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Message Title: Not expert but .... |
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Posted by:
Steve T on 2002-12-17 at |
Message:
I've always found the wheels
are most likely to hop right about at the optimum slippage point or just a
little higher. You want the wheels to spin at about 10-15% higher than the road
speed for optimal traction (generalized for road tires), not taking into
account alippage that needs to be induced so that
when the tires DO hook up the rpms are well into the
power range of the engine.
If you overshoot this point too much the wheels will
just spin and you won't make enough traction to get enough force into the
suspension to start wheelhop.
Obviously massive generalization. Different cars will
react differently.
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Message Title: Re: Wheel Hop! |
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Posted by:
Steve T 2002-12-17 at |
Message:
M5 has 2 rear subframe braces not on 540i. They'll definitely fit on the
body side but you'd have to replace the lower subframe
bolt with an M5 one since its tapped through the bolthead (or tap it yourself if you've got as lathe).
I'm not sure if these will prevent wheelhop but they
couldn't hurt and seem to be minimally painful to install.
Ever BMW I've ever driven has wheel hop. Bogging sucks. You might be able to
tune most of it out with a set of adjustable shocks and different tire
pressures. Hell if you're just doing 1/4 mi stuff screw around with pressure
and see where it gets you. No good for the street :(
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Message Title: Haven't dragged my E39 yet... |
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Posted by:
Steve T on 2002-12-17 at |
Message:
Probably won't, its too nice to beat and I haven't disabled the idiotic CDV
yet. Race cars save wear on the road car.
Experiment. Every suspension is different and so is
every tire. Wheel Hop is a complex phenomenon related to a lot of different
compliances and suspension geometry. You need to keep the wheels and the forces
generated at the wheel away from whatever natural frequency of the suspension
they are exciting. Its not as simple as matching spring rates to tire rates
either since you have anti-dive and anti-squat effects to account for (which
are a function of geometry, forces and applied torque).
The adjustable shocks will yield a lot of control over the situation, much more
so than the tire alone. They will allow you to change the natural frequency and
also the rate at which the oscillations damp out.
A stiffer sidewall tire may also help since it won't allow the tire to hook up
quite as hard
Malachi,
Interesting
I believe the best way to combat wheel hop/spin is to vary the tire pressures
(this is not good for the street obviously). But in so doing, each track
and each condition (temp/humidity,ht
above sea level, track preperation i.e. how well
prepped the track surface is) will have its own unique tire setting that will
work on any given day.
Adjustable shocks are very nice, and do help in 1/4 mile times. I have
them on my Race Car, and can "dial in" the car given the track
conditions. They were also useful on my street car, b/c I like it firm
when I drove, yet my GF could (if she wanted) adjust the shocks so they
provided a more comfortable ride.
Not sure on the M5 subframe braces... haven't looked
at the differences that closely. But I am still a firm believer that
practice makes perfect. And that before Anyone
starts "modifying their car", that they learn to drive it to its full
capabilities before they attempt any mods. (My
BMW mechanic can beat most everyone in his unmodified BMW, b/c he knows every
limit)...
Same goes for drag racing. Practice, Practice, Practice is my
motto.....Learn all one can about techniques and by watching others, and by
trying new 'combinations'....
A good friend of mine practices so much, he has been able to be .5 seconds
quicker on almost every car he drives (.5 or more sec quicker than the
owners....) We use him to find our cars best capabilities... then we have
a "goal" to shoot for. :o)
I sure hope Revhigh sets up a 1/4 day in our area in
the spring, b/c I would love to see the different times the 5series can
produce..... and would love to watch and learn, as
well as help those "new" to the drag race scene.
Wheel Hop can be caused by a number of things.... Tires/tire
pressure/suspension set up just to name the "major" ones".
In some cases wheel hop occurs just before your tires spin (like when you do a
burnout)....The "hopping" is your tires getting some traction then
not, traction then not etc. If you launch at too high an rpm... you
usually get wheel spin (not enough traction for the power, or in some cases
your tires may be overinflated or wet-from water box
at track)...
If you are getting wheel hop, then you are probably just a "hair" to
high on the launch rpm, or you may need to reduce tire pressure by one psi, or make sure you don't have to
much water left on them if you rolled through the water box. The key to
eliminating wheel hop (or spinning) is to change one thing at a time (if you
can get enough 1/4 runs in, which is sometimes very hard).
I would try dropping your rpms' down slightly on
launch, assuming you keep your tire psi the
same. (the perfect launch usually has no tire
spin, no hop, and maybe just a small 'chirp' of the tires where they slip for a
split second then find grip)
Also, b/c our cars are rear wheel drive, when we launch, the weight shifts from
the front of the car to the back, so the weight transfer puts itself over the
drive wheels... improving traction on the launch (that is why most rear wheel
drive cars w/ the same hp and weight will be faster than a front wheel drive
car.) Front drive cars experience a lot of wheel hop b/c not only do the
have the same issues w/ tires and suspension, but upon lauch,
all of the weight goes off the front end, and the front tires no longer have
weight over them providing more grip.... hence wheel hop (my 500+ hp acura integra can have some
violent wheel hop if not launched properly)
Again, I hope this helps.
Happy Holidays....
Go ahead if you think its worthwhile. One thing I forgot to mention is that a lot of this is also due to geometry of the rear suspension. BMWs have considerable amounts of anti-dive and anti-squat geometry built into their suspensions for stability under accel and braking and this will tend to amplify wheel hop as you are putting an undamped force into the body (in other words some vertical force goes directly through the suspension links not through the shocks and springs).
The tire stiffness recommendations are mentioned only to point out that different tires will have other effects. With our sidewall being so short it may be very difficult to find stiffer tires. My thought on this is that it may keep the contact patch from hooking up so firmly which, although it may slighty increase wheelspin, will lessen the hop as a function of transients between kinetic and static friction. The whole point of which being that the tire will be allowed to spin a little longer at lower rpms until the roadspeed has had a chance to catch up to the engine speed where there is a good amount of torque. And it raises the natural freqeuncy of the tire/suspension comination a bit. Of course spinning too much is bad so its a compromise just like anyhting else in racing and takes a lot of experimentation. I've found a lot of times that something I thought to be true didn't prove out because its very difficult to take everything into account.
Anyway Happy New Year. Let me know if you've got any other questions or thoughts.
Steve