Weight Distribution

[DB6]

I have pondered this for some time:

 

What is the front to rear weight ratio on a typical 7.

 

Then, what is the effect on that ratio by adding or subtracting driver,passenger, gas in the tank and spare wheel ?

 

What is the subsequent change in handling characteristics ?

 

I am guessing it must make big changes in such a light car and wondered if anyone has experiences or experimented on a track with this.

[Davemk1]

My Birkin has 51% of the weight on the rear with my butt is in the seat and with a 1/2 tank of fuel. Fuel doesn't weight enough to change that in a big way in my case. A full tank would add about 30 pounds to the rear.

 

I don't notice a difference in handling with changes in fuel load. I use my car for street and autocross and it's not like track racing where the change of fuel load happens during a run and one might feel the difference. Each autocross run feels different due to other stuff like temperature and fuel load doesn't change much during a day of crossing.

 

I've never driven the car without my body weight in it and I'm not sure how I might get that to work

 

Dave

[scannon]

My Caterham SV had a 50.6% front to 49.4% rear unloaded weight bias. Add my not inconsiderable bulk and it changed to 47.8% front to 52.2% rear bias.

 

The weights were taken on corner weighting scales with 5 gallons of fuel in the tank, no spare on the rear, no weather gear and nothing in the boot. It was also before I installed adjustable coil spring perches so corner weighting has changed the bias a little but I don't have the data sheet for that one handy.

 

I have since subtracted some lightness: FIA roll bar, Rhino lining the wings and water/alcohol injection to name a few of the heavier items. It's not a pure race car seeing mostly street use with some track lapping days.

 

Adding a passenger makes a noticeable difference on the track or the street.

 

Skip

[DB6]

My Birkin has 51% of the weight on the rear with my butt is in the seat and with a 1/2 tank of fuel. Fuel doesn't weight enough to change that in a big way in my case. A full tank would add about 30 pounds to the rear.

 

I don't notice a difference in handling with changes in fuel load. I use my car for street and autocross and it's not like track racing where the change of fuel load happens during a run and one might feel the difference. Each autocross run feels different due to other stuff like temperature and fuel load doesn't change much during a day of crossing.

 

I've never driven the car without my body weight in it and I'm not sure how I might get that to work

 

Dave

 

David,,

You mean you do not have the remote control option?lol.

[DB6]

My Caterham SV had a 50.6% front to 49.4% rear unloaded weight bias. Add my not inconsiderable bulk and it changed to 47.8% front to 52.2% rear bias.

 

The weights were taken on corner weighting scales with 5 gallons of fuel in the tank, no spare on the rear, no weather gear and nothing in the boot. It was also before I installed adjustable coil spring perches so corner weighting has changed the bias a little but I don't have the data sheet for that one handy.

 

I have since subtracted some lightness: FIA roll bar, Rhino lining the wings and water/alcohol injection to name a few of the heavier items. It's not a pure race car seeing mostly street use with some track lapping days.

 

Adding a passenger makes a noticeable difference on the track or the street.

 

Skip

 

Skip,

Do you think that would pretty much hold true for all 7,s, I guess it would not be far off.

It,s odd cuz when I look at the car it would appear more nose heavy, but then again I guess if you plop a couple of hundred pounds almost on the back axle it should balance things out hey.

[scannon]

Skip,

Do you think that would pretty much hold true for all 7,s, I guess it would not be far off.

It,s odd cuz when I look at the car it would appear more nose heavy, but then again I guess if you plop a couple of hundred pounds almost on the back axle it should balance things out hey.

Hard to say since I only have data on my car but since the motor is behind the front axle, the transmission is in the middle and the diff plus heavy deDion suspension in the back it does seem right.

 

I think the mostly empty nose cone hanging out front contributes to the nose heavy look. Looking at the car without the bonnet and nose cone changes the nose heavy look. I don't have a good side shot of the car without the nose cone but this one sort of illustrates what I mean.

http://memimage.cardomain.com/ride_images/1/2545/3361/6361680220_large.jpg

Skip

[JohnCh]

With 190lb of ballast in the driver's seat and about 4 gallons of fuel, my car came in at 46.9% front, 53.1% rear.

 

-John

[scannon]

I shot a side view of the car that better shows where the weight is. The front pulley on the engine is nearly 3" aft of the center line of the front axle.

[slngsht]

I have not weighed my car yet, but took a detailed account of components that went into it. When it's all said and done, I think it'll be 1800 lbs.

 

This thread (http://www.usa7s.com/vb/showthread.php?t=2621&highlight=1800) has a breakdown of component weights and distributions on my seven. Even with the oversized engine and tranny, I should still have a rear weight bias.

[Mondo]

I just did mine (S2K) a couple weeks ago... car weighs 1450lbs with 3/4 tank.

 

We got 50.00% front/rear with me in it... less than 3lbs difference side to side. The guy at the race shop was really impressed!

 

The sad part is I can't tell how it handles because I've only taken one little shake down ride in the foothills last weekend.... haven't pushed it yet.

[slngsht]

I just did mine (S2K) a couple weeks ago... car weighs 1450lbs with 3/4 tank.

 

We got 50.00% front/rear with me in it... less than 3lbs difference side to side. The guy at the race shop was really impressed!

 

The sad part is I can't tell how it handles because I've only taken one little shake down ride in the foothills last weekend.... haven't pushed it yet.

 

On the plus side, fixing that is much easier than fixing weight distribution.

[jwduncan]

Another data point... my Ultralite came out to 48% front / 52% rear with driver and fuel. And about 12-15lbs heavier on the driver side. I've noticed that as the fuel burns off the car gets looser.

[DB6]

Whre do you get your cars weighed and with front to rear and side to side balance.

All I can do here is sneak on the DOT truck weigh points when no one is around,lol.

[Davemk1]

Whre do you get your cars weighed and with front to rear and side to side balance.

All I can do here is sneak on the DOT truck weigh points when no one is around,lol.

 

Corner weight scales. They are a bit spendy but 4 of us autocrossers went in on a set and it works out very well.

 

Dave

[pi7ot]

I have a Zetec Birkin. I run a square wheel tyre set up with a 175ish driver.

I don't notice the effects of the fuel load, but when I have a passenger on the track, I have to add 1- 2psi to the rear tyres. The car has demon understeer with a passenger, as the added rearward bias adds Uber rear grip which heats up the rears faster than the fronts.

The understeer scares me so much that I'll add 1psi even on the street with a passenger.

I've never felt so imPOtent as when the car caught me by surprise and speared off the track. The passenger was impressed:rolleyes:.

 

m

[Davemk1]

I have a Zetec Birkin. I run a square wheel tyre set up with a 175ish driver.

I don't notice the effects of the fuel load, but when I have a passenger on the track, I have to add 1- 2psi to the rear tyres. The car has demon understeer with a passenger, as the added rearward bias adds Uber rear grip which heats up the rears faster than the fronts.

The understeer scares me so much that I'll add 1psi even on the street with a passenger.

I've never felt so imPOtent as when the car caught me by surprise and speared off the track. The passenger was impressed:rolleyes:.

 

m

 

The understeer is surprising. Have you ever taken tire temps across the width of the tires in front? I'm wondering about your camber and toe.

 

The only way I can get my Birkin to understeer badly is to completely overdrive it with lots of throttle and a stab of steering on turn entry. Otherwise it's neutral or oversteering.

 

Dave

[Kitcat]

I added a 250lb passenger at a track day last year and handling was unfazed. But, as I quickly discovered, braking distances were a bit longer and brake pedal pressure went way up. Nonetheless, the car stayed totally neutral and toss-able in the curves.

[eVox]

My Locost- 50.2% rear bias with me in it, and no front fenders. 1470lbs without driver.