Spring time in Montana

[Davemk1]

It's been a very cold and snowy spring here in Bozeman but we have had some days to get out on clear roads. The Bridger Mountains are in the background and my home is near the base of the mountains as viewed from this spot. Really good driving round here.

 

I've done a bunch of work on my Birkin in the past month and it's all coming together just in time for our first autocross race this weekend.

 

I've recently -

 

1) installed an emergency cut off switch

 

2) installed driveshaft safety hoops (fun drilling rivets!)

 

3) made and installed a front splitter

 

4) corner weighted and balanced

 

5) made a passenger side cone deflector for the rear fender

 

6) made new ABS fender spats for the rear fenders to replace the ratty alloy versions. The added side benefit is that they are quieter when rocks whack them.

 

 

 

Now it's time for a fun summer.

 

Dave

 

 

http://www.usa7s.com/aspnetforum/upload/1546558906_DSC_9025.jpg

[11Budlite]

Looking good Dave! :thumbs:

 

Do you have any close-up pics of the front splitter?

 

Bruce :7drive:

[yellowss7]

Dave, any photos of the drive shaft hoop install?

 

I'm jealous, great roads, great Skiing and great Fishing. Too bad I can't convince the wife to move out there. Tom

 

 

[Davemk1]

The driveshaft hoops come from Woody at MSI. My era of Birkin had the driveshaft tunnel riveted in so I got to drill out 48 rivets. A fun day.

 

I made the splitter out of 1/8" ABS sheet and angle aluminum. The whole deal just bolts on and weighs less than 3 pounds. I have no idea if it "works" but I doubt it hurts and I was underweight for Dmod anyway. Another way to look at it might be to say it must work as I haven't slid off the road yet!

 

Thanks for asking.

 

dave

 

P.S..........the little grill in the splitter is to keep big bugs out of the fresh air intake I've run from the splitter to the foot box. I hate when bees go up my pants!

 

 

 

 

 

 

 

http://www.usa7s.com/aspnetforum/upload/608352650_DSC_9010.jpghttp://www.usa7s.com/aspnetforum/upload/1146541531_DSC_9046.jpghttp://www.usa7s.com/aspnetforum/upload/1214039026_DSC_9049.jpg

[twestrup]

Dave,

 

What was involved in fabricating the ABS "fender spats"?

 

Where did you purchase the ABS sheet material?

 

How did you affix them to the fenders?

[gjslutz]

Dave,

 

You mentioned "corner weighted and balanced". What is your weigh at each wheel, & what formula did you use to arrive at the proper target weights? I assume you had your weight loaded into the seat for this. You also said you had to add weight to get to D mod. What is your EW?

 

I thought EW in D Mod was 1280 With driver? I plan on doing the corner weight on my Birkin today, but didn't know the target % at each.

 

Gary

 

[Davemk1]

Dave,

 

What was involved in fabricating the ABS "fender spats"?

 

Where did you purchase the ABS sheet material?

 

How did you affix them to the fenders?

 

It's dead simple and really cheap. I used black ASB sheet from McMaster Carr. If you haven't heard of them they are the giant of industrial supply online. I used 1/16" sheet and it cost about $10 for enough material for both sides.

 

Making them was simple. The Birkin has aluminum spats that are riveted on. I drilled out the rivets and removed the spats. I then taped them down to the ABS sheet and traced with a sharpie marker. I also drilled the ABS for the rivets by just passing the drill bit through the old and into the new pieces.

 

I then cut the ABS with tin snips ( I think heavy kitchen shears would do it). Now all you need to do is rivet the new ones on and have a good dark beer while sitting alone in the garage. The last part is optional.

 

have fun.

 

Dave

 

 

 

[Davemk1]

Dave,

 

You mentioned "corner weighted and balanced". What is your weigh at each wheel, & what formula did you use to arrive at the proper target weights? I assume you had your weight loaded into the seat for this. You also said you had to add weight to get to D mod. What is your EW?

 

I thought EW in D Mod was 1280 With driver? I plan on doing the corner weight on my Birkin today, but didn't know the target % at each.

 

Gary

 

A few autocross friends and I bought some scales for corner weighting so it's really simple. Put the car on the scales, put the read-out next to the car where you can see it, plunk your butt in the saddle and see what you've got.

 

In a perfect world the corner weights would be 50% and the right-left balance would also be 50%. But the world isn't perfect so while it's easy to get a 50% corner weight it's near impossible to get a 50% right-left.

 

So I just concentrate on the corner weight. The only way to offset my near 200# of driver and helmet would be to add a lot of weight to the passenger side. I've removed as much weight as possible from the driver side to help but it's a token effort really.

 

So corner weighting - I get the car set up EXACTLY how I race it. Race slicks set up with proper air pressure, 1/4 tank of fuel, my helmet on my head (yes it feels silly sitting in your car standing still in your garage with your helmet on), etc. Look at the read out and make a plan as to what spring to adjust, make the adjustment and recheck. The scales are very sensitive and can tell the difference between my hands on the steering wheel and my hand in my lap. Not that it matters really but why not go all the way. Tweak, check, tweak, and repeat. It's very easy really.

 

My car weighs 1241 (in race trim) without me in it and 1431 with me in the car. I have a 50% corner weight and 49.4% front and 50.6% rear. 53.6% of the weight is on the driver's side and 46.4% on the passenger.

 

As for the car weight and the SCCA. It's a bit convoluted and you need to look closely at the rule book. The min weight for DM (weight with driver) is 1380 with a 2.0L motor. But there are two additional weight considerations. One is that the Birkin ( and most Sevens) incur a 40# modified tub penalty. So the min weight then becomes 1420 for a Birkin. But (there's always a 'but' isn't there?) there is another more obscure rule that says IF you have less than 51% of the weight on the drive wheels you can deduct another 35#. So with my car, since it's weight distribution is less than 51% rear I can consider removing weight from the car AS LONG AS I don't end up with 51% or more of the weight on the rear. In my case this means I can effectively remove weight from the middle to the rear of the car, get the weight down to the adjusted min of 1385. For me this means at large important events I'll remove the windshield (and gain a big aero advantage- 11 pounds) and the rear luggage tray (4.5 pounds) which will bring my weight down to 1415 and still have less than 51% of the weight on the rear. Due to the variable weight of the fuel and the fact that not all scales read the same I want to stay about 20 pounds over the limit just in case.

 

All this said I think the whole weight deal is only important at large events where they actually impound the car and weigh it. If you just want to compete locally and have fun then corner weight the car and don't worry about the overall weight. No one else will care and you will no doubt be above the min unless you try hard to not be. But if you want to compete on a national level it all matters. So far I've done well locally but have never been to the big show in Topeka. I hope to go this year. I'm sure I will get my butt kicked with the HP I'm running (145) but that makes the whole weight deal even more important. Plus the whole thing is interesting and fun for me.

 

I hope that helps.

 

dave

 

 

 

[11Budlite]

Dave. Great write-up on corner-weights. Hope you make it to Topeka!

 

Bruce :7drive:

[twestrup]

 

Now all you need to do is rivet the new ones on and have a good dark beer while sitting alone in the garage. The last part is optional.

 

 

I would consider the dark beer as mandatory! Thanks for the info.

 

Tom

[gjslutz]

Dave,

 

Thanks for the weight info, my scales just came in yesterday. Your info has me rethinking what I'mm doing to the car.

 

Gary

[locostv8]

It seems like when I was going thru the rules that the weight of the driver isn't actual but rather a fixed amount.

[Davemk1]

It seems like when I was going thru the rules that the weight of the driver isn't actual but rather a fixed amount.

 

I don't believe that's true. I'd have to double check but I'm pretty sure it's the actual driver.

 

When you are pulled into impound at a divisional or national event they have you (your helmet and all that) get in the car when it's weighed. If it was a flat amount for driver weight they'd just weigh the car and then add that flat amount and come up with a total.................. now as I think of this if the driver weight was "fixed" then you wouldn't have any mention of it in the book. It would just be car weight since driver weight wouldn't matter.

 

Does that make sense?

 

If you go to the SCCA site you can download the rule book and see what it says.

 

dave

 

 

 

[yellowss7]

Dave's explanation of the weight requirements for DMOD is correct with one exception. For those of us with 1800cc's or less the minimum weight is 1280 pounds, plus the 40 pounds for the modified tub, for a total weight of 1320 with driver. Tom

 

For those of you with those HUGE 1801-2000 cc motors, you pay a penalty of 100 pounds bringing your minimum to 1380 plus 40 for the modified tub, so 1420 is your minimum weight. :_deadhorse:

 

Tom

[Davemk1]

Dave's explanation of the weight requirements for DMOD is correct with one exception. For those of us with 1800cc's or less the minimum weight is 1280 pounds, plus the 40 pounds for the modified tub, for a total weight of 1320 with driver. Tom

 

For those of you with those HUGE 1801-2000 cc motors, you pay a penalty of 100 pounds bringing your minimum to 1380 plus 40 for the modified tub, so 1420 is your minimum weight. :_deadhorse:

 

Tom

 

Eggsactly...........but remember that if your rear end carries less than 51% of the weight you get to pull off another 35 pounds.

 

Dave

 

 

 

[yellowss7]

With my fat butt sitting over the wheels, that ain't going to happen. I am the heaviest single part of the car. The engine only weighs 89 KG. :ack: Svelt guys like you might have a shot at it, especially with that boat anchor up front. Tom

[Martin]

Dave,

 

The car looks great and I have to admit I'm jealous of that background!

Having mountains like that around must be wonderful and provide for awesome drives. I'm sure you will enjoy it to the fullest.

 

Martin