CrashDummy
CrashDummy Reader
7/28/14 7:25 p.m.

How big of a difference would a reduction of 50 pounds make on a typical 60 second autocross course? Lets say the car is something a little under powered like a Miata; I imagine that the weight savings would be less significant on a car like a corvette. I think it will be an interesting discussion but I am trying to figure out approximately how much there is to gain by deleting some creature comforts from my car (like AC). I'm also wondering how big of a penalty it is to take passengers along on runs.

irish44j
irish44j PowerDork
7/28/14 7:28 p.m.

depends on where you take the weight out (back, front, up high, etc). Less weight is never a bad thing. 50lbs is about a 3% weight reduction on a miata, so your p/w ratio just got 3% better.

I have about 200-250lbs out of the e30 and it feels a LOT lighter. But even when I just took out the OEM seats I could feel the difference. YMMV.

Will
Will SuperDork
7/28/14 9:15 p.m.

Has GRM ever run an article on testing this question via ballast? Because I'd read that article.

I do seem to recall a GRM article in which lighter wheels didn't make an appreciable difference, which surprised me.

bgkast
bgkast GRM+ Memberand SuperDork
7/28/14 9:19 p.m.

My miata could probably stand to lose some weight around the gut.

Duke
Duke UltimaDork
7/28/14 10:07 p.m.

Yeah, 50 pounds right off the driver is where I'd like to start...

Mr_Clutch42
Mr_Clutch42 HalfDork
7/28/14 10:54 p.m.

Reducing weight can bump you into a higher class since you can't remove stuff in the Stock or Street Touring classes. The weight penalty from passengers also depends on your suspension setup and the tires you are running. I have rode with many people in different setups and most of them don't feel a difference. Some people also note that a passenger balances the weight of the chassis which helps them.

wbjones
wbjones UltimaDork
7/29/14 6:03 a.m.

well you CAN remove some weight in ST.

some OEM seats are HEAVY (like 60 - 70# each) … race seats (ballast up to a total of 25# each seat, which should include all the mounting hardware, brackety, etc …) can save you decent amt. of weight … if the car (in that trim package) was avail. without AC, then the AC can be removed ….

in Street, maybe the AC …but the vehicle has to be available (your trim level) without before it can come out)

tpwalsh
tpwalsh HalfDork
7/29/14 7:22 a.m.

I can tell you that putting 10% on a kart makes a HUGE difference. A couple years ago I ran a friends ICC kart at blytheville, with and without the weight vest to make me legal. 30-35lbs made a huge difference in both lateral and and longitudinal grip/G's. That 10% was probably 0.5s if I had to guess.

pinchvalve
pinchvalve GRM+ Memberand MegaDork
7/29/14 7:52 a.m.

When you do the article on ballast, try removing 50 pounds versus moving 50 pounds around in the car (to lower CG or increase traction). I am getting more interested.

scardeal
scardeal Dork
7/29/14 8:18 a.m.

Once or twice I've gone faster with a passenger in the car than without. I think it might have to do with less overdriving as a result, though.

noddaz
noddaz GRM+ Memberand Dork
7/29/14 8:27 a.m.

It depends on how many cones you miss that you would normally hit because the car handles better...

I would estimate that it would mean tenths of a second at most. Maybe hundredths on a car otherwise sorted out. But that IS what can win races...

wbjones
wbjones UltimaDork
7/29/14 8:47 a.m.

my car is in need of re-corner balancing … if I get a small person (100 - 120#) to ride with me, I usually end up going quicker

alfadriver
alfadriver UltimaDork
7/29/14 9:50 a.m.

My car lost 50lb. But that was because I went from 230lb to 180. The car was faster, but some of that may also be me being in better shape. totally legal.

T.J.
T.J. PowerDork
7/29/14 10:49 a.m.

Ignoring any rules and/or classing issues with this, I know from when I auto crossed that I was not good enough of a driver to see much gain (if any) from losing 50 pounds from anywhere. I would say if you are going out there and nailing all of your runs where all of them are within a few tenths then maybe see what losing 50 lbs does for you. If you are like me and usually improved dramatically over my runs as I learned the course better, then work on seat time and keep your AC, soundproofing, seats, or whatever else you are thinking about sacrificing to lose 50 lbs. If you go with Alfa's suggestion of losing weight from the driver, then that is a no lose situation. Try that.

Jerry
Jerry SuperDork
7/29/14 10:54 a.m.

My last run of my first autocross in the Abarth 2 weeks ago was my fastest, and the only one I had a passenger with me. I think maybe I was trying to show off?

Matt Huffman
Matt Huffman New Reader
8/17/21 9:13 a.m.

one data point... Had my 250 pound dad a ride in my 3100 (w/ me) RX8 at the autocross last weekend. He's around 8% of that weight.  when i kicked him out, i could only manage to go about 1.3% faster (37.4 versus 37.9).  and some of the additional speed was likely from driving better because it happened on my 6th run.

So, 8% weight difference netted likely less than 1.3% performance improvement.

i overlayed the videos and didn't see much difference.

His weight hurts acceleration by 8% (that's simply F=ma), but the extra weight puts extra normal force on the tires and they can generate additional lateral and longitudinal traction for stopping and turning.  you can research it and it's not linear, but 8% weight reduction does NOT mean you go 8% faster around corners...

cyow5
cyow5 Reader
8/17/21 9:30 a.m.
Matt Huffman said:

 

 but the extra weight puts extra normal force on the tires and they can generate additional lateral and longitudinal traction for stopping and turning.  you can research it and it's not linear, but 8% weight reduction does NOT mean you go 8% faster around corners...

The lateral force required to go around a corner at the same speed goes up by the same amount as the normal force increase, canceling the gain out. 

frenchyd
frenchyd UltimaDork
8/17/21 12:10 p.m.

John Force was faced with just such a question. Taking weight out of a dragster gets really expensive real fast. I forget the details but it was probably around $5000 to lose 15 pounds. 
   He lost weight instead. 

Tom1200
Tom1200 SuperDork
8/17/21 12:13 p.m.

I have a 1652lb Datsun, 50lbs on an autocross course maybe worth 1-2 tenths on a given course. Years ago I manged to take 109lbs out of the car..........wait for it.................4 tenths on a 3.4 mile road course, that was it.

Increasing the horsepower from  76whp to 99whp was worth 3.5 seconds per lap on a 3.4 mile course.

The reason why driver weight reduction is so effective is because human beings are top heavy; you lower the center of gravity (most Formula car have lay down driving positions). If you can shed some pounds work on that. Note I weigh 143lbs so I understand not every driver can lose weight.

Frankly I'd work on car set up before I went with carving 50lbs out of a 2200lb car. Are the dampers/shocks the best they can be? Is the alignment the best for your driving style etc? Put the effort into set up.

My .02

 

 

LeftLaneLoser
LeftLaneLoser UltraDork
8/17/21 4:16 p.m.

I can tell you moving the battery from the front drivers side (in front of the wheel arch), to behind the passenger in the floorboard, was an incredible wake up moment. I've now done it on several cars. 

AnthonyGS (Forum Supporter)
AnthonyGS (Forum Supporter) SuperDork
8/17/21 6:30 p.m.
Duke said:

Yeah, 50 pounds right off the driver is where I'd like to start...

Me too.  

Mr_Asa
Mr_Asa PowerDork
8/17/21 7:50 p.m.
CrashDummy said:

How big of a difference would a reduction of 50 pounds make on a typical 60 second autocross course? 

We did simulations on this when I was in FSAE, then backed it up with actual track time.  There is a linear relationship with weight and track times.  For other factors, such as grip or horsepower then there is an exponentially decaying relationship where it starts to take massive changes in order to get better results

Linear graphs look like this, so there is a direct step-for-step change with weight lost 
Linear Functions and Their Graphs

 

Exponentially decaying graph: Add HP, add grip, or other changes and you eventually reach a limit where you no longer improve your times

racerfink
racerfink UltraDork
8/17/21 10:33 p.m.

About 27 years ago, there was a semi-professional Improved Touring race on the club course at Sebring.  To simplify the run groups, and make sure there were  enough entries to cover the payouts, they combined ITB and ITC as one class, and made the ITB cars add 50lbs.  Our ITC SR-5 Corolla held the track record on the short course, and usually was around the race for 3rd-5th in ITB.  Randy Pobst borrowed a friends ITB Rabbit, dropped 50lbs of chain in the right rear corner, and proceeded to break the track record by nearly one second on his way to the win.  He had driven that very Rabbit several times before, but talked about how much better it felt with that 50lbs in the right rear.

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