mount can compare to the rotational force exerted during launch. Don't split hairs, for asphault, rigid frames seem to give more predictable resultant forces IMO!

 

Have you read the rest of the thread?

 

Splitting blond hairs IMO.

[wheelman]

Some people aren't convinced 50/50 is 'ideal'... Porsche, Ferrari, myself...

 

Not to jack the thread but I've been discussing this with one of the members of my auto-x club. He is planning to make some pretty major mods to his 240SX to reach the magical 50/50 mark.

So what are your feelings/opinions on this subject Ron?

If you don't want to polute this thread with the response then PM me or include a link to prior threads where you've stated you view point.

 

BTW: I did some quick calculations of the weight distribution of my Z with me in it. I weighed it sans myself so I'm making the assumption that 90% of my weight ends up on the rear. Is this reasonable?

Car weighed 2540 with 1320 on the front and 1220 on the rear for a 52/48 distribution.

With me added:

Total 2705, 1336 front, 1369 rear for a 49/51 distribution.

 

Thanks

Wheelman

[johnc]

Jack stopped... There are a few threads that already discuss weight distribution.

[johnc]

Another possible way to look at it... Because there is a direct link from the contact patch to the crossmember, when you turn the wheel the crossmember (carrying the engine) moves laterally. The only reason the chassis follows is because of the four crossmember bolts, and to a lesser degree, the TC rods and upper strut mount.

 

I'm confused about the point you're trying to make. Are you saying:

 

1. That a more direct connection between the contact patch and the engine has an effect on handling?

 

2. That a more direction connection between the contact patch and the engine has a greater or lesser effect on framerail loading?

 

I'm not being argumentative, I'm just confused.

[RTz]

I'm not being argumentative, I'm just confused.

 

That makes two of us, my friend. I read your arguments as constructive and I'm hoping that you're doing the same with mine. I think there's value in understanding loads and load paths so this discussion interests me. If I prove to be wrong, so be it. I will walk away with a better understanding.

 

 

1. That a more direct connection between the contact patch and the engine has an effect on handling?

 

 

Nope, sorry to give that impression.

 

 

2. That a more direction connection between the contact patch and the engine has a greater or lesser effect on framerail loading?

 

 

Getting closer.

 

I thought presenting the case from a lateral perspective would have been easier, but thats not proving true. Allow me to start over from a vertical perspective, from the point of severe bottoming out.

 

To exaggerate, put 20,000 lbs of weight on top of the engine (stock mounted). The suspension will be on its stops and the four vertical bolts through the crossmember will try to tear out (vertically). The lower rail is seeing a lot of focused stress (in tension), but its being held to great degree by the upper rail due to the strut tower. A fairly robust load path.

 

On the other hand, assume the engine is mounted to the lower rail, roughly in the middle. Again, putting the same 20K lbs of weight on the engine. The suspension is still bottomed, but now the lower rail is bowed downward because the only thing attaching it to the upper rail is the inner fender and its not efficiently built for that load. Also, there is no longer ANY stress on the 4 crossmember bolts. In fact, you could remove them and move the crossmember up and down by hand. The load path changed.

 

In both cases the amount of stress is the same, but its re oriented. The loads on the rail changed from tension to compressive and the loads on the crossmember are removed entirely.

 

In retrospect, laterally speaking, its more complex, but still has the same 'issue'... a re-arranged load path.

 

If any of that makes sense, and you agree, I'll take another stab at the lateral scenario.