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CMC17
04-27-2006, 11:01 AM
This post is get a feel for the scales and how accurate they are at the track. The reason I ask is I cannot believe I missed my weight/lap ratio by more than 40lbs. I've never been off more than a couple of pounds at most in my calculations.

Setup for R3:

47lbs ballast added (thx Glenn!)
Full tank gas

Car weight w/driver: 3233lbs. (weighed before race)
Race length: 24 laps (CMC)
Fuel ratio: .5 gal/lap (formula derived from previous race (4.5 gal. used in 9 laps)
Fuel used: ~12 gallons
Weight per gallon: 6lbs.
Total fuel weight lost: ~72lbs.

Car weight after race: 3198lbs.
Difference from total pre-race setup: 35lbs.

Something isn't right and I was wondering if anyone else thought something was off. Again, this is only for discussion and to see if there is an issue that needs to be addressed (constructively).

mitchntx
04-27-2006, 11:58 AM
My car gained 35lbs, somehow ....

I had been coming off at 3312 or so in previous races and I was at 3347 post qual.

I filled the car to the brim (about 5 gallons) and ran 4 or 5 laps for Sunday's qual and came off at 3367

I should have weighed post race, but just never thought about it ...

The car didn't use as much fuel as I thought it would during the extended race.

donovan
04-27-2006, 12:03 PM
Last year I was coming off the scales after a 20 min race, that I started with a full tank of fuel at 3150-3160 many times...

this past weekend, I dropped 24lbs off the car, started the 40 minute race with full tank of fuel and after the race weighed 3180... Huh?

donovan

CMC17
04-27-2006, 12:09 PM
Thanks for the input guys.

I'm hoping others will chime in also with their weight reports. Something is definitely not quite right and with enough information we should be able to present this to those responsible for the scales and maybe get something done before the next event.

If I base my next setup on the weights used for this past event and then the scales somehow revert back to the "old" way, then I will be totally screwed and DQ'd. However, I'm going to keep my same weight log book and stay on course with it.

If the scales are off, we might as well not weigh the cars since it looks like no one would even be close to the minimum.

.02

Thx again!

/cheers

Rob Liebbe
04-27-2006, 12:45 PM
I thought the scales were reading 40 - 50 lbs heavy as well.

Normally, I come off a 20 minute race at 1/2 tank and weigh 3170 lbs. This was consistent in the last couple of events.

R3 came off with under 1/2 tank indicated and weighed 3198. Seems wrong.

Remember, we had scale problems at Hallet last year as well. If I remember correctly, we had Johnny Phillips' Miata on and off the scale for back-to-back measurments and got varying results.

AI#97
04-27-2006, 01:22 PM
Only input I can offer is that in Houston and Cresson last month, after filling the car to the brim and doing 20 minutes, I consistantly came off course at 3190 in Houston, and 3170 last month. I have since pulled 40 pounds off the car which would have put me at 3130 with the same fuel setup in a 20 minute race.

I was 2 gallons shy of a full load in R3...12 lbs, more than twice the minutes on track and came off at 3130... I usually use about 5.5-6 gallons in a 20 minute session so lets assume given my previous numbers and weight loss, I SHOULD have left the track at 3100 even....my minimum was 3069 so I was safe.

Before this weekend I was thinking of going after that other 10 hp and 10 tq I still left on the table but with the scales being off....I tend to want to play it safe.

michaelmosty
04-27-2006, 01:41 PM
When I weighed after R2 I was exactly where I thought I would be, 3164. In Houston I weighed and came in @ 3175 but the race was about 3 min. shorter.
I didn't get to weigh after R3 last weekend. :cry:

Todd Covini
05-01-2006, 09:04 PM
Check the specific gravity of super unleaded.
I've been using 7lbs/gal.

That'll account for some, but not all of your "missing weight".
Sounds weird...wish I was there.

-=- T

oz98cobra
05-02-2006, 12:24 PM
Remember that for the scales to be accurate, they MUST be levelled - all at EXACTLY the same height. From what I have seen at NASA events, I don't think enough attention is paid to this, so some variation from event to event should be expected.

Interestingly, the CCRs only allow a 5 lb leeway on first weigh in and zero leeway on subsequent weight ins - 5lbs is only about 0.15% on a typical CMC car, and is not nearly enough to compensate for the innacuracies that could arise if the scales were not properly leveled and calibrated (and is probably not even enough to cover the tolerance of their scales!).

Good reason not to sail too close to the wind on weight.

mitchntx
05-02-2006, 12:46 PM
Good reason not to sail too close to the wind on weight.

You suthunurs crack me up ...

Mike Bell
05-02-2006, 02:07 PM
Remember that for the scales to be accurate, they MUST be levelled - all at EXACTLY the same height.

Is that true? Would a car that is on an uneven surface gain mass? I'm so confused............ :?:

I thought the total weight would still be accurate, only the corner weights were skewed in an uneven surface?

marshall_mosty
05-02-2006, 02:58 PM
Mike,
I'm sure it has to do with the actual force vector acting on the scale pad... okay engineer talk off...

If you look at the banking at TMS and have a car driving around, gravity will act straight down on the track, not the car. This same thing happens with the scale pads, but at a much smaller angle.

If one set of pads is higher, the weight will skew "sideways" and not record all the true vertical load properly properly.

CMC17
05-02-2006, 03:12 PM
The scales were in a different place this time at MSR-C. They were set back farther. Slope of the floor that much different? One of the pads not reading correctly? Since the scales will be the same ones we will be using at MSR-H (assumption), I will be visiting them before practice to note any difference.

cmarvel
05-02-2006, 03:44 PM
That is always a good idea. As a 20+ year veteran of the tech shed, we always gave the same answer about the scales.

"These are the scales you need to make weight on today, so we suggest you weigh on them."

Mike Bell
05-02-2006, 04:21 PM
Mike,
I'm sure it has to do with the actual force vector acting on the scale pad... okay engineer talk off...

If you look at the banking at TMS and have a car driving around, gravity will act straight down on the track, not the car. This same thing happens with the scale pads, but at a much smaller angle.

If one set of pads is higher, the weight will skew "sideways" and not record all the true vertical load properly properly.

That would make the scales read "lower" than actual weight, no? Not what the examples above indicate from what I'm reading. I can't figure how uneven scales can cause a "higher" than actual reading.

I think there is something else causing this, more like an actual difference in scales rather than uneven pads.....

Also, the car is at rest and has a suspension, right? The tires also can change shape depending on what direction the path vector is loading them, yes? We shake the car to unbind the suspension, thus giving it the ability to adjust/shift prior to weighing the scales, no?

Need more coffee I guess........

oz98cobra
05-02-2006, 04:57 PM
Perhaps we all just ate and drank too much at Saturday night's grill? The cars and scales were fine, but the drivers were all porky for R3? ;)

jeffburch
05-02-2006, 05:46 PM
those things are temperture sensitive.
Cool morning vs. afternoon directly in the sunlight.

jb

marshall_mosty
05-03-2006, 07:27 AM
Mike,
You're right... I didn't think about it enough before opening my yapper. It should read lower. Now I'm out of ideas.

Mike Bell
05-03-2006, 08:59 AM
I think you made a good point about why the scales should be level in order to get the proper reading, I hadn't thought that part through myself. Oh well, too much for my little pea-brain to handle.

cmc#5
05-03-2006, 09:28 AM
Just to add fuel to the fire...the scales at Infineon this past weekend also felt about 30-35lbs heavy relative to the weights Nick Steel and I got for our cars at our first race.

I don't think a 1% or so margin is that horrible for this level of equipment though. I think the best advice has already been given: weigh in on the scales early and adjust as necessary.

CMC17
05-03-2006, 09:44 AM
I started this thread to see if anyone else had experienced the same thing. Up until this event I never had an issue and was always withing 2lbs. of what I thought CMC17 should weigh coming off the track. It definitely made me think and I tried to rationalize what could be different. Was it my setup? What was I missing? Could the scales really be that far off? I was also afraid of using the new numbers to base my weight off of and kept my calculations the same and not going by the scale reading. I wasn't sure if moving the pads around could affect the scales and if I had changed my weight it could lead to coming in 30-40lbs under.

I agree that it's a good idea to head to the scales with calculated numbers off a setup that is a known from previous events, then see how much difference there is with the scale.

I used to play with fire and come in between 3150-3160. Not anymore. lol

AI#97
05-03-2006, 10:07 AM
I forgot to add that the NASA scales were 20lbs heavier than the PST scales when I cornerweighted on that friday.

Mike Bell
05-03-2006, 10:23 AM
Does NASA TX always use the same set of scales? I can't recall if they were purchased or are they borrowing a set which may differ from event to event?

Hey, scales reading high is a good thing, no? :lol:

RichardP
05-03-2006, 10:46 AM
"A man who has one clock knows what time it is. I man with two is never sure." (Unknown)


Since this issue has been brought up a few different times I thought a more technical discussion of the topic was in order. This particular note is about the "official scales" at an event. It's important to understand that I'm writing this note as a competitor and not as a NASA Official ('cause I'm not one nor do I wish to become one). NASA may or may not agree with any or all of what I am saying here but the generic concepts are pretty universal over all racing organizations I've worked with.

Since I'm a nerdy engineer weenie, it seems appropriate to start this discussion with an understanding of how we are doing the measurements.

The most basic scale is a balance scale.
http://www.lovson.com/gifs/round-brass-pans.jpg
Known weights are added to the opposite side of a scale until the weights equal what is being weighed and it balances out.

The next most common scale is the spring scale. A spring of a known stiffness is deflected. The amount of deflection is measured to give you the mass being weighed. A linear "fish scale" is the most obvious of these. Sometimes a mechanism is attached to the movement to change the direction of movement or to amplify it. The standard bathroom scale with the dial that spins to a number that you don't like or agree with is an example of this.

The scales being used at the track to measure the weight of our cars are electronic. To do the measurement, strain gauges are used. When any load is applied to any object, the object deflects. Strain gauges are used to measure this deflection. When a wire is put in tension, it gets slightly longer and it's cross sectional area is reduced. This changes the resistance of the wire. Strain gauges use this principle, along with fancy electronics and a bunch of math to determine weight.

Here is a picture of a typical strain gauge.
http://www.omega.com/literature/transactions/volume3/images/strainpic1.jpg
These things are small, generally 1/2" x 1/4" or so. You can see that the wire is zigzagged back and forth several times so that the effective length change of the wire is magnified several times.

Explanation of the strain gauge:
http://www.omega.com/literature/transactions/volume3/strain.html
More details of implementation and error compensation (temperature changes, etc.):
http://www.omega.com/literature/transactions/volume3/strain2.html
Even more really nerdy details:
http://www.omega.com/literature/transactions/volume3/strain3.html

Strain gauges are bonded to precision machined structures inside the scales that we use. Although the scales appear to be fairly rigid, they do "squish" ever so slightly when a car is rolled up on them.

The key relevance to us is the total range of loads that the scale is expected to see. The structure inside the scale has to be strong enough to take the highest load it will ever see without damaging it (plastic deformation of the structure or delamination of the strain gauge). Unfortunately, the stronger you make the internal structure, the less it deflects under light loads. Since we are measuring deflections, our accuracy also goes down.

Digging into the articles linked above (which I'm sure very few of you did), you can see that under ideal conditions strain gauge accuracy can be as good as 0.1%. What does that mean to our scales? Well we have cars like Joe Johnson's Cobra R that tipped the scales at just a hair under 4000# in race trim (ouch). Since his front/rear weight bias isn't 50%, the two front scales saw over 1000# each. A common race scale increment is 1500# per pad and I would guess that is what we have been using. Since the scale is "rated for measurement" at 1500#, it realistically has to take a load much higher than that without sustaining damage. Lets call it 2000# but who knows what the number really is?

Doing a little math, we find that each pad can be expected to do no better than being within 2 lbs. of actual. Since we are using four pads for our measurement, we're now up to no better than 8 lbs. Realistically, production scales with normal manufacturing tolerances and some level of use and abuse aren't going to meet "ideal conditions."

It's not uncommon for scale pads, especially from different vendors, to vary from each other by about 10 lbs. Since I personally have seen at least a 40# total difference between the borrowed scales we used early last year, the NASA supplied scales used now, and scales I have available to use at home, my personal experience bears this out. Although that's only around a 1% variance, it's certainly more than anyone would like. BTW, I have no idea which set of scales were "correct." (Hint: The correct answer is realistically "none of them.")

OK, if these things suck so bad, why do we bother??? Although the accuracy of the strain gauge scales is not spectacular, the repeatability of them has been found to be pretty good. That is the basis on which the whole weighing thing works. Scales in good working order have been found not to vary much at all over the course of a weekend.

From a rules standpoint, section 17.8 of the CCR, 7.6.1 of the CMC rules, and section 6.0 of the AI rules cover this. I particularly like the wording in the CMC rules. It includes words like "the scales located in the impound area are the official scales" and "it is the competitor's responsibility to know the weight of their vehicle."

The key here is that it is the competitors responsibility to know the weight of their vehicle on the official scales for the event. It does not matter if the scales are off. Realistically, there is no way that the scales can be "correct." They are going to be off by some unknown amount. They are quite likely to read differently than other scales that you used to set your car up before the event. But the official scales for the event are the official scales. Your compliance with the rules is going to be determined on the official scales. If you are playing the weight game with little margin then you really need to weigh your car on the official scales before the first qualifying of the weekend to make sure you will not be surprised. You have to be compliant on the official scales for the event. Your actual weight, your weight on a different set of scales, or your opinion of your actual weight is quite irrelevant.

I've worked tech in a few different series. When the scales are set up, there is a variety of different things that are done to make sure they are functioning properly and “accurately.” Sometimes it is just a known weight set on each different pad. Sometimes it is a bunch of weights that are closer to an actual corner weight that are moved around to each pad (what a pain that is). Often times it's just a person that stands on each corner to verify that they all read the same. It is certainly best if the scales are set up on a flat and level surface. Realistically, though, that's hard to come by at some tracks. Even if the surface is not flat and level, the set of scales will still repeatably show the total weight of the car. What will be off will be the corner weights.

So with all that rambling, I still haven’t addressed Eric’s question. Did the NASA scales change between events or more importantly, did they change in the middle of the event??? That’s a very interesting question. I certainly hope they didn’t. The whole thing only works if the scales are repeatable. I know that my car came in heavy for race three but it was clear that we still had a bunch of fuel on board. The car ran two full sessions last weekend on the fuel left over from the last race. I didn’t pay all that close of attention to the numbers because realistically, the only way we were going to come in light is if the engine somehow fell out of the car between the checkered flag and impound.

From my experience, the NASA scales have been repeatable in the past. Hopefully they haven’t sustained any damage and are still good. It does make me cringe when they let us drive on and off the scales. It certainly is more convenient but it is so much easier on the scales to be pushed on and off. There are cabling issues that could cause errant readings but they are much more likely to show up as obvious errors rather than a little light or a little heavy. I guess we should keep an eye on it but I’m not too worried.


The only exactly accurate measurement device I've seen is a stopped watch. It is exactly correct twice a day. :-)


Richard Pedersen
#91 LX 5.0

CMC17
05-03-2006, 11:36 AM
Thanks, Richard!!!!

Very well explained and it actually raises more questions. lol

I will not take a chance on the spinning wheel to land on "safe". I will take the known good road and will only verify that I'm "legal" at the scales not paying attention to anything other than being over the minimum weight.

It would be unfortunate to see someone DQ'd and lose the chance at the championship.

Worst case scenario would be to weigh "X" after practice, setup car for race by the scale weight from practice and then something somehow changed since the last time weighed only to come in light.

I'm keeping my clock that is right twice a day.

mitchntx
05-03-2006, 11:58 AM
From a rules standpoint, section 17.8 of the CCR, 7.6.1 of the CMC rules, and section 6.0 of the AI rules cover this. I particularly like the wording in the CMC rules. It includes words like "the scales located in the impound area are the official scales" and "it is the competitor's responsibility to know the weight of their vehicle."

The key here is that it is the competitors responsibility to know the weight of their vehicle on the official scales for the event.

What sucks is when the scales aren't set up BEFORE the event begins ....

Adam Ginsberg
05-03-2006, 12:12 PM
Does NASA TX always use the same set of scales? I can't recall if they were purchased or are they borrowing a set which may differ from event to event?

Late last year, NASA Texas bought their own set of scales - that's what we've been using since the TWS event in 2005.

Section 8.2 of the 2006 American Iron rules state that all weight measurements must be done using American Iron approved weight scales, or with specific approval of the Series Director.

Section 7.6.1 of the 2006 CMC rules clearly state the scales located in the impound area shall be the official scales.

In both cases, the NASA scales at impound are the approved scales.

That's precisely why we always recommend folks get on the scales as soon as possible, especially if they are in the habit of playing the weight game. Granted, NASA doesn't always get the scales setup early enough for people to weigh ( say, on a Friday evening ), but we're working with them on this.

Chris Marvel said it best:


"These are the scales you need to make weight on today, so we suggest you weigh on them."

GlennCMC70
05-03-2006, 01:21 PM
i hounded NASA to get the scales set-up. it took them till just prior to qual to do it. once it was set-up i went down there and they had 15+ cars in line. i had no time to wait as i would have missed qual. its not a big deal w/ my car, but it was for the #8 we had. we did weigh it the day before @ PST, but sice its not official, it didnt hold alot of value.
NASA also does not need to have the scales in the TECH line. alot of those cars getting tech'ed were not weighing. but i would have had to wait just the same. that covered paddock needs to be clear of the "sevens, berkins, in the way car, whatever you want to call it. no reason they should have had that whole area taken up like that.

basically dont tell me its my responsibility to check my car and hold me liable if the scales are not made available. had i been light after qual, there would have been a problem.
Adam, i'm not bitching @ you. but this needs to be addressed. tech should never open, till the scales are set-up. period. all class's of racers should be saying this to NASA.

Mike Bell
05-03-2006, 01:33 PM
Did I miss a meeting? Since when do we weigh the cars after qual? I know it makes sense (theoretically somone could go out very light and blister a qual lap to start up front) but, hey, there's only so many hours in a day so I never questioned anyone on the issue.

Did something change? :?:

GlennCMC70
05-03-2006, 02:52 PM
i always assumed qual was also subject to impound just as any other race. there are points for pole. its minimal in CMC, but AI its like 5 or so.

Mike Bell
05-03-2006, 03:02 PM
Hey, I'm not in disagreement with you Glenn - it was just news to me. Sounds like a good idea, like I said, keeps all the car's in the qual session at min weight or above!

Adam Ginsberg
05-03-2006, 06:18 PM
Did I miss a meeting? Since when do we weigh the cars after qual? I know it makes sense (theoretically somone could go out very light and blister a qual lap to start up front) but, hey, there's only so many hours in a day so I never questioned anyone on the issue.

Did something change? :?:

Nope - nothing has changed. We have indeed weighed after qualifying before, I just can't recall the event we did it at. It may have been TWS last year, but I'm not 100% certain.

Essentially, ALL cars have to make weight, for qual or race. If a competitor gets the pole, and is ~50lbs light, he/she don't deserve it, since they are under the minimum weight.

Glenn - you are absolutely correct regarding scales at the last event. The Sevens had taken over an area of the covered paddock, thereby making it impossible to have separate tech/weigh-in lines.

It's something we'll cover with NASA going into the MSR-H event.

Todd Covini
05-03-2006, 09:44 PM
Whoa.

Doesn't matter if "things change" in the weekend procedures or not.
Just because typical routine was to not tech cars after qualifing doesn't mean folks can take advantage of that!?!?!

This discussion makes me want to change things up even more. Are we getting complacent? The rules are the rules...not just at times when we're checking for them.

Ultimate responsibility rests with the racer to be in compliance. The "reasonable person" test comes in again and I think it's only fair not to ding someone on the scales at 11 AM when they were just set up at 1030AM.

For this reason, NASA has purposefully given plenty of time after the scales are set up before "roll-ups" are done. This has normally been after Race 1 or Saturday afternoon.

The whole discussion is moot if you have a 40-50lb cushion. I've never worried about car weight. Sometimes I'm within 10 lbs....sometimes within 60. If I'm going to play the weight game, then I'm going to have to worry about all the variables that will come with that....and not try to blame NASA for my being under the limit.

Personally, I prefer not to live and die by the sword!

-=- Todd

PS- We've had the same scales that NASA purchased for the 2005 TWS event when Eric tried his triple gainer in T1.

cmarvel
05-03-2006, 10:32 PM
Since when do we weigh the cars after qual?

When I ran the tech shed for SCCA, I frequntly called the entire feild of Formula Vees of Spec Racers to impound and weighed them after qual. The first time we did it we caught several people who knew better cheating on their qualifying weight. They were moved to the rear of the feild as a penalty.

Mike Bell
05-04-2006, 05:03 AM
Whoa.

Doesn't matter if "things change" in the weekend procedures or not.
Just because typical routine was to not tech cars after qualifing doesn't mean folks can take advantage of that!?!?!

This discussion makes me want to change things up even more. Are we getting complacent? The rules are the rules...not just at times when we're checking for them.



LOL, sometimes you crack me up Todd. I've been racing with NASA TX since what, 2003 and I haven't been weighed after qual. Is that an example of me getting complacent or more like I've never seen this done? :wink:

Granted, I missed a bunch of events in 2005 with a blowed up AI car so if this was added to the "weekend procedure" at one of those then I missed out. Since I haven't seen it done so far it was news to me, that's all. Like I said, I AGREE it is a good idea to keep the qual sessions on the up and up. :P

I also agree with your perspective about weight, now that there are some 40 min races I don't want to have to be doing the math in my head (I'll mess that up everytime) so I'm running the car a bit heavy. I'm not losing races because of an extra 40lbs, I've got other "challenges" to overcome first. :wink:

mitchntx
05-04-2006, 05:18 AM
#8 was an isolated case.

We knew the car was under weight. We were trying desperately to get the car weighed so we could add ballast to make it legal BEFORE qualifying.

RichardP
05-04-2006, 08:05 AM
Are we getting complacent?

You don't really want me to answer that, do you?


We have been checked for weight after qualifying a couple of times before. They come by on grid and let everyone know to go to impound after the sesson. The people who forget and go back to their pit because there is no one pointing them to the right place get penalized.

When I worked tech with a different group we had a white board that we would write instructions on for people comming off of the track (eg. "Top three to impound, Top 5 CMC to impound, All purple cars to impound, etc.). It was very effective.


Running under weight here is not a very bright thing to do considering it is the only perfomance based rule that is ever checked.


Richard P.
#91 LX 5.0

mitchntx
05-11-2006, 10:12 AM
Reading and re-reading the CCR ...



17.8 Minimum Weight
Each car that that is checked for minimum weight will be subject to the following policy:
Each owner / driver will be given a standard five (5.0) pound leeway under the minimum
published weight for their car during the first time the car is weighed for that event
(weekend). After the initial weighing, the competitor must meet the exact published
weight with zero (0.0) pounds leeway for the remainder of that event. This policy should
compensate for any discrepancies between scales, margin of error, and imperfections in
ground surfaces. No other tolerance will be given.


So the first time the car is weighed you get a 5lb fudge. After that, it's the published weight.

Mike Bell
05-11-2006, 12:49 PM
Good catch Mitch, but I thought class rules took precedence over the CCR's?

Todd Covini
05-11-2006, 09:37 PM
Reading and re-reading the CCR ...



17.8 Minimum Weight
Each car that that is checked for minimum weight will be subject to the following policy:
Each owner / driver will be given a standard five (5.0) pound leeway under the minimum
published weight for their car during the first time the car is weighed for that event
(weekend). After the initial weighing, the competitor must meet the exact published
weight with zero (0.0) pounds leeway for the remainder of that event. This policy should
compensate for any discrepancies between scales, margin of error, and imperfections in
ground surfaces. No other tolerance will be given.


So the first time the car is weighed you get a 5lb fudge. After that, it's the published weight.

That doesn't mean that if you ignore the scales all Saturday morning and get a podium during race 1 that you get to claim "fudge"!

It means if the scales are set up at 10 AM...and you are pulled onto the scales after qualifying at 11 AM...you can can claim "fudge"!

Oh...and "fudge" is 5 lbs...we've reweighed folks 2 and 3 times to make sure fudge is fudge.

We all (racers & directors) don't leave our common sense at home when we head out for race weekends! :roll:

CMC17
05-11-2006, 09:52 PM
Don't be so close to the minimums!! :D

If you need to add some dead weight... get one of the GM boyz to hop in. 8)

David Love AI27
05-11-2006, 10:20 PM
Don't be so close to the minimums!! :D

If you need to add some dead weight... get one of the GM boyz to hop in. 8)

I'm available now that Amber is driving....

CMC17
05-12-2006, 07:47 AM
Don't be so close to the minimums!! :D

If you need to add some dead weight... get one of the GM boyz to hop in. 8)

I'm available now that Amber is driving....

<kidding!!> 8)