PPF reinforcement

[Mokesta]

The PPF is meant to react diff torque and hold up the tail of the gearbox. These loads are both effectivly in the vertical plane so the PPF is in bending and shear.

Engine and diff torque about the longitudinal axis are reacted by the engine mounts and diff ears. These react at different rates so the diff and engine need to rotate different amounts. The PPF is therefore flexible in torsion.

The PPF is the ideal section shape to be stiff in bending and shear but flexible in torsion; a "C" section.

Assuming that the diff ears and engine mounts can only move a certain amount until they contact solid stops and there isn't a huge amount of chassis twist, the PPF won't see much more twist as engine power is increased. It will see a lot more bending and shear. If it's limits are reached it may need more capacity in these two modes. The device shown at the start of this thread would be an inefficient way of adding this capacity.

There are some gyroscopic effects that can load the PPF in the horizontal plane on a sharp clutch dump but I expect these would be second order loads.

If Mazda intended the PPF to transmit a torsional load, the unit would be a box or tubular section and the diff wound't need ears, just a simple hanger. That is, the arrangement between a Porsche 928 engine and box.

M

[orx626]

....and here endth the lesson!!!!

'Z'-sections aren't bad either.

[Sasso]

ok but that's still just words. Meh I don't care, I'm studying electrical eng anyway...

[zoomzoom]

Just words, yes, but I am very sure these guys have a fair idea of what they are talking about so I would certainly take their word for it.

[Keith]

I just went to the link at the top of this thread and saw the price. $214 US ( plus freight) for a simple C section !!!Strewth. Im in the wrong business!If I wanted one,( which I dont) Id get it bent up locally for a fraction of the cost.
I reckon Mokesta has nailed it.
Its a cost benefit analysis( leaving aside the $$$). A famous yacht designer, Uffa Fox, said about 70 years ago "Weight is only of use in a steamroller". So the cost is that you lose speed, and particularly acceleration, in a direct proportion to the weight you add to the vehicle- as well as increasing fuel consumption every time you drive it..but there doesnt seem to be much of a benefit to performance.
The other thing Id add its this: Mazda engineers went to a lot of trouble and expense to design & produce a reasonably high tech ( for a mass produced economy car of its day) lightweight alloy ppf. This clumsy bracket is just a bent up mild steel c section.Mazda could have done that for a fration of the cost. Surely you wouold pay to get rid of it, not to add it!!!
Interesting thread!

[orx626]

ok but that's still just words. Meh I don't care, I'm studying electrical eng anyway...

That's a shame Sasso that you feel that it is "still just words" and you "don't care". Mokesta's written assessment of Mazda's PPF design and the 'aftermarket stiffener' is based on a wealth of knowledge and experience. I know that you're studying Electrical Engineering, however, having an understanding of basic engineering principles in other fields will certainly be to your advantage as a professional electrical engineer. Neither of us were trying to belittle you, we were just trying to help you and others on the forum understand the engineering principles behind Mazda's PPF. Bugger.....I'm starting to sound like Mr Morlock .

Cheers,
Danny

[Sasso]

What I mean by still just words is that I like to see pictures and numbers as evidence. I haven't really looked into the whole ppf thing, all I know is that it stops my gearbox falling off and looks like a big FM frame rail. I was never really convinced by the brace, I just though that it might be funny if it turned out to be good. The reason I don't care is that I'm not goning buy one and since everyone thinks it doesn't do anything I'm not going to persue it further. See thing is, mazda may have spent alot of time and money designing the car, but that doesn't mean its going to be good. For example the FM frame rails make a Huge difference, why didn't they make those stronger from the factory?
Oh I do have a reasonable understanding of mech eng too, ironically the highest mark I have gotten so far was a mechanical subject, electrical is soo freaking hard, but in this case I wasn't interested so didn't bother researching.


So did the PPF design change from '89 to now?

[orx626]

Sorry that weren't of any assistance to you.

[Mokesta]

Just about all undergrads and new grads think they know all about engineering... until you ask them to design something.

I can't wait for my next batch of 3 grads to start after their final exams in November. {sigh..} More noses to wipe. Another couple of months of shot productivity.

[AJ]

LOL @ Mokesta, back to the start of your cynical cycle hey mate??

[zoomzoom]

Just about all undergrads and new grads think they know all about engineering... until you ask them to design something.

I can't wait for my next batch of 3 grads to start after their final exams in November. {sigh..} More noses to wipe. Another couple of months of shot productivity.

Good to hear its 4 years well spent. I can't wait till I get out, I am pretty certain I will not know much of use either, at least I am around plenty of knowledgable people to help guide me.

[Hellmun]

I doubt average Joe Bloggs would get in a car like the mx5 and think the chassis rails aren't stiff enough. It wasn't worth the money to Mazda. Replace frame rail with say, refrigerated cubby hole as it's essentially the same notion. It's bad service to force things on customers they don't need or want and charge them for it.

[Sasso]

Average Joe Bloggs don't buy mx5's and yes if they did they would notice the flex (on NAs anyway), just not be able to go, "oh it needs frame rails."

Sorry that weren't of any assistance to you.

I never said that, you and mokesta helped confirm all the doubt about the brace, and thank you.

Just about all undergrads and new grads think they know all about engineering... until you ask them to design something.

I can't wait for my next batch of 3 grads to start after their final exams in November. {sigh..} More noses to wipe. Another couple of months of shot productivity.

I never implied that I know everything, but I do know not to blindly trust everything I read or hear, I'm just a sceptic because in my experience most things you read and hear are wrong, not that I think you're explanation is wrong, I just don't like when people make assumptions and don't provide evidence or pictures and calculations (referring to the posts on m.net).

And regarding grad studends, the reason most of them don't know anything is because they basically give you the degree without having to know anything. Fortunately if you do electrical at UTS there is a lecturer that fails nearly everybody because he only wants people that can actually demonstrate knowledge to pass so he sets hard exams and makes you go out and find ways to learn and understand the material. Unfortunately he only teaches electrical.
Sorry to hear about your grads, but don't be so fast to blame the student.

[mmm]

Everyone is touchy.............

The way I see it...the 5 (NA/NB in paticular) is a convertable, so its bound to flex.

Flex or no flex, I will just drive that car until it snaps in two. I won't even spend more than $100 on bracing. Let it snap.

I suppose if I had wanted a ragtop with less flex I would go for the NC or the S2000 but $$ is an issue. But you see....at uni, 13 years ago, I failed everything and got asked to leave (seriously!).

Previously I had a brand new Nissan SI5 and spent money on all aftermarket braces that were available (7 in fact...front/rear strut, room bar, front lower brace, front lower frame, rear lower bar, brake stopper brace). On the track it did handle better after each one and lap times dropped accordingly. However, in hind sight (which is always 20/20) I shouldn't have bothered and should have spent the $$ on other life neccessities which would have made my overall life experience better. Those improved lap times, afterall, were just some numbers. When the car was sold they now mean jack.

I would have had just as much fun with the car bending and twisting.

I know that I will part with the Mx5 one day so as long as the experience was fun, the car would have served its purpose. So I won't bother with the engine mounts on the 5. When I get into the wrong gear at Wakefield (from 2nd to 5th) I will just clutch in and change again.

I will just enjoy my humble NB for what it is and not get too caught up in theory/worry.