Scales and corner weighting.

[Davemk1]

Interesting... If your weights are not the same on the 2 front wheels, wouldn't the expected behavior be for the wheel with more weight to lock up after the one with less weight?

 

Absolutely. It makes sense to me but in practice they lock up at the same time everytime.

 

My records show that the left front is carrying 50lbs more than the right with me in the seat. Only thing I can figure is that there is enough weight transfer to make that 50 pounds a smallish percentage of the overall weight during hard braking.

 

I'll toot my own horn a bit and say that with this set up I just two weeks ago won the Top Gun Meet in Helena MT. The Top Gun event is our state championship and pits the three SCCA regions in MT against each other. Our region (SCCA #52) took the most "first in class wins" and so won the overall trophy for the 3rd year running. Not bad for the smallest (and fastest growing in the country!) SCCA region in the USA. I won my class (DM) and took FTD both days of the event. That was fun. The course on day tow had a 4th gear 85mph slalom. That was the shit.

 

Dave

[xromad]

Well, I'll be durned. I just assumed all sevens had the behavior. My bad. Tell you what, I"m going to quit commenting until I get a chance to break all the scales out and set it up again.

 

That way I'll have real numbers to discuss. I can not find the sheet back from when I set it up last time.

 

This way we will also have real numbers for the scales, repeatability, standard deviation, etc...

 

I still think bathroom scales are fully capable of being accurate enough for the needs of many seven owners. However, I think we can all agree that careful attention to procedure and an understanding of exactly what the measurements mean is vital to getting the best results.

 

 

[Davemk1]

Well, I'll be durned. I just assumed all sevens had the behavior. My bad. Tell you what, I"m going to quit commenting until I get a chance to break all the scales out and set it up again.

 

That way I'll have real numbers to discuss. I can not find the sheet back from when I set it up last time.

 

This way we will also have real numbers for the scales, repeatability, standard deviation, etc...

 

I still think bathroom scales are fully capable of being accurate enough for the needs of many seven owners. However, I think we can all agree that careful attention to procedure and an understanding of exactly what the measurements mean is vital to getting the best results.

 

 

I agree 100% - the bathroom scales should be more than accurate enough to get reasonable results. It's interesting to me.......in retrospect it seems surprising that my car doesn't have issues with this. It completely makes sense that it would. But, in my car's case, I don't have an issue.

 

It's a curious thing.

 

Dave

[xromad]

Well, I'm not ignoring this topic. I've been thinking and reading.

 

I actually found a direct reference for my argument about corner weights and setting up a wedge to offset the driver's weight and get equal weight on the front wheels.

 

Direct quote:

 

"With single seaters you should be able to get it to about five pounds. With cars in which the driver's weight is offset from the centerline things are somewhat more difficult. Normally it is more important to have the front wheel weight equal and to accept a discrepancy at the rear."

Carroll Smith, "Prepare To Win", Page 137, Corner Weight, Paragraph 4

 

Exactly what I found in my car.

 

Also:

 

"Adjust the bias bar until the fronts [brakes] lock first - but just barely."

Carroll Smith, "Prepare to Win", Page 82, Brake Bias, Paragraph 3

 

Also exactly what I found in my car.

 

And:

 

While it is difficult to just grab a direct quote, Carroll Smith discusses Longitudinal Load Transfer in detail in his book "Tune To Win". The basic summary is that there is a basic formula to calculate load transfer. It's primary variables are: deceleration (g),weight, cg height, and wheelbase.

 

                                      weight(lbs) * cgHeight(inches)
weight transfer = deceleration(g) * ------------------------------------
                                             wheelbase(inches)

 

All of those variables except deceleration work against a 7 transfering a significant amount of weight in braking, they also prevent a 7 from transfering a lot of weight under acceleration at the dragstrip. Acording to his discussion, this is not as bad as it might seem. Many of the things that weight transfer does equal bad dynamics. Examples of this are: Unloading the rear wheels causing less traction. Loading the front suspension causing ride height changes, camber changes, gyroscopic procession forces in the steering, bump steer, etc.... These issues are all quite effectively avoided in sevens because of the light weight, low cg, and long wheelbase (for its size).

Carroll Smith, "Tune to Win", Page32, Longitudinal Load Transfer

Carroll Smith, "Tune To Win", Page34, Anti Dive and Anti Squat Geometry

 

Oh, and before anyone jumps in with spring rates, saying a stiff spring or a soft spring allows more or less weight transfer. Carroll specifically states that neither spring rates nor anti dive significantly affect the weight transferred (notice they are not in the equation).

 

Off to do some more reading

Brent