Wheel and tire weight

[jstoezel]

Hi:

How much do an SCCA FV slick and a rim weight? I run in the Canadian FV series (WCMA) where we run radials, and I would like to know what the weight difference is with the SCCA tires.

For the info, the spec tire we use is a Falken Azenis RT615 in size 195/60R14. The rim and tire weight 35lbs.

Jean

[SR Racing]

Wheels can go from about 12 to 14. Front tires weigh about 10lbs and rears about 12. So you are looking at 22 to 24 for fronts and 24 to 26 for rears.

[jstoezel]

Hi Jim:

Thank you for the reply. Hum that's a substantial weight difference for an unsprung component... I guess with heavier wheels stiffer shocks (like the stock ones) would be necessary?

Jean

Wheels can go from about 12 to 14. Front tires weigh about 10lbs and rears about 12. So you are looking at 22 to 24 for fronts and 24 to 26 for rears.

[hardingfv32-1]

It is not clear from the literature what the implications on high sprung weight is on the shock setup. Basically, all race car research you find is for properly designed race cars with low unsprung weight when compared to sprung weight.

Brian

[brian]

going from radial street tires, which have a wide and predictable adhesion curve, is quite different from a racing slick which has, while higher grip, a very short and sharp adhesion curve. Street shocks are more about handling all the body sway and bouncing where as racing shocks are designed to maximize the tire patch. Maybe my head is up something, but frankly with brake drums and our wheels, I don't obsess about unsprung weight.

[cendiv37]

Jean,

At one time I weighed most of the front suspension stuff on my Vee. Using the general rules of thumb, I estimated the total unsprung weight of each front as approximately 50 lbs. It looks like your wheel/tire combo would add about 10 - 12 lbs to this, or 20+%.

It would seem logical that this should take a bit more shock stiffness to keep under control. At the same time, it's my understanding that most guys who have run both US slicks and Canadian series radials on their car did not have to make too many changes to make the car work fairly well.

Ours are pretty crude vehicles and therefore not all that sensitive to minor changes in set-up, at least to the majority of drivers. I once drove a competitive race after bending a right front trailing arm in qualifying and not having time to fix it before the race. After being launched off of that same RF in a turn one wreck, I continued the race and came back from last (about 25th with no-one in sight) to eighth at the end and within a couple tenths of fast lap. Back at my shop, I measured the RF camber as +2 degrees. As Lybarger always told me when I got too technical, "It's just a VW. Stop over-analyzing and go drive it." Some of the best advice I ever got.

[hardingfv32-1]

1) We can not obsess over unsprung weight because there is almost nothing that can be done other than turning your brake drums. It would be nice to know how it could effect the shock settings when you go to setup your shocks. What might be different from a low unsprung weight car shock setup?

2) I'm not sure stiff is better. A typical wheel rate formula shows that the wheel rate needs to go up when you decrease the unsprung weight to maintain the same characteristics.

3) This forum is all about over-analyzing a topic. Some understanding of what is going on may be helpful when you make your driving decisions.

Brian

[jstoezel]

Hi:

In all honesty I don't know much more about unsprung weight than what I read from wikipedia. I started looking into it after I read that some people install shocks upside down so the body of the shock would actually switch from the unsprung to the sprung weight side.
When I bought my car one of the shocks was damaged. I never really bothered to replace it but I want to do it this year. I've read several opinions from you guys for what should be used for front shocks, and I want to know if your shock advices have to be derated based on the wheel weight difference that exists with the Canadian FV.

I came to realize that most of the Canadian FV I've seen use stock shocks, from the CIP website. My car for example came with these types of shocks. I've only driven my car with the busted shock, and I don't have much to compare it against. I've done gokart racing before and I don't truly understand how suspension setup works. At the club for instance everybody runs stock shocks, and when you ask it just seems that everybody uses them because they are cheap. Since everybody pretty much runs the same setup there's not really a way to check if this is the way to go, except than trying with a different shock setup.

I totally agree with you Bruce, these are old machines and they may not be worth over thinking over them.
At the same time, for once (and please don't take this bad), I fully agree with you too Brian H.. This is a forum, we should be able to ask questions and hopefully receive answers. If the final answer is that unsprung weight is of no consequence then so be it. It would be good to understand why though.

Jean

[Rickydel]

Jean,

The short answer as to why low unsprung weight is desireable is;
the lower mass can react to road irregularities faster than a larger mass, thus keeping the tire on the pavement more.

[hardingfv32-1]

A FV can function with stock shocks or even steering dampers as shocks. There is even a small chance that they might have correct values to function very well. In general though, you will see better performance with a reasonably setup racing shock. This is a budget vs how competitive do want to be issue.

Brian

[jstoezel]

Well these are 2 different shock setups though. The stock shocks are way stiffer than the steering dampers. By exerting a similar amount of pressure (don't have a dyno) it takes almost twice to compress and extend a stock shock compared to a steering damper. The stock shocks seem way harder in compression as they have that built in spring that the steering damper don't have.

How do you mount the steering dampers by the way? Like the stock shocks, the shock eyes have a smaller diameter than the mounting bolts (to the torsion arms and the towers). On the stock shocks I was able to drill the eyes igger, but the steering dampers don't seem to have enough material to do this.

A FV can function with stock shocks or even steering dampers as shocks. There is even a small chance that they might have correct values to function very well. In general though, you will see better performance with a reasonably setup racing shock. This is a budget vs how competitive do want to be issue.

Brian

[hardingfv32-1]

1) "they have that built in spring", do you mean the shocks are gas pressurized? Gas pressure is not generally considered a major tuning option/variable.

2) Off coarse compressing the shock by hand has almost no evaluating value.

3) Using the steering dampers is very old school. It was used by some front runners for aero reasons. I have no idea about the mounting issues.

Brian

[cendiv37]

Jean,
Sorry if I appeared flippant. It's just that you will see a huge array of different set-ups, some close to or actually stock and others quite exotic and expensive. There isn't necessarily a good (inverse) correlation with the amount spent on shocks and the lap times achieved.

You will definitely want to have two "good" shocks whatever route you chose. One broken shock is not the hot set-up!

I think I understand why, but using all "stock" parts, you shouldn't have to ream out the shock bushings to fit the bolts. The stock shock bushings are 12mm and fit the lower arm stud and the upper bolt fine (when a 12mm bolt is used at the top). Most racing shocks use a 1/2" spherical bearing at each end. To use with the stock, 12mm fasteners with the 1/2" shocks, it is common to make a shim (pop can material cut to size and rolled up works well - it takes a couple wraps). Some people don't bother with the shims.

Alternatively, you can re-tap the upper shock mount to 1/2" thread so you don't have to shim there. With a true racing shock, you will also need spacers to move the shock center-line out to clear the tower. The bottom is trickier since the stud is pretty hard to remove and replace (in my experience). Many of us "off set" adapters to mount the shock behind the shock tower for aero reasons. this opens up other possibilities on how the shock is fastened.

This is speculation from what you've said, but it sounds like at least the top mount is using a 1/2" bolt. I don't know about the stud.

Shock choices in order of increasing cost:

Stock replacements: typically Brazilian made. I believe you can remove the outer sleeve, most do I think.

Steering Dampers: cheap, maybe cheaper than stock shocks. You will need to make on adapter or weld on an extension to the upper mount. I don't know how you fit it at the bushing end. I think they have 10mm bushings. You have a few choices: Type 1, Type 2 and 3 if you can find them. The type 2 and 3 look more like a shock with bushings at both ends, but they are 10mm bushings I think and I don't know if the max and min lengths would necessarily be workable.

Intermediate performance, non-adjustable shocks OR
Intermediate performance adjustable shocks (usually limited adjustments possible)
(I don't know what's available here in these two categories. Maybe Jim from SR has some options.)

Used Penske shocks, probably "blacks": These are still the preferred set-up for many since they are small and light. I believe Penske still makes parts for them, but they don't sell complete units. They are kind of precious to those who have hoarded them over time so they won't come all that cheap, maybe a couple hundred $ a piece, but probably more. They give you the some adjustments and are completely rebuildable, etc.

New or used full size Penskes: These are the most common "new" set-up . They give you the most adjustments possible and are completely rebuildable, etc. I haven't bought one recently, but I'm guessing they are $700 each or more today.

Of course these are just options, not a recommendation. My recommendation would be to start with the cheapest shocks that are commonly used on the cars you will be competing with. That way you get a starting point and can work your way into it without breaking the bank. Once you've driven the car and gotten a feel for what it's like, maybe you can borrow someone's "hot" set-up shocks to see if you like them better. Or you just decide want to have some adjustability and are willing to spring for something fancier (and more expensive).

I don't think I'd try the steering dampers first. My hunch is that those heavier wheels and tires would be more than they could control.

my $.02

Bruce

[tiagosantos]

Speaking of tire weights.. Anyone bothered weighting a hoosier wet tire? Finally went through the scales at the last race, was 10lbs over the limit on rains.. I'm wondering if the slicks are lighter than rains or if I can safely get rid of another 5lbs or so.

[Edward Schubert]

If you are obsessed with unsprung weight mount your shocks upside down (body up)....but then you are putting the shock body in the wind and now you will obsess about aerodynamics. Maybe using 2 torsion bars with 'sprint car" style stops in the top beam tube and eliminating the shocks altogether would be neat! Oh yea...it just a Vee!

[hojo]

Jean the easy answer you were looking for is here

video by Greg Rice comparing SCCA and F1200 tires - should be same rim tire combo (falken)

http://www.youtube.com/watch?v=dC8aiqaO7A8