[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: DYNO RESULTS -- STOCK vs AFTERMARKET WHEELS
At 11:50 AM 4/20/2000 , Jonas Karlsson wrote:
>Question: Did this Dyno automatically recalculate the figures into HP at
>the crank?
Nope-- these were hp measurements at the wheel. The first time I dynoed
the car, I did a coast-down measurement, and once plugged in, peak HP came
out to 123 (i.e. stock) at the engine.
>I don't know much about Dyno's but it would seem to me that if the dyno
>took some readings while everything was spinning down, then it could
>calculate the effective crank HP figures.
Bingo.
>If done right this would throw out the losses/gains due to wheel weight
>(theoretically). The result would show no HP loss due to the heavier wheel.
Bingo. :)
>It would be great to get some 0-60 and 1/4 mile times to compare the
>acceleration with the two setups. 60-0 braking distance would also be
>interesting.
That's another story entirely, because you're adding one more variable into
the equation, namely grip. With the 185/60-14s, I could get the wheels to
chatter a bit at full throttle in 1st, but with the 195/50-14s,
nothing. Dropping the clutch with 185s at 2000rpm caused massive wheelspin
-- with the 15s, it would cause a stalled engine. In fact, with the (far
grippier) 195s on the car, I have to drop the car from at _least_ 4000rpm
to get any appreciable wheelspin. Therefore, since launch techniques are
so different, you can't really compare the 0-60 and 1/4 mile times
directly... unless you did launches with no wheelspin-- like Car & Driver's
"Street Start". That would be a quite accurate measurement in and of
itself-- but again, you're entering the "human factor" into that equation--
maybe on the runs with the 195s on it, my average shift time was 0.2
seconds quicker (for whatever reason)-- and therefore that result could be
interpreted as quicker acceleration.
For that reason, I chose not to use the G-Tech, but the dyno itself to
obtain the results.
>But none of this would mean anything unless the rotational inertia of the
>two wheel/tire combos is known. This can be measured, a crude test would
>be to let the wheels roll down an incline and
Very true. My body tells me that the 15s are heavier than the 14s, but
there's no way to obtain the rotational inertia of the two wheel/tire
combos... and, in theory, the 15s could have *less* rotational inertia.
But, I digress, I didn't do a proper "scientific study". More important
for me to know was whether there was an appreciable difference in power put
to the ground with *my* 14s versus *my* 15s.
>My point is this. Heavier wheels will not reduce the HP output of your
>motor. Heavier wheels (actually wheels with greater rotational inertia)
>will hurt performance, no way around it. Torque output from the motor will
>be used to turn the wheels rather than accelerate the car. More rotating
>mass will reduce the deceleration caused by your brakes. More unsprung
>weight will alter your suspension's behavior.
That is very true-- engine HP and torque output will remain the
same. Using lighter wheels and tires is synonymous with using a lighter
flywheel-- output is unchanged, but since the engine spools up more
quickly, acceleration can be improved in lower gears.
In fact, the best test for the difference between the wheels would have
been doing a run in 1st gear, where the wheels are accelerating as quickly
as possible. Of course, having 500lb wheels versus 1lb wheels isn't going
to make much of a difference in 5th gear when you're accelerating at a very
slow rate. So what I looked for on the dyno results was an increase in the
difference in wheel output as the torque curve rose (and therefore "a" was
greater). Since the two curves remained as close when the engine was at
peak output as they were when at idle, I concluded that there was no
significant loss in effective power output due to the additional weight of
the wheels.
I agree with everything you've said-- of course, including the effects of
unsprung weight on handling and braking. However, in my case, bumping up
tire size from 185/60-14 to 195/50-15 produced such an immense improvement
in grip (0.84g to 0.95g) [of course this is as a result of the tire as well
as the size increase] that any adverse effects of increased unsprung weight
are, at best, negligible.
As far as the effects on braking are concerned: That's another story
entirely. It's very difficult to tell what kind of improvement or
reduction there was in braking efficacy. My gut feeling is that my car
brakes better with the 185s. It's doubtful (in my mind) that this has
anything to do with the mass of the wheel/tire combo, as I can immediately
lock up the 15s, meaning that the brakes themselves are capable of enough
force to overcome the rotational inertia + grip almost
immediately. However, I attribute the perceived increases in braking
distances to the *shape* of the contact patch rather than the weight of the
wheel. Why? We had a debate a while back on this (I think it was on the
MB list)-- but physics will prove time and again that the *area* of the
contact patch is quite consistent regardless of what size tire you're
wearing. Despite me fighting tooth and nail about this, the engineers
(including a tire engineer) on the list sat me down with pressure formulas
and proved it to me. So, at least in theory, the contact patch area stays
constant. So, I'll reduce that for real-world application and state that
the contact patch stays "relatively consistent" in terms of area.
What happens when you add width to the tire is that the contact patch
becomes elongated (relatively speaking, of course) laterally, and shortened
fore and aft. It's my hypothesis that a laterally-elongated rectangular
shaped contact patch would help with lateral forces, and diminish the
tire's capability for longitudinal forces (i.e. braking or acceleration
traction) slightly. Likewise, using a narrow tire would help longitudinal
forces by having a longitudinally-elongated rectangular contact patch, and
thus would help with traction on acceleration in braking, and hence its use
in snow and ice applications.
Regardless of that debate (which I'm *still* not sold on), I find my 195s
*much* more difficult to modulate than I did the 185s. If my car had ABS,
I would most certainly go and do before-and-after 60-0 deceleration runs
with the G-Tech... but I don't have a few sets of tires to burn through to
practice modulation with both types of tires to the point that I can
perform reliably consistent braking.
My Mercedes has ABS, and I went +1 with the tires on that car. In it's
case, braking improved from the stock 132 feet (published) from 60 with
185-65-15s to a consistent, repeatable 120 feet from 60 with 195-60-15s. I
never did any tests with *my* car with the 185s, so I can't be sure how
significant the decrease is-- but I'll tell you that the car brakes like a
motherfucker-- and like nothing I've ever driven before.
>A dyno would only be able to detect the change in torque output to the
>ground. We (of all people) should know that there is much more to the
>performance of a car than it's HP output.
Exactly. And that's not even the whole picture. It's more than the torque
output to the ground, it's the torque output to the ground on
*acceleration*. Having 20lb wheels or 50lb wheels won't make a difference
in torque put to the ground up a constant speed, full-throttle hill. The
decrease in effective output would only be present upon acceleration
because of the changing rotational speed of the wheel.
>food for thought
See what you started?!?!?!
:)
Jason
>Jonas
>85 Scirocco 2.0
>For direct reply: jonas@metabolex.com
>http://www.geocities.com/scirocco2l8v
>______________________________________________________
>Get Your Private, Free Email at http://www.hotmail.com
>
----------
1987 Scirocco 16v
1989 Mercedes 190E Sport Euro
http://members.aol.com/rocco16v
--
Email problems to: scirocco-l-probs@scirocco.org To unsubscibe send
"unsubscribe scirocco-l" in the message to majordomo@scirocco.org