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Guys, talk to me about this as a possible setup for my A-Mod.



I know, it doesn't apply brakes to both rear wheels...

Other than that, isn't it just effectively an open differential?

_________________
Too much week, not enough weekend.

OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216

I think it's just half an open diff. In a straight line you would get half the acceleration. Is that a picture of an oval track car or is the drive shaft just temporarily removed? For low power I suppose it would work.

Formula Fords run open diffs, I only ever found one place that it would spin up the inside tire just a bit. If the inside tire was the only one getting power I think it would have been easy to spin it up.

Autocrosses are slower, so lower gears and easier to spin up the tire. My FF had 106 HP and a 56 rear weight bias...

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
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It's my A Mod with one axle temporarily removed. I haven't scaled the car, so I don't know the weight bias.

With the spool rear end she is a tail happy beast. The tighter the turn the more she crabs and the looser it gets. What I learned yesterday in our 2 tight turns was that I had to brake, turn in, coast through, and not apply power until I had completed the turn.

If I started rolling on power at the apex she'd spin out.

I'm toying with the idea of running without a left axle at the next event.

_________________
Too much week, not enough weekend.

OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216

First thing that comes to my mind is it will want to dart a bit on braking. Then it will behave completely differently depending on whether the turn is a left or right hander. Have you no option for an open or LS diff?

In a right hand turn you're just going to spin the only drive wheel as weight transfers laterally off of it. If it was a good idea I think we'd see more cars doing it.

My Sprite's differential is welded and once you get used to it the spool isn't bad. I'd keep learning it.

How progressive or linear is your throttle pedal? Might be worth playing with it to make the throttle less sensitive from idle to half throttle.

Any chance its also just mid-engine car stuff? What you're describing doesn't sound too odd.

_________________
-Andrew
Build Log
Youtube

I'm not too terribly concerned about movement on braking as I'm heavily front biased. I may feel it or I may not, only way to know is to test.

Spinning the inside tire on a right hander is my primary concern. I can add sway bar to force plant the inside tire.

I think it's pretty linear. I haven't looked closely at the tb cam though. I modified my accelerator pedal to lower the ratio a while back and give much more forgiving throttle response. It has 3 inches of travel from idle to WOT.

I'm honestly not sure Andrew. I was not expecting this to be a point and shoot car. It goes like a bat out of hell in a straight line but won't accept power in a turn.

_________________
Too much week, not enough weekend.

OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216

I'll most likely end up building a differential sometime this year. Right now I'm searching for a unit small enough to house within the constraints of the existing carrier.

_________________
Too much week, not enough weekend.

OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216

That seems like a reasonable amount.

It has a throttle position sensor right? Can you figure out the voltage at idle, voltage at WOT, then map the voltage for every 1/4" or 1/2" of pedal movement and plot it in Excel? I'd be curious to see if it does anything odd.

Do you have any spare sprockets you can play with? Dropping a few teeth off of the rear or adding one or two to the front may calm things down.

_________________
-Andrew
Build Log
Youtube

Good idea on throttle mapping. I'll see what I can do with that.

This is a smaller sprocket than what was on it. Of course this is a different surface than my 3 1/2 runs at Roebling a few weeks ago. Freestyle is pretty coarse.

I took on of the better runs and slowed the back half of the video down to 1/4 speed.
It's boring but you can see my hands and the steering inputs. Can easily see when it loses grip.

https://youtu.be/LzQvEmlKvzs

_________________
Too much week, not enough weekend.

OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216

So let me get this straight: You've got a high power to weight ratio mid engine car car that brakes well, accelerates well when not turning, and turns well when not accelerating, but does not accelerate well while turning at the limit...And you want to reduce deceleration and stability while braking, cut the straight line acceleration in half, and make trying to accelerate while turning one direction into not accelerating while turning as a result of uncontrollable inside wheel spin that will then need to be rehooked before accelerating (half as hard) in a straight line again, all in the name of a possible slight improvement in accelerating while turning in the opposite direction?

This combined asymmetry and loss of power application does not sound like a net gain to me.

And no, it's not effectively an open differential. In addition to allowing them to rotate at differing speeds, an open diff (basically) still applies equal power to both wheels right up until one wheel loses traction. This would not even do that.

_________________
-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

The only thing I would change about your statement is turns well when not accelerating. I learned during my runs that I could coast it through a tight turn without looping it. It wasn't happy and it wasn't fast.

Still, overall opinion is this a really bad idea. I'm going to take it to my work parking lot and make a couple of hot laps. Specifically tight right handers.

It only takes 20 minutes to put the axle back in. Then I'll be looking at camber and toe adjustments to see if I can maintain some grip and softening rear shocks to increase body roll a little.

_________________
Too much week, not enough weekend.

OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216

Intersting. Your previous description actually sounded to me a bit like what I have seen from some exceptionally fast cars/drivers. I would think that there are other ways to tune some of this out, like a more progressive throttle linkage or F vs R roll stiffness...Although I'm not actually sure whether more front bias to plant the inside rear, or more rear bias to allow just the inside rear to slip easier with the spool. Some of it may also be the learning curve separating your current driving style vs the cars preferred driving style.

_________________
-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

Turns out this is a REALLY BAD IDEA. I didn't have to take it for testing, just grabbed the wheel and pushed / pulled and was able to move the bearing in the hub.

I'll slap the axle back in later this evening and think about geometry to help keep the outside wheel planted and allow the inside to slip.

_________________
Too much week, not enough weekend.

OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216

Oh, by the way...You'll want to bolt an outer CV housing into the hub before taking it for a drive in this configuration.

_________________
-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

Yep. That could have been bad.

_________________
Too much week, not enough weekend.

OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216