Black Art Dynamics - Tyres and load transfer, Baseline spring specification

[whataboutbob]

Couple of interesting pages I found as a result of a friend's FB post:

http://www.blackartdynamics.com/Chas...d_transfer.php

http://www.blackartdynamics.com/Chas...es/Springs.php

Thoughts and discussion anyone?

[dem00n]

I'd like to point this part out. A bit off topic to our cars...

An oversprung car will also be very quick to respond to driver inputs, sometimes to the point of being twitchy and difficult to balance on the edge of grip.

Could someone explain why FWD cars with aftermarket suspension, thus having a higher spring rate, tend to be so twitchy? I do understand it would have to do in part that the front wheels are the power source and acting a forward force, but i can never really find a legitimate explanation in this.

[Racecomp Engineering]

Originally Posted by dem00n

I'd like to point this part out. A bit off topic to our cars...

An oversprung car will also be very quick to respond to driver inputs, sometimes to the point of being twitchy and difficult to balance on the edge of grip.

Could someone explain why FWD cars with aftermarket suspension, thus having a higher spring rate, tend to be so twitchy? I do understand it would have to do in part that the front wheels are the power source and acting a forward force, but i can never really find a legitimate explanation in this.

FWD cars set up to be fast are a different beast because they have to be in order to put power down. You need a soft front and a very stiff rear. On entry they'll be twitchy and oversteer. But when you hit the gas you won't just spin the inside front tire as much.

The natural frequency thing is not a one size fits all thing...real motorsports engineers are not always trying to optimize for the "flat ride" in every situation.

- Andy

[Hoosier Daddy]

Originally Posted by Racecomp Engineering

real motorsports engineers are not always trying to optimize for the "flat ride" in every situation.

- Andy

They never do actually. We tune for speed and consistency only. What that entails will depend on the configuration of the car. Flat ride is an OEM philosophy and if that's what's required to be fast then it's soley by coincidence.

[Racecomp Engineering]

Originally Posted by Hoosier Daddy

They never do actually. We tune for speed and consistency only. What that entails will depend on the configuration of the car. Flat ride is an OEM philosophy and if that's what's required to be fast then it's soley by coincidence.

And yet you see every armchair internet forum engineer go for it without a second thought.....

- Andy

[Shankenstein]

Originally Posted by Hoosier Daddy

They never do actually. We tune for speed and consistency only. What that entails will depend on the configuration of the car. Flat ride is an OEM philosophy and if that's what's required to be fast then it's soley by coincidence.

From your testing, what would you expect to be the bias on our cars for road race tuning?

Looking at the RCE Tarmac 2 Clubsport (as an example), they use 400/400 spring rates, which puts a ~23% biased towards the front (using the motion ratios from my data thread). Stock is ~27% biased toward the rear.

I've always heard that autocross and racing with RWD tends to work well using a stiffer front than rear. You get a planted feel during slaloms, but better power delivery. There is a penalty with regard to weight transfer, but it's faster because each end of the car can do its job more effectively.

[Hoosier Daddy]

Originally Posted by Shankenstein

From your testing, what would you expect to be the bias on our cars for road race tuning?

Looking at the RCE Tarmac 2 Clubsport (as an example), they use 400/400 spring rates, which puts a ~23% biased towards the front (using the motion ratios from my data thread). Stock is ~27% biased toward the rear.

I've always heard that autocross and racing with RWD tends to work well using a stiffer front than rear. You get a planted feel during slaloms, but better power delivery. There is a penalty with regard to weight transfer, but it's faster because each end of the car can do its job more effectively.

I don't have a twin yet. What I would expect are wheel rates that are fairly balanced with respect to the weight distribution. I am not contradicting myself as it's a function of many things including the amount of power, tires, ARBs, downforce (not on this car) etc and ultimately the course layout. For a twin, if the spring/shock motion ratio in front is closer to unity than the rear, which I would assume is the case, I'd expect the spring rate bias to be somewhat rearward. What are the stock spring rates?

Slow course layouts such as autocross and slaloms have different requirements compared to road racing. Power-down, drive-off, or whatever you want to call it is of the utmost importance I would think especially for cars that are massively traction limited (big power) rather than momentum cars when navigating around cones. Then you would want, all else equal, to move the spring rate bias more forward to help with longitudinal acceleration performance.

[Gunman]

A car that goes thru the corners flat, is better for aero though.

[Shankenstein]

Originally Posted by Gunman

A car that goes thru the corners flat, is better for aero though.

I know there's been active aerodynamics for a while. Speed-based wings on Porsches. McLaren has a brake assist system. Bugatti and others switch to low drag during top-speed runs.

Has anybody tried to do active aero for steering assist? It seems like an obvious thing... add downforce to the inside wheel to combat roll. This keeps the car level and the tire wear even.

[Gunman]

Originally Posted by Shankenstein

I know there's been active aerodynamics for a while. Speed-based wings on Porsches. McLaren has a brake assist system. Bugatti and others switch to low drag during top-speed runs.

Has anybody tried to do active aero for steering assist? It seems like an obvious thing... add downforce to the inside wheel to combat roll. This keeps the car level and the tire wear even.

Early on, there was talk that the Deltawing had some active aero steering in the rear underbody, like "flaps". It was either outlawed, like all active aero, or didn't pan out, because I never actually saw it on the car.

[Racecomp Engineering]

Originally Posted by Gunman

A car that goes thru the corners flat, is better for aero though.

The aerodynamics on the front of the car are usually more sensitive to pitching, so a higher front ride frequency generally helps aero.

There's good explanation of "flat-ride" and spring frequencies here:

http://www.optimumg.com/docs/Springs...Tech_Tip_1.pdf

Great quote in there:

"The above theory was originally developed for passenger cars, where comfort takes priority over performance, which leads to low damping ratios, and minimum pitching over bumps.

Racecars in general run higher damping ratios, and have a much smaller concern for comfort, leading to some racecars using higher front ride frequencies."

- Andy

[CSG Mike]

Originally Posted by Shankenstein

I know there's been active aerodynamics for a while. Speed-based wings on Porsches. McLaren has a brake assist system. Bugatti and others switch to low drag during top-speed runs.

Has anybody tried to do active aero for steering assist? It seems like an obvious thing... add downforce to the inside wheel to combat roll. This keeps the car level and the tire wear even.

If you have that level of downforce, you'll get more benefit by adding grip to both wheels.

[bholliday]

Originally Posted by Racecomp Engineering

There's good explanation of "flat-ride" and spring frequencies here:

http://www.optimumg.com/docs/Springs...Tech_Tip_1.pdf

Andy, thanks for the suspension theory link! That paper, along with the others in the series, helped me better understand spring frequencies, sway bar balancing, and shock curves. The explanation on setting up the shock curves was especially helpful, and answered several questions I had after reading other internet sources on the subject.

If anybody else is interested, the 6 part series can be found here:

http://www.optimumg.com/technical/technical-papers/

[Captain Snooze]

Originally Posted by Shankenstein

Has anybody tried to do active aero for steering assist? It seems like an obvious thing... add downforce to the inside wheel to combat roll. This keeps the car level and the tire wear even.

Doesn't the Pagani Huayra do this? Independent front left and right and rear left and right spoiler activation. There is also a thread somewhere on these forums about an aftermarket manufacturer with separate left and right sections.

Stop looking. This crowd:http://aeromotions.com/products/s2-dynamic