jkstill

iTrader: (11)

So what is considered an ideal weight distribution?

I am assuming that the discussion is for cars used on road coarse.
A car used for drag racing benefits from much different weight distribution than a car used for road racing.

I've seen 50/50 touted as the ulimate goal, and the FD is near that. Not sure about FB and FC. This ignores corner weighting however, which should also be balanced.

It would seem that the ideal weight distribution might also depend on the kind of track that a car is driven on, tight twisty corners, or wide open sweepers.

A static weight distribution would be a compromise.

Does adding some weight to the front of an RX-7 actually harm handling?
Or can any ill effects be ameliorated with suspension tuning?

And how many of the drivers here are fast enough for it to matter?
( me included )

I can check what Carroll Smith says about it later.
(books are at home)

jgrewe

iTrader: (1)

I don't know if there is an 'ideal'. I'd try to get as much to the rear as I could with a front engine car. Also getting it as low as possible is important as well. There are so many variables and constants that you are going to compromise no matter what. Right/ left balance and cross percent are other issues.
So unless you are starting with a clean sheet of paper you play the hand you're dealt and move what you can to where it does the most for you. The rest you have to play with spring rates, anti-roll bars, dampers and tires.
You can have a car that is enjoyable to drive on the street that doesn't look good on the scales. There will always be that on/off ramp or twisty section of road that you might get near the limit while being stupid but I always looked at it as being able to avoid an accident with the superior performance if needed.
Until you run the car on track and maybe within a rule structure against other cars you won't care if there is 100 more pounds on one corner or one end of the car. Driver skill will have more effect on the car than that 100lbs on the street. At a track day a good driver will feel it.

sub9lulu

iTrader: (7)

for stock 255hp 50/50 might be ideal, but when u get to the 400+ hp level its a total different ball game, 45/55 might be better depends on how much traction you can get ( or how much weight u can put at the back without hurting the total weight of the car)
because you need to do everything you could to get the the rear hooked to the ground.

you would want to actually move as much weight to the back as possible, because thats the easiest way for traction. you might say the rear tires will wear out faster, but it'll be just as bad if you are doing burnout at the stop light

like jgrewe said its totally pointless for a horsepower queen that cant get any traction.

Rotary_Powerd

Yes, but then you might understeer when your foot is planted down ( like an older 911)

jgrewe

iTrader: (1)

Any car will push if you try to accelerate while still turned. They will do it until you overpower the rear wheels and they start to slip sideways. The fast way through turn is to get the car rotated and only squeeze on the throttle as you unwind the steering. The tires can only give 100% and if you are using all of it to turn, there is nothing left when you try to make them work harder by pushing them the direction the rear wheels are pointing.

Asterisk

iTrader: (3)

lol, THE only piston engine swap I actualy think is a good Idea. That, and that it's not American made.

Asterisk

iTrader: (3)

That's because... those DRIVERS were fast. They didn't need a faster/better/whatever engine to make up for their lack of skill...

BryanDowns

iTrader: (2)

Actually, I'd say all of those RX7's in those classes were faster than engine swapped Rx7's because an engine swapped Rx7 wouldnt be in that class.

Its a self fulfilling comment really. Its like saying the fastest FD's in at autox in ASP and SM2 still have rotaries in them. WELL NO DUH! If it had a piston engine it would be in XP or EM.

WKZ (Works)

Ding!Ding!Ding! We have a winner; there's a reason the engine is as far back behind the front axle as it is. Every V8 swap I've ever seen has the 1 and 2 cylinders (1 and 5 on a Ford V8) sitting way in front of the front axle. Proper weight distribution can be made, but it will require extensive cutting and welding; not just motor and tranny mounts like most people do it. Cutting the firewall, subframe, tranny tunnel; etc. (acutally, you may just be able to get away wth the subframe on many swaps) is neccesary to do the swap right if you care about anything other the point A to point B street driving or drag racing. Same is true with 20B.

The point is: TUCK THAT MOTOR BACK!

suckerfree

sometimes i feel like jimlab pays people to write these statements just so he can prove them wrong. or maybe im just being optimistic for the rotary community.

BryanDowns

iTrader: (2)

No ding ding ding... While you are correct that where the engine is matters, as previously stated its an entire system. Changing one variable doesnt ruin the system.

You were fine until you said:
Which is completely contradictory to all the guys successfully running their V8 swaps in motorsports that involve turning. Real world results dont tend to lie.

MikeFB

That's oversteer.

Understeer is when the front tires begin to lose grip and your front end plows forward, forcing the car to drift to the outside corner.

Rotary_Powerd

Nope Under steer. Acceleration moves the weight back. The front tires will have a smaller contact patch and therefore suffer understeer.

mar3

iTrader: (8)

Short answer...real world competition and GRM "$200x Challenges" have proven that the 2nd and 3rdgen cars do not suffer from weight distribution problems, especially if an aluminum V8 block is used.

1stgen cars, the SA and FB's do suffer bad weight re-distribution and are rendered uncompetitive except as drag race cars.

jgrewe

iTrader: (1)

While its a correct description of understeer, 911's understeer with slight throttle and SNAP oversteer when you lift. If you get out of the throttle totally near the limit its like somebody installed faster tail lights and the lights want you to follow them around the turn.

Boostmaniac

The questions I have I think Jimlab has the answer to.

Since the tranny can sit lower, does this negatively affect the angle of the driveshaft to the rear end or do you just go with a custom rear end setup?

It is also understood the center of gravity IS important, but between these 2 motors, the difference is most likely negligible. In track racing, polar moment of inertia is just as important if not more so than center of gravity, so does the V8 negatively affect the polar moment of inertia compared to a 13B or have there been no quantatative inquiries or measurements to draw any conclusions at this point?

And for the above comment, understeer is when the front tires exceed their maximum limit of adhesion before the rear tires do.

-Maniac

jgrewe

iTrader: (1)

All you have to do is keep the crank/output shaft of the tranny parallel to the pinion. If the tranny is pointing down in the back, have the pinion pointing up the same # of degrees. A couple degrees is better than none, it lets the u-joints work a little.

1stgenNutt

iTrader: (3)

I dont understand why anyone would take out the rotary engine to begin with ive seen 1st gen swaps and there useless as road cars and really dont fair that well as drag cars. they just make alot of tire smoke . the 1st gen has a very well balance wieght distribution to begin with chopping one up is the ultimate sin..
the 8th deadly sin just in front of lust.

BryanDowns

iTrader: (2)

More responses that arent really needed....

btw, lmao at not fairing well as drag cars. I've saw multible FB's running 9's in teh 1/4 with V8's in them. How does that not fair well? Either you are A) straight up lieing, or B) you have saw a really shoddy conversion

On a side note, autox'ed my ls1/fd again yesterday

mar3

iTrader: (8)

With a four link in the back, any car can be a great drag race car...even a brick like a '55 Chebby or even a truck w/o a bed.

RX7SIGN

I spoke to a Golden Cup Porsche owner the other day who knows quite a bit about Rx7's and does ALOT of professional SCCA racing. He is probably in his 40's and has been around cars for awhile. He basically spoke against the V8 swap due to the amount of high performing 13b and even 20b Rx7's he has seen, I want to try and get a more in depth explantion of why he doesnt like the V8 swap for **TRACK** use.

aussiesmg

iTrader: (1)

There is a lot of myths and unsubstantiated stories being spoken of in here now, YOUR choice as the whether a V8 should be in MY car doesn't concern me and is not the reason for this thead, so get back on point or stay quiet.

The overall center of gravity is tunable, while lowering the engine height helps this is not the only method to lower the overall center of gravity, further it is possible to lower the center of gravity too far, this can cause problems such as bump steer.

The pinion angle does need to be adjusted for a v8 in most cases as the rotary angle is around 6 degrees whereas the V8 is around 3 to 4. Due to the height of the transmission.......lol

American Rice

Yes it is the lower centre of gravity that you are after then. And you can make other adjustments to your suspension if you want your car to be more "loose" or "tight" in the corners.

American Rice

That is a great point. I a sick of hearing all the 50/50 talk. It mostly matters for what you want to do with your rex. Like my weight ratio should be in the neighborhood of 40-45/60-55 when my cage gets put in. Cause mine is going to used for drag racing and just a street legal power car that I MADE to launch in a straight line not take a corner at 90 mphs and have full control. lol **** my car doesn't even have power steering or a front sway bar in it.

toplessFC3Sman

iTrader: (1)

the center of gravity has nothing to do with bump steer. bump steer is a purely geometric thing based off of the way the suspension moves. The only possible connection a lowered CG will have with bump steer is taht a lowered CG will make the car roll less, giving less bound of the outside wheels and jounce of the insides, thus giving less bump-steer. For steady-state handling, assuming the same size tires are being run front and back, a slightly rearward weight distribution is ideal (btwn 45/55 and 40/60) because as a car corners, it slows as well (tire scrub, energy lost in gaining & maintaining slip angles, tire heating), so weight is shifted sideways and forwards. Ideally you want all 4 tires as evenly loaded as possible due to tire load sensitivity. Essentially, the coefficient of friction (mu*Fn = Ff, mu= coefficient of friction, Fn is normal force, Ff is frictional force) is inversely dependant on load, so that as load increases, mu decreases (but not nearly as quickly). So although the frictional force still increases with greater load, it increases less than proportionally, meaning that the total proportional grip goes down. Transient handling is a completely different matter, and a description of that would get very out-of-hand very quickly, so suffice it to say that it is dependant on both weight distribution and polar moment of inertia.

wow, i got sidetracked a bit.