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So, by applying the gear ratios (end to end) simple multiplication turns torque at the flywheel into torque at the axle, and due to the 24" diameter of the wheels that means each foot/pound of torque at the axle equals one pound of lateral thrust at the contact patch.
Gear Ratios, including Final Drive
Which yields this Thrust Chart!
Note real world thrust is limited by traction...
This interests me for a few reasons:
1. the car is traction limited on the bottom two gears on anything but the stickiest road
2. as usual, max thrust is achieved by using all RPM before selecting the next gear
3. throttle induced wheelspin is not only possible, but also frequent in the bottom two gears - feather or spin!
4. adding even more power underscores the underlying predicament that on acceleration weight transfer is the enemy!
Acceleration values over .5 G are unlikely in the real world, at least in my experience. Hoosiers or drag slicks with 9" + wheels can do it, but it is not easy! This again underscores the point of diminishing return for engine power...
On my S a 10# reduction in weight per wheel is really noticeable in acceleration. There is also a slight improvement in ride from the reduced unstrung weight. You are really going to love them.
It is winter. Days are short, and temperatures are cold. Oh woe.
What to do?
Well, if you are (as I am) a borderline geriatric child motor head frustrated wannabe, then you spend the winter in a driver's course so as to prepare for spring time.
What course is that d'ye say?
Why the on-line tutorial recently published by fellow SCCA member (and National Champion) Vivek Goel.
Who has published a focused, relevant, well footnoted, and unique set of materials aimed exclusively at autocross.
These materials are based on Vivek's first hand experience, but the bibliography has also been a tremendous resource.
At present, and I've ground yet to travel I am trying to ingest and internalize two key points - one from each of these two tomes:
From The Lost Art - drive collaboratively (which focuses on my street driving which features short following distances and quick decisions - not pretty), and
From the Perfect Corner - consider the Euler Spiral! Believe it or not Pierro Taruffi has this (at least on exit) in the "Technique of Motor Racing" 1958 but I had missed it completely!
And to practice?
Well, I am one of the last OG hold-outs for the "Grand Prix Legends" and hone my skills (read, make thousands of mistakes) in that domain. Specifically I practice with a 1967 Ferrari F1 car on the Nurburgring, which is a merry challenge.
Look ahead? Yup that applied big time.
Be smooth? Yup, that too...
Feel the weight shift? Oh hell yes!
See the spirals? Actually, yeah - I am starting to connect it all.
Anyway, Vivek Goel is the first top rank auto-crosser to write anything down that I could grab a hold of, and I am most grateful.
It is winter. Days are short, and temperatures are cold. Oh woe.
What to do?
Well, if you are (as I am) a borderline geriatric child motor head frustrated wannabe, then you spend the winter in a driver's course so as to prepare for spring time.
What course is that d'ye say?
Why the on-line tutorial recently published by fellow SCCA member (and National Champion) Vivek Goel.
Who has published a focused, relevant, well footnoted, and unique set of materials aimed exclusively at autocross.
These materials are based on Vivek's first hand experience, but the bibliography has also been a tremendous resource.
At present, and I've ground yet to travel I am trying to ingest and internalize two key points - one from each of these two tomes:
From The Lost Art - drive collaboratively (which focuses on my street driving which features short following distances and quick decisions - not pretty), and
From the Perfect Corner - consider the Euler Spiral! Believe it or not Pierro Taruffi has this (at least on exit) in the "Technique of Motor Racing" 1958 but I had missed it completely!
And to practice?
Well, I am one of the last OG hold-outs for the "Grand Prix Legends" and hone my skills (read, make thousands of mistakes) in that domain. Specifically I practice with a 1967 Ferrari F1 car on the Nurburgring, which is a merry challenge.
Look ahead? Yup that applied big time.
Be smooth? Yup, that too...
Feel the weight shift? Oh hell yes!
See the spirals? Actually, yeah - I am starting to connect it all.
Anyway, Vivek Goel is the first top rank auto-crosser to write anything down that I could grab a hold of, and I am most grateful.
Happ New Year y'all.
Cheers,
Charlie
PS: I have a spreadsheet which records the approximate time of cataclysm and root cause (usually not looking ahead) for my myriad deaths on the Nurburgring... There should be 50 rows, but I am lazy. I can do a 9 minute timed lap now (necessarily preceded by a warm up also 8 minutes), but only one before my concentration wavers in some fatal manner!
PPS; A good simulator with wheel and pedals is even less painful than autocross (at least in the winter time).
PPPS: An autocross is usually 70 seconds or less, so.......
So winter has arrived with sharp teeth, an my hibernation continues.
My dear landlord allowed me to hide in the garage for a couple of particularly nasty evenings, and the tires work pretty well around zero...
The driver's course continues, now encompassing a half dozen good books and about 60 Nurburgring laps in that immortal 312. I'm finally able to do three laps without some hysterical (and usually fatal) mistake at high speed. I claim I've learned the course, which just leaves the challenge of driving it quickly and consistently. My lap times are now in the 8:42 to 8:50 range, which is close to my all time best performances of 8:39 (race) and 8:41 (qualifying).
And the winter parts disease has set in, as I confess I just ordered up some camber plates for next year.
I reckon those nice 225s on the 8" rims will just love about -1.7 front, and -1.8 rear. I'm not tempted by coil-overs this time, as the sport springs and B8*s taken with the NM bar are quite capable, and I like the ride height just fine.
Hard to get a body roll number, but that's kinda fundamental when considering optimum static camber in a McPherson Strut front end.
I found an On-Screen Protractor (love SourceForge) that acts as a "top layer" when reviewing photographs, and then pulled out 129 Slayer's best photograph of Gollum IV turning in at 9 tenths.
Lean and mean...
From this analysis I do reckon that the max positive camber change on the outer edge of the car will be in the 2.5 to 3.0 degree range.
So it's a straight up (pun intended) trade off between best cornering (more camber) and best braking and acceleration (less camber).
Charlie,
Years ago Grass Roots did a one car (R53 race car, on a track) test of camber and worked their way up to -3.5 deg. If I remember correctly, anything above -3.0 was a looser in lap times because of the loss of braking capability, even though they were seeing higher cornering g’s. From their one car test, they concluded that -3.0 (maybe it was slightly above that) was the most someone would want to use. Of course, you could lower that number by adding some caster to the suspension. But I don’t know how to do that on a F series MINI.
I know the Grass Roots article is still on line, but I don’t have a link (sorry). It is an interesting read.
Charlie,
Years ago Grass Roots did a one car (R53 race car, on a track) test of camber and worked their way up to -3.5 deg. If I remember correctly, anything above -3.0 was a looser in lap times because of the loss of braking capability, even though they were seeing higher cornering g’s. From their one car test, they concluded that -3.0 (maybe it was slightly above that) was the most someone would want to use. Of course, you could lower that number by adding some caster to the suspension. But I don’t know how to do that on a F series MINI.
I know the Grass Roots article is still on line, but I don’t have a link (sorry). It is an interesting read.
I haven’t seen anyone talk about roll angel in ages (including that Grass Roots article). Back in the day... Road and Track would publish pictures of the cars they were testing with the roll angle marked on them. I’m not sure anyone knew what to do with that information, though, and they stopped doing it.
As for your analysis, remember that the suspension compression is adding a little bit of negative camber. It may not make a lot of difference, other than having you be more comfortable with a setting of -2.5 deg and not thinking you should go higher. I have friends who used -2.5 on their street cars for running on the track and liked it. They felt the added tire wear on the street was minimal.
Acceleration values over .5 G are unlikely in the real world, at least in my experience. Hoosiers or drag slicks with 9" + wheels can do it, but it is not easy! This again underscores the point of diminishing return for engine power...
That's because you are omitting weight transfer, the bane of FWD. The faster a FWD accelerates, the more weight transfers to the rear. That's also why you see hypercars like the McLarens keeping up with AWD ones now - the acceleration is so high that the front wheels are hardly contributing.
I have an Excel sheet that pulls in CG and wheelbase to estimate rear weight transfer, and a FWD with reasonable tires will really struggle to get below 5s 0-60. Also, an easier way to calculate contact patch thrust is from power, not torque - F=P/V. Just calculate what rpm you are at for a given velocity, and then you can get the force. Just be careful with units and the divide-by-zero.
11-27-2018, 04:35 PM
