Randy McMahon and others with loggers

Discussion on Data Loggers, how to read your logs, what to improve, etc.

Moderator: RPM

Post Reply
Message
Author
bracketracer
Posts: 329
Joined: Wed Oct 29, 2008 7:14 pm
Location: New Jersey

Randy McMahon and others with loggers

#1 Post by bracketracer » Tue Nov 18, 2008 11:48 am

Here is a screen shot of a pass with a new right rear shock sensor. To me it don't look like
the shocks or maybe springs are correct. This is a link car with a strut up front and 14"
Strange double adjustables..On this pass it went 8.73 with a short time of 1.234...
Looking at the drive shaft rpm it looks like to me that the tires are unloading just after
the release of the brake..What do you think???? Thanks

Image

bracketracer
Posts: 329
Joined: Wed Oct 29, 2008 7:14 pm
Location: New Jersey

Re: Randy McMahon and others with loggers

#2 Post by bracketracer » Wed Dec 10, 2008 9:49 am

I guess Randy don't come around to much :( :( :scratch:

User avatar
John_Heard
Site Admin
Posts: 5684
Joined: Thu May 12, 2005 11:20 am
Location: Resume Speed, Kansas
Contact:

Re: Randy McMahon and others with loggers

#3 Post by John_Heard » Wed Dec 10, 2008 11:18 am

Yes, according to your Driveshaft RPM and GForce it's spin/hook/spin/hook a little bit at the hit. Wouldn't hurt to experiment with the shock settings.

RPM
Posts: 50
Joined: Fri Oct 07, 2005 9:47 am
Location: Oklahoma City
Contact:

Re: Randy McMahon and others with loggers

#4 Post by RPM » Wed Dec 10, 2008 2:12 pm

Sorry for being so late in responding.

You are correct, the data indicates that the rear end extends at the hit and plants the tire, then the car squats down as expected. But then, the rebound reduces the actual weight on the tire and you get more tire spin. As evidenced in your G meter by the dip after the first peak, the spin did have an adverse effect on the launch. This loss of traction also reduces the torque felt by the suspension, which limits the effectiveness of the suspension and the process repeats, giving you the "oscillations" seen in the shock travel. Ideally, if you could prevent the "overshoot" when the car squats, there would be no rebound to unload the tire. Tightening the compression on the shock will reduce these effects by distributing the energy of the squat over a longer time and reducing the overshoot. The downside is that the tight shocks will reduce the cars ability to follow track irregularities downtrack. This is one of the advantages of programmable shock controllers. They leave with the shocks tight, then loosen them up after the suspension has settled.

Four link settings will also have an effect on the launch profile. From the readings, after the oscillations settle out, the car is very close to static ride height, so your intersect point is probably close to the squat line. This setting is more prone to develop oscillations. With the equipment and data you have, it should not be too dificult to get this working right. What you are after is getting rid of the oscillation. You will (obviously) be working with the leave RPM, 4 link and shock settings to get this working. The initial extension will always be there, but you want the transition to the final set to be slow and smooth with minimal overshoot. This may result in a longer, smooth, controlled spin, but that is not necessarily a bad thing. Pay close attention to the G meter when evaluating the leave. Getting the dip out of the G meter will pick up the 60 foot.

Randy

bracketracer
Posts: 329
Joined: Wed Oct 29, 2008 7:14 pm
Location: New Jersey

Re: Randy McMahon and others with loggers

#5 Post by bracketracer » Thu Dec 11, 2008 6:05 am

Thanks for the reply Randy, I had changed the 4 link setting by lowering to top bar one hole and going to a 110 lb rear spring. When I did this I didn't have the rear shock sensor so I was not really sure that this helped but short times did suffer but they were more consistent.

I think what Im going to do is put the upper 4 link bar back up one hole and give it a try. I did have 125 lb springs in the rear.

I did try different shock setting with the rear the way it is now but really could not make it much better than the graph above..

Any suggestions that you may have???? Thanks

User avatar
John_Heard
Site Admin
Posts: 5684
Joined: Thu May 12, 2005 11:20 am
Location: Resume Speed, Kansas
Contact:

Re: Randy McMahon and others with loggers

#6 Post by John_Heard » Thu Dec 11, 2008 6:18 am

Have you experimented with more tire pressure and/or tubes to help stiffen the sidewall?

bracketracer
Posts: 329
Joined: Wed Oct 29, 2008 7:14 pm
Location: New Jersey

Re: Randy McMahon and others with loggers

#7 Post by bracketracer » Thu Dec 11, 2008 6:23 am

John I have tried any antwhere from 5 to 6 1/2 lbs and it likes 5 1/2 lbs. This car is only 2250 lbs with me in it so I was told it dont need a stiff sidewall for bracket racing from Hooser...

User avatar
John_Heard
Site Admin
Posts: 5684
Joined: Thu May 12, 2005 11:20 am
Location: Resume Speed, Kansas
Contact:

Re: Randy McMahon and others with loggers

#8 Post by John_Heard » Thu Dec 11, 2008 6:34 am

I've been running into something similar with mine that occurs from .23 to .7 into the run, tire pressure is one of the things I want to experiment more with. You're car is very light so yeah I can see why the lower pressure.

Have you checked to see if anything is binding in the rear suspension? Bad Shock/s? Take both shocks off and run it through the travel and see if anything is going on. I found out that when I lowered the rear of my car, the anti-roll bar heim nut was hitting on the arm of the anti-roll bar. Not sure how much it affected it, I haven't had a chance to run mine again. Just something to look for.

bracketracer
Posts: 329
Joined: Wed Oct 29, 2008 7:14 pm
Location: New Jersey

Re: Randy McMahon and others with loggers

#9 Post by bracketracer » Thu Dec 11, 2008 6:39 am

John, The car is new and only has less than 30 runs on it :shock: ....But I will check it again

RPM
Posts: 50
Joined: Fri Oct 07, 2005 9:47 am
Location: Oklahoma City
Contact:

Re: Randy McMahon and others with loggers

#10 Post by RPM » Fri Dec 12, 2008 5:17 pm

Lets consider some other factors for a bit.

With the power you have, the converter initial stall is about 5500. This converter, from 4000 to 5000, it is not coupling nearly as much power.

The initial tirespin, in the first tenth or two is aided by the fact that when the tire "wads up", the distance from the wheel rim to the tire surface gets shorter. Instantaneously, this tries to pull the tire off the track, reducing traction until the rear end starts to "fall" out of the car. What I mean to say by all this is that it is easier (takes less power) to generate the initial spin.
Once the rear drops (shown by your shock extending) planting the tire, the traction increases and the tire hooks up. Then the car starts squating on the rear (shock compressing). At this point we need to consider another factor. If you have enough torque, and the setup is right, the twist in the rear end is trying to prevent the squat, and making it take longer, keeping the tire planted. If this is not happening, it squats too deep and rebounds.

This is what I think may be happening in your car. You aren't leaving with enough torque to make the chassis work. If you look closely at the data, once the engine reaches stall (its best torque coupling) the tires start to spin again AND the G meter picks up. Your initial tirespin is not excessive, so raising the leave chip isn't going to blow the tires off, and it will bring in the torque sooner.

If you leave higher, it may very well power on through and maintain a reasonable spin, which will actually be better than hooking up. This increased torque may also tame some of the oscillation in the shock readings.

I know that the general thought is that "tire spin is bad". In reality, if you have enough power to sustain a controlled spin, at the right level, it will be much quicker than a dead hook. If you look at the data from any door car running a 1.10 or better 60 foot, you will see considerable tirespin for the first second. Idealy it will show a smooth curve up from launch, then it will roll over and slowly hook up until ground speed catches up with tire speed, then it will turn up and follow ground speed.

I would be very interested to see the data on your car leaving at 4500 and 5000. My bet is that you would see better 60 foot times.

Regards,

Randy

bracketracer
Posts: 329
Joined: Wed Oct 29, 2008 7:14 pm
Location: New Jersey

Re: Randy McMahon and others with loggers

#11 Post by bracketracer » Sun Dec 14, 2008 6:16 pm

Thanks for the reply Randy, I live here in N.J. so racin is done till March unless Atco opens if it gets warm so I won't get to try your suggestions till then.........Thanks again

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest