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Game Forum / Simulators / Car Simulators / July 2005Chassis Rigidity
What is an acceptable ratio of chassis rigidity compared to wheel rate? > What is an acceptable ratio of chassis rigidity compared to wheel rate? How does one measure chassis rate? Is it the deflection relative to the origial plane? Does it take into account how the other corners are deflected relative to the original plane or is it measured, statically, with the other 3 held in their original position? PS - I initially had a smart arse answer here since I though "WTF would the units of chassis rigidity be FFS" and thus WTF would ratio mean? :) Then I saw "pdotson", a familiar name, and thought a little more seriously on the subject. Glad I did! :) Anyway, all I know of this subject is that the word always seems to be the more rigid the better. I imagine the reason is that the more rigid the chassis, the more accurate the handling can be adjusted via springs/shocks/sways? I'd imagine "acceptable" is greatly subjective. Byron, glad you remember me. I haven't been doing much in the sim world lately. I'm hoping rfactor will change that :) I'm racing karts right now, which is the reason for my question... I'm building a new kart chassis. It's designed already, and I've welded the basic structure together. The idea of the design is to apply the idea of a rigid race car chassis to a racing kart. The center section of the kart is designed to be rigid. All the flexing in the chassis will be isolated to the front and rear tubing. So this is new gound I'm breaking here in terms of karting, but I figured there might be some car chassis design guidelines regarding chassis rigidity for varyious types of cars. And if there is, then someone on RAS would know. This information is probably in Milliken, but I don't have access to that book right now. I could add as much structure to the center section as necessary, but don't want to go overboard. As for measuring chassis rigidity, it's a common thing. The units would be force per unit of deflection - like lb/in or kg/mm, same as wheel rate. Rigidity is measured like you guessed, pin down three corners, apply a force to the fourth corner, and measure deflection. A car with a stiffer suspension would need a stiffer chassis to properly distribute roll torque. I know stiffer is better. But, stiffer is also heavier, more expensive, more time consuming, and more restricitive to the design. If I knew that street cars typically have a chassis rigidity of 1% of wheel rate, then I'd use that number as a target for my kart chassis. If the ratio is more like 1:1000, then I'm in trouble :) BTW, what is FFS? Thanks, Pat > Byron, glad you remember me. I haven't been doing much in the sim > world lately. I'm hoping rfactor will change that :) I'm racing karts [quoted text clipped - 26 lines] > wheel rate, then I'd use that number as a target for my kart chassis. > If the ratio is more like 1:1000, then I'm in trouble :) So in the case of this kart of yours, this "centre section" is like the chassis and the tubing is like suspension. Or is there seperate suspension? How do you make adjustments? ie oversteer/understeer? > BTW, what is FFS? For frigs sake! :) Yes, the center section is the equivalent of the rigid car chassis, and the tubing at the rear and front is the suspension. It operates like any other kart, and will use mostly the same adjustments use on other kart chassis. Weight percentages mostly as this one is designed for ovals. Caster, camber, and front ride height is also fully adjustable. This chassis is different in that it has some provision for adjustment of its overall stiffness. The roll couple distribution, which is already radically different compared to other kart chassis', can also be adjusted. Pat Dotson www.kartcalc.com > Yes, the center section is the equivalent of the rigid car chassis, and > the tubing at the rear and front is the suspension. [quoted text clipped - 9 lines] > Pat Dotson > www.kartcalc.com To just guess, I'd say if the chassis is already pretty stiff then the most important thing would be the stiffness of the front tubing compared to the rear so as to get a neutral handling kart. But I guess that's pretty obvious. I suppose what you're asking is how stiff (thick or long/short) should the tubing be relative to the chassis? I'm betting trial and error for both general stiffness and handling balance will be required. Even in sims, you can be as theoretical about it as you like, but simply making adjustments and trying them out is all that counts ultimately. So is this kart a kit? Any instructions/recommendations? How much lattitude is there to stiffen chassis. What sort of tubing options? I imagine that wheel/tyre type is a big part of all this too. My idea would be to keep it all as tight as possible up to just below the point where boucyness = poor grip. Sorry for the rambling and lack of a particular answer - I haven't thought too much on this issue b4. :) I wonder if chassis flex is moddeled in sims so far? Byron, Right, the front/rear relative stiffness is the key to getting balanced handling. That's the easy part. I'll be able to get whatever I build to handle well by adjusting weight percentages, etc. If by "is it a kit?", you mean did I buy plans and materials from someone, then no. I designed the chassis, bought the tubing, then cut, notched, and welded it together. Seems like I once noticed a "chassis rigidity" variable buried in ISI's F12XXX .ini files. Although, maybe I just dreamed that... Maybe there is, and I can just compare that value to the wheel rate in the INI file. Thanks for the idea! I'll check it out. > Seems like I once noticed a "chassis rigidity" variable buried in ISI's > F12XXX .ini files. Although, maybe I just dreamed that... Maybe there > is, and I can just compare that value to the wheel rate in the INI > file. Thanks for the idea! I'll check it out. LOL - even though this was not what I meant to suggest, I'll gladly take the credit for it! :) Here is an interesting discussions with a picture of one test rig: http://fsae.com/eve/ubb.x/a/tpc/f/442600868/m/89010136711/r/26610330521 Pat Stock Car Racing Magazine did a series by Sleepy Gomez (apologize if spelled wrong) that covered this in a neat and clear way. Was this spring I think. Either way Mr. Gomez is, IMHO, a set-up genius/guru and his test rig would really work for this cart test. Glenn > Byron, > [quoted text clipped - 10 lines] > is, and I can just compare that value to the wheel rate in the INI > file. Thanks for the idea! I'll check it out. > Stock Car Racing Magazine did a series by Sleepy Gomez (apologize if > spelled wrong) that covered this in a neat and clear way. Was this spring > I think. Either way Mr. Gomez is, IMHO, a set-up genius/guru and his test > rig would really work for this cart test. "Was this spring I think". Had to think about that for a sec! :) lol...i had to too after re-read... >> Stock Car Racing Magazine did a series by Sleepy Gomez (apologize if >> spelled wrong) that covered this in a neat and clear way. Was this spring [quoted text clipped - 4 lines] > > Had to think about that for a sec! :) > Byron, glad you remember me. I haven't been doing much in the sim > world lately. I'm hoping rfactor will change that :) I'm racing karts [quoted text clipped - 31 lines] > Thanks, > Pat I don't think comparing kart and cars chassis is a good idea. Cars have suspensions, karts don't. > I don't think comparing kart and cars chassis is a good idea. Cars have > suspensions, karts don't. I would tend to agree with this statement. A Kart uses a SOLID rear axle... this requires that the chassis flexs enough to allow the rear inside wheel to lift (leave the ground) so as to allow it to turn more rapidly, since the outside wheel has to turn more times than the inside wheel to make a corner. Without the chassis flexing enough to let the inside wheel (lift) turn faster.... your turning radius is going to grow. It will feel like serious understeer. My 2 cents Cheers ==--== If someone can actually answer this, they should be making WAY too much money to be roaming around in RAS :) -Larry > What is an acceptable ratio of chassis rigidity compared to wheel rate? First of all, every kart DOES have a suspension. The suspension is the chassis itself, which absorbs bumps and helps keep the tires on the track. The difference between a kart and a car is that there is no rigid structure separating each of the four corners of the chassis Secondly, there are lots of race cars that use a solid or locked rear axle that don't rely on lifting/unloading the inside rear tire to turn. The chassis that I am building at the moment is for oval racing. Rear stagger is used on ovals to allow the solid rear axle to roll around the corner WITHOUT lifting the inside rear wheel. Lifting the rear wheel on an oval is usually a bad thing, and a ticket to the back of the pack. When I build a sprint chassis, it will be designed to lift the inside rear wheel. The oval chassis won't lift the left rear. ...so I guess no one has the answer :) Pat |
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