Inboard shocks/Sprung-unsprung weight

There's a pretty decent thread on sprung/unsprung weight at this site, though it's really oriented more to the straight-line guys. Follow the links from there.

I always thought that horizontal/levered shocks were done that way due to space/packaging issues, but I could be wrong

 

Solly,

I'll take a wack at your questions.

1) Repositioning of the shock inside the body work improves local aerodynamics. A link tube is much smaller in diameter than a shock/spring assembly and also lighter so local air flow is improved, which reduces drag and improves heat transfer to down stream radiators, and unsprung is reduced, which increases the natural frequency of the supension.

2) A reduction in unsprung weight increases the natural frequency of the suspension allowing it to move more quickly and remain in contact with the road longer. Ultimately, this provides more grip on the pavement and increases cornering speeds. If the road is perfectly smooth there is no benefit. Real roads and tracks are not perfectly smooth. Saving 20 lbs on a 2000 pound competition car will increase accelleration about 1%. Saving 10 pounds of unsprung weight on a suspension corner that may weigh 30 pounds is a lot. The increase in natural frequency also depends on the spring rate. The advantage occurs if one can exit a corner at a higher speed. If the exit is onto a long straight, the average speed running down this straight will increase saving time and should result in lower lap times.

 

In addition to the excellent description above I have found less reciprocal weight is easier to control and allows a smaller dampener to be used thus reducing weight overall.

Suspension links also allow progressive ratios to be used similar to those used on the rear 'mono-shock' dirt bikes. The more pronounced the suspension movement the stiffer the suspension ratio becomes through the use of eccentric bell cranks etc......

Suspension links also allow adjustment to the suspension at each corner of the car to balance the chassis.

 

Skimming one of Carroll Smith's books last night, he talked about progressive versus non-progressive suspension (look at 308 non-p) versus 360 (p). Interestingly, and IIRC, he opined about 20% at the front and rather less at the rear. Really easy with bell cranks. Much harder with vertical shocks.
Philip

 

The reason for inboard mounting is a mechanical advantage on the shock itself.

There can be a 5:1 lever actuating the inboard shock, so a 50lb inboard shock would only have the 'weight' of a 10lb shock at the hub.

Wheras a direct-mounted 50lb shock will have a 50lb 'weight' at the hub.

 

Solly, I think you've got some really informed and concise answers here.

As ever, this is an excellent site.

I particularly take my virtual hat off to (no-profile?)BILL308

 

Minimizing unsprung weight eases the work for the damper, allows the suspension to work more efficiently, may reduce rotational weight (wheels, brake rotors, bearings), increases the ease of initiating directional changes. Vastly improves ride and handling.

Many advantages to remotely locating the spring/damper. Primarily the ability to tune the response curve more accurately, especially for aero cars. Rising rate suspensions are easy with push rod or pull rod suspension. Mono shock suspension. Separation of suspension modes, such as soft in one-wheel bump (improving mechanical grip), stiff in dive or heave (stabilizing the aero platform) becomes easy.

BTW, a conventional coil-over suspension attached to lower control arm and chassis (like virtually every Ferrari), can be (and are) designed to be rising rate, or at least not decreasing rate.

Best regards,

Rob Schermerhorn

 

Hey Rob, good to see your input here. Carroll Smith provides a line drawing that could almost be the Ferrari 308 suspension and indicates it is decreasing rate. I assume it depends on the "z" axis location of the upper pivot point versus the lower and as the upper pivot moves further out from the car centerline relative to the lower, it'll tend towards rising rate(?)
Philip

 

Solly
Imagine 1 inch increments in wheel movement in compression. For each inch of movement, the force exerted on the spring either increases or decreases depending on the motion ratio. For example, it make take 200 lbs of force for the wheel to travel the first inch, but 240 lbs of force to travel the next inch of compression. This is rising rate. The corresponding situation where the first inch requires 200lbs and the second inch 180 lbs is decreasing rate. Whether a car has rising rate or decreasing rate suspension is driven by the pick up geometry. Of course there are modified (progressive) springs where people try and simulate increasing rate suspensions.

Carroll Smith's illustration led me to compare the 308 front upper shock mount point (inline with the A arm pivot) with the 360 (higher than the upper shock mount).

Smith's experience guided him to indicate about 20% rising rate at the front was reasonable and somewhat less at the rear. He found, for example, that having a steeply rising rate suspension at the rear of the car led to chronic (fast, unpredictable) oversteer...

His books are great reading for non-engineers (me) and (I suspect) engineers alike.

Philip

 

Solly, others are more knowledgeable than me at this, but, with that caveat, I don't think rear rising rate suspension would cause what you are describing. The snap oversteer from RRR is more caused by compression at the rear which, as it firms, becomes skate board like and loses any compliance, hence the oversteer. This would result from an accelerative force compressing the rear. By comparison, in a lift, the weight shifts forwards.

In most cars, a lift causes a weight shift which unloads the rears with respect to the front. If a significant unload results, and the car is cornering, it'll try and spin. Sometimes it'll succeed. And, sometimes it happens so fast we only have time for the expletive before we're in the dirt.

(Too) Soft springs can do this, as can poor choice in ARBs and many other factors. I think the issue with the 360 Challenge cars is complicated (a lot) by downforce and, others have told me, as you experience, they will get "squirrelly" with a lift. Someone with significant experience of the cars should be able to guide you on set up for predictability (which may not be the fastest, but may be more enjoyable...)

Now, where's Rob to correct our (mis-)understandings...

Philip

 

(with all due respect to Rodney Dangerfield..."so I go into the doctor and say, 'hey doc, it hurts when I move my arm this way'...so he says, 'well then don't do that'...no respect I tell ya..."

So don't completely lift in mid corner while driving 10/10ths. Not surprised it spins. The 360 Challenge is basically a somewhat softly sprung proper race car, and the aero works. It is quite ride height sensitive. You know you have it wrong when it porpoises down the straight at 110+ mph, too low in the rear. Raise in small increments until it doesn't slam down on the bump stops and release suddenly. Chassis must be balanced for mechanical and aero.

I do recommend all of Carroll Smith's (RIP) excellent publications. Well written, and not the sleeping pill that Milliken & Milliken is.

More recommended reading here: Technical Reference Books I Keep Close By

Best regards,

Rob Schermerhorn

 

Solly, I have some goodbooks here I can show you at the next track event I see you at. We'll sit at the CC truck and go over some of the stuff. I can also show you someof the stuff on my computer tobetter visually illustrate the ideas thebooks discuss...sometimes 2d pictures are not enough.

I assume you are asking about all this due to your sports racer shopping

 

I can't remember my physics but I seem to remember with springs F=KD. So per pma1010 increasing rate would be any increase in travel. So how can you ever get decreasing rates with a spring unless the spring constant can change...but then it would not be a "constant". O.K. what am I missing?

 

Personally, I don't understand Ferrari. They claim to do all this techno stuff but do not seem to understand that even in competant hands it would be just a lot easier to drive the 360C if it had a wing. O.K it is an issue of the technology of gaining aero with no drag from a wing but lets face facts...even the best challenge driver is not the best pro driver like even a guy on the minardi team. If they really wanted to make a racecar where is the factory fuelcell, full containment HANS compatible seat, side nets, full cage welded and welded to the suspension points, kneebars etc.. etc... ? If they are so picky about the drag a wing causes vs a body that is aero why not be just as picky and use a monoball suspension?

 

Solly,

I am kinda where you are. The question is do I keep dicking around with my 348 trackcar trailer queen or move to a 355C or get a real racecar? My problem is that sometimes I just want to go out to the track. I have three within 2 hours of me so I just look at the web and see who is playing and I join in the fun. I can always get on a track with a modified streetcar. The problem is that once you go to a C/DSR FF F Mazda SR3's, Storer etc. you can't just go out and play. You have to get out when similar cars are running and sometimes the races are very specific like this is a DSR in NASA with all the crap that that entails. So I'm still seriously considering a pseudo real racecar I can take anywhere... a GT3 cup car. It actually seems to have the most bang for the buck. If you took a E46 BMW or even a Ferrari and tried to set it up as a real racer with proper welded cage etc... you would start to see the GT3 Cup as a bargin in a nearly sorted out racecar right out of the box.

 

Solly,

Before you sell the 360 why don't you just dump a wing on it?

 

FBB,
Suggest you look at one of Carroll's books: as you change the a-arm inclination, the motion ratio (MR) [ratio between wheel movement or wheel rate (WR) and spring movement, or spring rate (SR)] changes.

As the MR changes, the WR changes too for any given SR. Net impact is rising rate or decreasing (wheel) rates as the wheel sees compression.
Philip

 

I read recently that SR on F1 cars are 3000 - 6000 lb/inch...

 

Steve, I tried to IM you before, But I know you are super busy.

It really depends on what you want to do with it.

As for maintaince, any Sports Racer will be considerably more tuning and maintaince then you are used to on the ferrari's. You will have to get alignment equipment (high level longacre style stuff) that has to go with you to every track. Obviously you'll need an enclosed trailer and truck topull it...and the required accessory equipment to run them. Stohrs are designed, built and made to win; So they require that much more effort to make fast. These are "true all out racecars" (if you will). They require a lot of fine tuning and playing with to get to runjust right...and that will change for each track, conditions, etc. Unless you go spend a full season with a winning Sports racer team, you'll need a small crew to run the sports racer with you (as goes with any other advanced racecar). High quality parts deliver extreme performance but cannot just be reused mostof the time (mostly little stuff, lock nuts, rod ends, bearings, etc), however you'll have to replace these parts, and check thoroughly between every event/day/run to make sure everything is ok. A small screw falling out can do a lot of damage, compared with the 360 where you can have two wheels fall off and still make it back to the pits

If you go with Ferrari's you will have the engine maintaince $$ bug which comes with any ferrari. Thats the only thing I would say against doing a 355CH. However, and this is a nice however, if you don't mind about the engine maintaince, you can make a 355ch MUCH MUCH faster then spec form. Penske/Moton/Sachs/Protrac/JRZ/Ohlins shocks, better aero, lose a couple hundred lbs (pretty easy) and you have a 360ch beater--easily. I would also put in a much more integrated and "complex" cage into the car to help chassis ridigity and safety. This is a lot of money but you can get a 355ch cheap...and since you'll be rebuilding the engine, and tearing the chassis down, a beaten 355ch will work just fine. All this comes with its extra risk factor with having problems evolve form the buildup, etc.

With all this being said, I think you'd be best off with a Gt3 cup p-car. You'll be AMAZED with the performance, very reliable, and running DE"S you won't be needing many engine rebuilds. Lots of available parts, tires, service shops, etc. Plus I'd assume a large fluctuation of gt3 cup car sales soon since the new 997 cup car is out and its UNREAL!

From being at the track with you a decent amount, I think the porsche would be your best bet. I know you just like to drive and hate having problems and worrying about messing with the car (like putting on pineapple cans on the muffler that was classic). With the porsche you can just drive the balls off, much less tuning to worry about, and well its a porsche factory racecar

I really don't think the Sports Racers are something for you, unless you want to race them (which takes even more effort on the tuning/maintaince aspect). Unless you get the Specific Track day cars (say like the one stohr is coming out with) you'll still need quite a bit of tuning, aligning and "massaging" of it to get it to be just right. Now me being an engineer in motorsport racecar development/designing/fabricating/tuning/etc I love that stuff But when you pay to drive for the weekend at a DE and you spend half the day on the scales trying to align the car after each adjustment... you will want to drive anything thats running at the track at the time

I'll try to contact you privately to discuss more.

Also about the Spring rate comment on F1 cars, its irrelevant to compare it since there are entirely different wheel rates. Spring rate is only half the equation, the leverage being implented and geometry setup creates the overall system (suspension is very complex to design on a formula 1 style car, this incliudes sports racers, and prototype style cars...since they are just bodied f1's haha).

Also Steve, you have seen how much fun I have been having with this BMW, and it costs less then half as much as the Stohr, and these car's do 59-60 at limerock And thats with only 275hp from a little I6 2.5-2.8L

Try to get to drive a gt3 cup or even gt3r (more maintaince, a bit more complex car) for a session or even a day, I know there are some guys who rent out thier cars...I am still waiting to get mine, someday...someday