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Engine, Exhaust, Transmission Discuss the FR-S | 86 | BRZ engine, exhaust and drivetrain.


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Old 09-09-2014, 05:59 PM   #29
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Originally Posted by DarkSunrise View Post
Yeah just to clarify, I meant that 50/50 would be faster than the Twins' 54/46.
My apologies, I misinterpreted.
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Old 09-09-2014, 06:32 PM   #30
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Originally Posted by DarkSunrise View Post
Yeah just to clarify, I meant that 50/50 would be faster than the Twins' 54/46.

Like you, I also suspect (although haven't seen it quantified) that a moderate rearward balance would be even better for pure performance. Something like the Cayman's 46/54 or even the Carrera's 38/62. You get:

- more even weight distribution under braking
- more rearward weight distribution under acceleration
- less weight upfront for better turn-in

The main downside I see is, if the rear end did let go, it would be a wild ride, as it was with the old 911's. But that might more specifically be a function of the polar moment of inertia rather than weight distribution. If you could centralize the weight sufficiently, you could minimize this downside.
You're right on two out of three. Less weight up front doesn't necessarily equate to better turn-in. In practice in a street car it usually does, but the question is one of yaw moment generated by the front tires vs polar moment of inertia. A car with a "dumbell" arrangement with the engine in the front, trans in the back (Corvette, Porsche 944) could have more of its weight on the rear wheels, but still not turn-in as well as a more front heavy car with more centralized mass (FT86, Mazda RX7/8).

To Captain Snooze's point, any engineer who says 50/50 is idea is basing their analysis on the most simplified of models. I.e. the bicycle model of cornering dynamics with "large" radius turns and no dynamic cornering (steady state cornering only). Under those conditions, 50/50 is ideal because it minimizes the maximum load on the tires. Since rubber is a non-newtonian material with a non-linear response of coefficient of friction relative to normal force (more load = lower friction) having the load as even as possible results in the highest cornering grip.

In the real world, however, we do more than steady state cornering on large radius turns. We transition from straight to cornering, we do quick left-right transitions, we brake into and accelerate out of turns. All of those favor having more weight on the rear wheels, how much more is dependent on many factors such that there is no "ideal" weight distribution. For a car with a given amount of power at a given racetrack there is an ideal, race engineers in professional motorsport play with the ballast all the time to get it. In practice the twins would be much closer to ideal if we drove them backwards. There's a reason everybody points to the Boxster/Cayman as the exemplar of balance.

Another wildcard is rear-wheel steering. Neglecting tire slip-angles the kinematic point of yaw rotation for a front-steer car is the center of the rear axle (another simplified model, though more sophisticated than the bicycle). With rear wheel steering you can move this rotation point forward and reduce the benefit of rear weight bias to transitional cornering, thus moving the ideal point toward the front wheels. Torque vectoring can have a similar effect. Most street cars have passive rear steering, though all I'm aware of have passive toe-in for stability rather than toe-out for quicker transitions.

Edit: As to tail-swinging with rear weight bias, I doubt a more centralized arrangement than the 911 is possible with 40/60 distribution. The heavy bits are all concentrated close to each other. The older 911s were widowmakers mostly because of the camber behavior of their trailing-arm rear suspensions, the weight distribution didn't help, but the suspension was the real culprit. See E30 BMW (and Z3) for similar problems in a car with 50/50 distribution.
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Old 09-09-2014, 06:46 PM   #31
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I believe the thinking is that 50/50 will get around corners fastest because the front and rear tires are equally loaded, and the car can then be tuned so that both ends use every bit of friction the tires have.

Only applies to steady state cornering. In reality as has already been stated, the acceleration and deceleration advantages of a moderate rear bias mean that *all else held equal* better lap times are possible.

The moderate front bias of the twins makes sure the front end stays in front making for a more forgiving, and importantly for Japan especially driftable car.

It just depends on what you want the car to do. The twins are meant to be fun above all else, and a moderate front bias is great for that.
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Old 09-09-2014, 07:28 PM   #32
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It is tempting to imagine a bigger engine in there. The engine bay is big enough to fit the flat 6 , 3.6. But unfortunately the steering rack is in the way and there's nowhere else for it to go in the current design. In the awd models the drivetrain is raised and further forward for the front driveshafts to fit and the steering rack sneaks in under everything as there is more room. No doubt that's behind the reversed front LCA on the BRZ.
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Old 06-04-2019, 01:32 PM   #33
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Old thread but point is still valid.

The weight distribution of my (AVO turboed) BRZ is even 56/44 (empty) and this is definitely a disadvantage in slaloms (autocrosses) because one cannot accelerate as quickly at the start and out of slow corners as the 4WD and mid-engined competition.

Also, if the engine was moved back (and the drive shaft shortened), the polar moment of inertia would decrease with this car and turn-in should be improved.

Maybe it'd be more difficult to control, but it'd be faster.

It's unfortunate that the steering rack cannot be adapted, as there's lots of space between the engine and the fire wall. Well, there may be an issue with the transmission and the transmission tunnel too.
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Old 06-04-2019, 02:57 PM   #34
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Why?
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Old 06-04-2019, 03:02 PM   #35
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Drive in reverse or switch the gears so you 6 reverse gears. You'll turn it into Porsche Boxster kinda thing.
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Old 06-04-2019, 03:07 PM   #36
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Drive in reverse or switch the gears so you 6 reverse gears. You'll turn it into Porsche Boxster kinda thing.
But then it will be front wheel drive. Rear steering though, kinda like a forklift with a torque dip.
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Old 06-04-2019, 03:44 PM   #37
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But then it will be front wheel drive. Rear steering though, kinda like a forklift with a torque dip.


You're right, but the torque dip will help as you can't get into a tangle with a Frankenstein Twin..
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Old 06-10-2019, 11:42 AM   #38
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Why?

Less wheelspin -> faster acceleration -> less time lost.
https://youtu.be/nGR3dsaB_LU
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Old 06-10-2019, 11:45 AM   #39
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Just put somebody in the trunk
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Old 06-10-2019, 12:17 PM   #40
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I actually did add 120 lbs of extra weight in the trunk.
It did help somewhat as far as initial acceleration was concerned. But it what was detrimental in turns and detrimental in acceleration after wheelspin stopped.
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