A word on Weight distribution
08-31-01, 04:37 PM
#1
A word on Weight distribution
Ideally you'd want 50/50 distribution between the front and rear wheels. But what actually happens to the performance when you don't have this?
Does a front heavy car understeer?
conversely,
Does a Butt heavy car oversteer?
Couldn't suspension and tires be adjusted to correct these problems?
I'm talking street.
Drag cars have their own set of concerns.
Does a front heavy car understeer?
conversely,
Does a Butt heavy car oversteer?
Couldn't suspension and tires be adjusted to correct these problems?
I'm talking street.
Drag cars have their own set of concerns.
08-31-01, 04:51 PM
#2
smog nazi destroyer
i donot know the answer, but i had been woundering that about your mg. once you drop your seven into it, it definatly will not have the desirable weight distro. and with how light you will be, not having that may make things a bit squerly.
08-31-01, 09:45 PM
#3
8krpm is not enough
generally speaking, yes to all three questions.
you can correct either understeer or oversteer with coil overs, sway bars, ballast, tires, ect. Of course, almost any RWD car is going to tend to oversteer, and any FWD car is going to tend to understeer, that's just life.
you can correct either understeer or oversteer with coil overs, sway bars, ballast, tires, ect. Of course, almost any RWD car is going to tend to oversteer, and any FWD car is going to tend to understeer, that's just life.
09-01-01, 02:33 PM
#4
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The classic Porsche 911 is tail heavy and they managed to get it to work, although it is a difficult car to "bring back" once the rear decides to pass the front.
Despite my tail heavy guess on the Midget, it is very stable and predictable with neutral handling in a steady state corner. It will understeer if you don't heel toe your down shifts while braking hard setting up for a corner, and it will also push out if you roll on the gas too soon (LSD doesn't help). But that is how most performance cars drive.
You can usually tune the car by adjusting spring rates, shock rates, sway bars and even stagered tire sizes (ala 911).
Despite my tail heavy guess on the Midget, it is very stable and predictable with neutral handling in a steady state corner. It will understeer if you don't heel toe your down shifts while braking hard setting up for a corner, and it will also push out if you roll on the gas too soon (LSD doesn't help). But that is how most performance cars drive.
You can usually tune the car by adjusting spring rates, shock rates, sway bars and even stagered tire sizes (ala 911).
09-01-01, 03:25 PM
#5
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Are midgets rear wheel drives??
Everything i learn about weight distrubution came from Gran turismo 3, The info isactually very helpfull and true in the real world too.
Everything i learn about weight distrubution came from Gran turismo 3, The info isactually very helpfull and true in the real world too.
09-01-01, 04:49 PM
#6
Old [Sch|F]ool
Ok, here's my rant:
Rear weight bias is GOOD!
It means the rear brakes can handle more of the braking duties instead of making the fronts do 90% of the work. It means the front end is nice and light for steering that is out of this world - it can communicate everything without requiring too much effort. Given that we only care about RWD vehicles, having more of the weight over the rear tires is a Good Thing for acceleration. While it does mean that the car's steady-state tendencies are for oversteer, this isn't necessarily a bad thing for a driver who is expecting that - in fact it can be turned into an asset for particularly tight corners. (Having excellent steering also means that oversteer is much less scary - see point #2)
50/50 weight bias being "perfect" reminds me of the notion that having the bore equal to the stroke of a piston engine (aka a "square" design) is "perfect", neither of which are true. Multivalve engines work best with an undersquare (smaller bore than stroke) design because it gives an optimal combustion chamber in the critical first third of the burn, two-valve engines work best with an oversquare (figure it out) design because they are valve limited, and racing engineers have found that the optimal weight bias is about 36/64.
"The world is going to end in 2000. That's because 2000 is a biiig and rooound number" - Dogbert
Rear weight bias is GOOD!
It means the rear brakes can handle more of the braking duties instead of making the fronts do 90% of the work. It means the front end is nice and light for steering that is out of this world - it can communicate everything without requiring too much effort. Given that we only care about RWD vehicles, having more of the weight over the rear tires is a Good Thing for acceleration. While it does mean that the car's steady-state tendencies are for oversteer, this isn't necessarily a bad thing for a driver who is expecting that - in fact it can be turned into an asset for particularly tight corners. (Having excellent steering also means that oversteer is much less scary - see point #2)
50/50 weight bias being "perfect" reminds me of the notion that having the bore equal to the stroke of a piston engine (aka a "square" design) is "perfect", neither of which are true. Multivalve engines work best with an undersquare (smaller bore than stroke) design because it gives an optimal combustion chamber in the critical first third of the burn, two-valve engines work best with an oversquare (figure it out) design because they are valve limited, and racing engineers have found that the optimal weight bias is about 36/64.
"The world is going to end in 2000. That's because 2000 is a biiig and rooound number" - Dogbert
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09-03-01, 12:44 PM
#9
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I was out at the SCCA National race at Heartland Park this weekend and got the chance to drive an FP Midget. The FP guys needed another car to start the race in order to get free tires from Goodyear. There was a guy there who had a Midget, but no National Competition license. I was there with my National Competition license and no race car (I hit a wall at 130 MPH a few weeks ago at Gateway, so my RX-7 is done for this year).
Anyway, I was thoroughly impressed with how the car handled. As compared to my EP RX-7, it was like driving a shifter cart. You don't need to slow down hardly at all to go around a turn. All I could think about was how cool that car would be with a good rotary in it!
But about weight distribution.....Ideally, you want as must rear weight bias as possible, as much of the weight to be inside of the 4 wheels, and as little unsprung weight as you can get. Another key thing to consider about weight is lightening and balancing any part that rotates makes considerable more difference than lightening parts that don't roatate.
Anyway, I was thoroughly impressed with how the car handled. As compared to my EP RX-7, it was like driving a shifter cart. You don't need to slow down hardly at all to go around a turn. All I could think about was how cool that car would be with a good rotary in it!
But about weight distribution.....Ideally, you want as must rear weight bias as possible, as much of the weight to be inside of the 4 wheels, and as little unsprung weight as you can get. Another key thing to consider about weight is lightening and balancing any part that rotates makes considerable more difference than lightening parts that don't roatate.
09-04-01, 07:51 AM
#11
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Originally posted by Chuck Clark
Anyway, I was thoroughly impressed with how the car handled. As compared to my EP RX-7, it was like driving a shifter cart. You don't need to slow down hardly at all to go around a turn. All I could think about was how cool that car would be with a good rotary in it!
Anyway, I was thoroughly impressed with how the car handled. As compared to my EP RX-7, it was like driving a shifter cart. You don't need to slow down hardly at all to go around a turn. All I could think about was how cool that car would be with a good rotary in it!
LBC - Little British Car
Good to hear that you got some track time while your ride is mending.
The FP car you drove was probably putting out about 120 FWHP in a national level car. Imagine that same car, same weight with a PP 13B
Even with my stock 13B, my friend that runs a regional GProd Midget said I could kick his but, and I have street tags! 
If you want a rotary Sprite/Midget (Spridget), feal free to e-mail me. I hope to have some motor mounts fabed up soon and for sale.
09-04-01, 12:51 PM
#12
Wow,
thanks for all the responses.
PeeJay, I think you have me convinced that a little more rear weight will not hurt, at least.
Jeff20B, I know how you feel.
RX-Midget, nice sig. How did the engine hunting go last weekend?
I just pull the 13B from my donor RX, and doing a little cleaning/modding before the transplant.
-Move oil pan drain plug for clearance.
-Switch shfter extension housing to that from B2000 truck.
-A little sheet metal work.
I originally posed this question to RX-Midget who has his engine a little rearward from stock. I suppose that if I install the 13B at the stock location, then it should be good enoungh (street car). But based on my measurements, I could set the engine back another 9 inches towards the firewall. Why, more work, but it'll be bugging me forever if I don't, unless I can find a good performance reason not too.
Thinking of a car like a dumbell, I would guess that having the weight centered of the wheels would be more beneficial. The car would be less like a dumbell and more like a top.
Phase 2 of this project will be the installation of a shortened GSLSE rear end. So I'll have vented disks and wide tires, completely oversized compared to stock. So it would seem that more weight in the rear would be better.
thanks for all the responses.
PeeJay, I think you have me convinced that a little more rear weight will not hurt, at least.
Jeff20B, I know how you feel.
RX-Midget, nice sig. How did the engine hunting go last weekend?
I just pull the 13B from my donor RX, and doing a little cleaning/modding before the transplant.
-Move oil pan drain plug for clearance.
-Switch shfter extension housing to that from B2000 truck.
-A little sheet metal work.
I originally posed this question to RX-Midget who has his engine a little rearward from stock. I suppose that if I install the 13B at the stock location, then it should be good enoungh (street car). But based on my measurements, I could set the engine back another 9 inches towards the firewall. Why, more work, but it'll be bugging me forever if I don't, unless I can find a good performance reason not too.
Thinking of a car like a dumbell, I would guess that having the weight centered of the wheels would be more beneficial. The car would be less like a dumbell and more like a top.
Phase 2 of this project will be the installation of a shortened GSLSE rear end. So I'll have vented disks and wide tires, completely oversized compared to stock. So it would seem that more weight in the rear would be better.
09-05-01, 07:30 AM
#13
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Ok, I did a little thinking this morning, and that can be dangerous!
Anyhow, see if you can follow my logic and math.
The Midget has a wheel base of 80”
All weight is distributed over this 80” span.
Estimated weight of 13B, trans and “stuff” – 300lbs
Reference is to 13B mounted in stock location.
If you mount the motor 9” further back from stock:
9”=11.25% of the 80” wheel base
11.25% of 300lbs = 33.75lbs that will be transferred to the rear wheels
My motor is about 4.5” back from stock (from memory), so:
4.5” = 5.625% of the 80” wheel base
5.625% of 300lbs = 16.875lbs
IF I am correct, the effort to move the motor back and additional 4.5” only gives you about 17lbs of weight bias. This does not seem to be worth the effort when you could easily move other items like the battery, or lighten up the front.
Please correct me if I’m wrong in my thinking. Time for some more caffine…
Anyhow, see if you can follow my logic and math.
The Midget has a wheel base of 80”
All weight is distributed over this 80” span.
Estimated weight of 13B, trans and “stuff” – 300lbs
Reference is to 13B mounted in stock location.
If you mount the motor 9” further back from stock:
9”=11.25% of the 80” wheel base
11.25% of 300lbs = 33.75lbs that will be transferred to the rear wheels
My motor is about 4.5” back from stock (from memory), so:
4.5” = 5.625% of the 80” wheel base
5.625% of 300lbs = 16.875lbs
IF I am correct, the effort to move the motor back and additional 4.5” only gives you about 17lbs of weight bias. This does not seem to be worth the effort when you could easily move other items like the battery, or lighten up the front.
Please correct me if I’m wrong in my thinking. Time for some more caffine…
09-06-01, 02:35 PM
#14
Ah, I'm sucking on a mountain dew 
For those who don't know about the history of my rotary midget, you can see it here http://www.geocities.com/cd23c/cars.html It used to have a twin dizzy 12A and top mounted starter tranny.
I mounted my 13B with the same modified RX-2 style motor mount(solid center, no oval to reduce weight
) but I had to move the bottom mounted starter tranny rearward by 20mm to upgrade from a 12A to a 13B. Otherwise, the front mount is bolted to the rearmost stock motor mount holes (they were drilled out and stock Mazda rubber motor mounts were used, but slightly cut to reduce overall size. Did that make any sense? Ah, more caffience)
I'm not sure how far back the engine is compared to stock, but I still have the option of moving the battery to the boot.
For those who don't know about the history of my rotary midget, you can see it here http://www.geocities.com/cd23c/cars.html It used to have a twin dizzy 12A and top mounted starter tranny.
I mounted my 13B with the same modified RX-2 style motor mount(solid center, no oval to reduce weight
) but I had to move the bottom mounted starter tranny rearward by 20mm to upgrade from a 12A to a 13B. Otherwise, the front mount is bolted to the rearmost stock motor mount holes (they were drilled out and stock Mazda rubber motor mounts were used, but slightly cut to reduce overall size. Did that make any sense? Ah, more caffience)I'm not sure how far back the engine is compared to stock, but I still have the option of moving the battery to the boot.
09-07-01, 03:13 PM
#15
et tu Jeff20B?
Jeff20B,
all this time I didn't know that you too had a rotary powered Midget? Very, cool. We almost have our own sub-club going here!
Nice write up. I have to work on my web page more. I'm not satisfied with it yet, but once I get the content up a bit more I'll post the ULR for it here on this board for all to see.
I started with a twin dizzy 12A, but took it out in favor of the 13B. The top mounted starter was nice, but I prefer being able to use the B2000 extension housing on the later tranny. I didn't know that you could swap out the front covers, so maybe I'll keep the engine around as a back up incase something happens like RX-Midget's Apex seal.
You mention Eric's MG12a project. I was corresponding with Eric back when he was getting started. He's been on the road for a few years now, and I'm still wondering about where to place the engine.
all this time I didn't know that you too had a rotary powered Midget? Very, cool. We almost have our own sub-club going here!
Nice write up. I have to work on my web page more. I'm not satisfied with it yet, but once I get the content up a bit more I'll post the ULR for it here on this board for all to see.
I started with a twin dizzy 12A, but took it out in favor of the 13B. The top mounted starter was nice, but I prefer being able to use the B2000 extension housing on the later tranny. I didn't know that you could swap out the front covers, so maybe I'll keep the engine around as a back up incase something happens like RX-Midget's Apex seal.
You mention Eric's MG12a project. I was corresponding with Eric back when he was getting started. He's been on the road for a few years now, and I'm still wondering about where to place the engine.
09-08-01, 01:30 PM
#16
Hey thanks! Yeah, the RE Midgets is what we are
Is your MG the 13B project midget on Eric's page that has been taken all apart?
Yes, my twin dizzy 12A has been modified to fit a '74-'85 front cover. One of the front cover bolt holes had to be tapped with a 14mm bolt installed because the front cast iron housing didn't have this bolt. however one of the other bolt holes ended up not being used. I suppose it's ok because the later front covers were able to support more power (like the 12A turbo and GSL-SE engine). However, I'll be doing some sort of 2nd or 3rd gen motor mounts in order to handle more power if I ever choose to increase the HP by any sort of large percent. Even the 15A had 6 front mounting bolts instead of the 12A/13B's 4 bolts.
I'll be putting my REPU engine back into my MG possibly sometime soon, but it'll have to be after I get something else running first. It sure will be interesting to drive this thing!
Is your MG the 13B project midget on Eric's page that has been taken all apart?
Yes, my twin dizzy 12A has been modified to fit a '74-'85 front cover. One of the front cover bolt holes had to be tapped with a 14mm bolt installed because the front cast iron housing didn't have this bolt. however one of the other bolt holes ended up not being used. I suppose it's ok because the later front covers were able to support more power (like the 12A turbo and GSL-SE engine). However, I'll be doing some sort of 2nd or 3rd gen motor mounts in order to handle more power if I ever choose to increase the HP by any sort of large percent. Even the 15A had 6 front mounting bolts instead of the 12A/13B's 4 bolts.
I'll be putting my REPU engine back into my MG possibly sometime soon, but it'll have to be after I get something else running first. It sure will be interesting to drive this thing!
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