Quick question. First, I am building a Altered Bantam hot rod based on late 60's Altered class dragsters. If you look at the image below, you can see the shackles on my front spring is about 45 degrees. I removed two leafs before installing the spring assembly. While I will add a little more weight in the front, I don't see the shackles changing that much. When finished, I will be able to adjust the rear end from about 55% antisquat for street driving to 107% from track days. (Current numbers based on estimates.) At 107%, under acceleration, the rear of the car shouldn't squat. But how does this react to the front of the car, which is setup at a 3% reverse rake? My question is do I want to keep the transverse spring as is, or readd the leafs I removed to stiffen up the front suspension? I have not found much on this subject and any thoughts will be appreciated.
No body here knows? Let me rephrase the question. In a Gasser style suspension setup where the rear end is set up with 100% antisquat, do you want the front springs stuffer with little give, normal for the suspension design or soft with more movement. Currently, I believe my setup on the front spring is normal with the shackles at 45 degrees. Do I leave it as is? Do I add back a leaf or two stiffing the spring assembly? Or do removed a leaf giving the front end more softness?
Traditionally, a ladder bar set up would be used on this type of car, if a true dragster. But ladder bars aren’t so street friendly. So I’m using a 4 link rear suspension mounted on the outside of the frame rail, due the space. With the top bar set at the upper bolt holes on the front mount, I should have around 56% anti squat. The bars are not parallel but the top bar does not have much of a drop. Lowering to the lowest bolt holes, extending the top bar an inch in length, should give just over 100% anti squat. Of course I have estimated certain numbers since the build is maybe 75% complete, weight. The exact amount will be determined once the car is ready to drive.
I would first get all of the weight added to the car. At that point you will be able to decide about spring leaves. You don't want too much spring on the front if it lifts the wheels you don't want a bounce.
I don't think you'll find a direct formula for a good answer. Front spring load and action on something like you're attempting to build is trial and error. Stiff flex is good. You want movement but not too loose. Until you are in total finish build you have no idea what kind of gross weight or corner weight you're working with. A spring weight rating also has little to do with actual spring growth under compression between wheel lift and contact. Multipurpose vehicles are always a challenge.
There’s many on the board that could probably figure things out( not me) for you, I’d suggest changing your title and ask about the information you need to catch some attention. @Kerrynzl comes to mind for suspension. @gimpyshotrods also comes to mind. Just trying to assist.
I'll try to relate my experience in setting up a drag roadster. My experience has been that the most often made mistake it setting up one of these puppies is that the whole car is sprung to stiffly. And it is also my opinion that shocks are set too stiffly, to go with the too-stiff spring rate. Someone once commented that "any suspension can be made to work...if it doesnt." I.E., bind everything up and it will be a rigid unresponsive sled. First let's talk about rear suspensions. Rear suspension tuning revolves around the instant center of the rear axle. In the case of ladder bars, that would be the front pivot point. In the case of 4-link it is the theoretical intersection point of lines drawn through the bars. Making that instant center point higher and further rearward will produce more anti-squat. It will plant the tires harder. It will also make the car prone to wheelstanding. Making the IC further forward and lower - generally speaking below the car's center-of-gravity - will make the car drive out with less weight transfer to the rear and less - pro-squat. As for the front, to answer your question, I vote for keeping it soft. Do not add back more leaves. And use a shock that takes advantage of the softness that is not too stiff. I also recommend a Panard bar on the front. You may need a steering damper too - like from a VW or Fiero. And try to get the rear roll center up high too - maybe with the use of a watts linkage. Good luck wth your project. Post pics.
What sort of steering do you plan on using [Side steer? or Cross steer?] Jack the frame up and see what position the shackles are in on "full droop" Ideally you want the shackles to be totally horizontal at full suspension droop [or you will need a panhard bar] The shackles are the easiest thing to make with different lengths between holes While setting up your car, I would recommend removing all the leaves except the main leaf [and use a steel block as a temporary leaf spacer] Then you can use a "G" clamp [or ratchet tie down] to "dummy" the ride height. Don't overthink "squat" and "antisquat" until you know what the CGH is [in fact many "experts" do not know the difference between squat and "overturning moment"] "Squat/Antisquat" is height change applied at the CG from "Axle Thrust" whereas "overturning moment" is lift at the front and dive at the rear during acceleration [or the opposite with decelleration] aka "weight transfer" They both have similar results [eg: a solid suspension car can have Zero squat, but still blow over] With ladder bars the instant centre is a THRUST LINE from the axle centreline to the front pivot [an imaginary straight line] It does not matter what shape the ladder bar is [they can be "J" shaped to fit around components if they're strong enough] If this thrust line is horizontal then the length dictates whether it is above or below the squat line [shorter ladder bars will create separation in the rear, and longer ladder bars create lift in the front] The shorter bars have a harder "hit" on the tyres, and longer bars will "ride out" further. [this can also be tuned with springs and shocks] An under tyred car wants a lot of overturning moment , so the CGH needs to be high and the front suspension stiffness soft [with 90/10 shocks] Then they usually have a short ladder bars above the squat line to create rear separation to hit the tyres and reasonably stiff springs to harness weight transfer from overturning moment. After the initial "hit" the car needs to get "up on the tyre" @THE FRENCHTOWN FLYER correct me if I'm wrong here ^^^ it has been 40 years since I played around with drag racing [ Turning Left ,Right, and Braking on Road Racing tracks is now my forte ] With Road Racing we still use CGH ,but what drag racers call instant centre [IC] we call the Roll Centre [RC] A car can have a front and rear RC which creates a roll axis. We prefer to have the RC at ground level to promote overturning moment [Bodyroll] then we harness this with suspension stiffness. This converts lateral acceleration into vertical forces on the tyre without increasing mass [The laws of physics still applies to straight line racing the same as cornering]
I don't do much for the straight line stuff. There are not too many straight lines where I live, or have lived. My local drag strip closed a year before I got my license. I was building what was later called pro-touring in the late 1980's
Great info here. I appreciate your responses. I used an online anti squat program to calculate the numbers I stated above based on my current setup and estimate gross weight. What I know about anti squat is basically what I have read online… which unfortunately means conflicting information. My target is a car I can drive around town, to shows but also one I track to a track day and race.
For street driving the fundimental stiffnes of each road spring should be set to give as near as possible the same stiffness to mass ratio, basicaly the same anount of deflection under the static weight of the car. It made a bit more complicated because a leaf spring is non linear but its a good enough as a starting point.
I didn't have time to add everything I wanted to add to my last post. KERRYNZL asked about steering. I'm using a side steer, which is more traditional for a car like this. I looked at a cross steering setup but decided against it because of the revered rake. The front end sits higher than the rear end. This would mean mean hanging the steering box way below the frame rails, which I do not like the look of. Plus, side steer is less prone to issues caused by sway on the front suspension, unlike cross steer. I will have room to add bar if needed, but for now I'm not adding one. I do have a panhard bar on the rear 4 link suspension as well as a small Speedway anti-roll bar. Also, the front shackles were horizontal before I mounted the engine and transmission to the frame, so that shouldn't be an problem. As for shocks, I have read on the Gasser forums that 90/10's are great for the track, not for the street. Some of those guys are using 50/50 or 60/40 on the front end for street. I already have a set of 50/50 shocks for the front I plan to use. Just waiting until the end of the build to mount the upper shock mounts. The rear shock will be coilovers. Again, I have to wait until the car is complete to weigh at the rear tires before buying the correct spring rates for the shocks. THE FRENCHTOWN FLYER, below is a current photo from the side after sitting the body on the frame. I am currently working on expending the firewall 7" for more foot room and to fill in the space between the cowling and the Buick engine. Btw, all of the main parts... engine, transmission, rear end, front suspension, they're all parts which could have been sourced in 1967. 67' Buick 430. Dearborn 3 speed from 65' GTO. Early Bronco 9". 1933 Ford straight axle. Even the Thrust D wheels (though new). I am using the 1967 NHRA manual to set up the car. Other than having two seats instead of one, the car should look like an Altered you might have seen in 67' or 68'. I watched many videos on youtube of both filmed 60's and early 70's drag racing and modern 'vintage' racing uses either original older dragsters or old cars rebuilt to resemble older dragsters. On some, the rear end squats. On many, the rear appears neutral, not dropping. As for the front ends, some do lift off the tract a little. Some way off the track... But many cars appear to keep all four tires planted throughout the run. This is, of course, what I am looking to achieve if I do go to a track day. Being that my car will be street first and track second, that might not be possible. Which is why I am here... Thanks again.
Explain in English please?? Do you mean stiffness to mass ratio is 1:1 so a 100 lb/in spring supports 100 lbs of mass [if so this is wrong] Maybe your wording is wrong. With suspension stiffness we use a "Wheel rate" , so if the spring is mounted directly onto a axle the motion ratio is 1:1 [a 100 lb/in wheel rate requires a 100 lb/in spring ] But if the axle has a 2:1 leverage against the spring aka "Motion Ratio" [eg: an A-Arm, or laid over coilovers] then a a 100 lb/in wheel rate requires a 400 lb/in spring. The wheel rate x Motion Ratio "squared" = spring rate ..... or SR ÷ (MR x MR) = WR to reverse this calculation. 400 lb/in ÷ (2 x 2) = 100 lb/in at the wheel A comfortable wheel rate for a high performance car should be approx 5:1 for it's supported weight [exclude unsprung weight] Most road cars are around 7:1 [which is why spring compressors/ spreaders are needed for installation] A stiff road racing car has a Load Ratio of approx 3.2:1 to 3.5:1 [stiffer on banked tracks] For simplicity I won't include driver weight. If you had a T Bucket that weighed 2000 lbs and had a 53/47 front rear weigh bias [1060 lb front and 940 lb rear] With the rear we would subtract 200 lbs unsprung weight netting 740 lbs sprung weight or 370 lbs each side Going on the 5:1 ratio mentioned above this would require a 74 lb/in spring each side. Noe if we now look at the front 1060 lbs but only subtracted 120 lbs unsprung weight would net 940 lbs sprung weight or 470 lbs each side. The same 5:1 ratio would require a 94 lb/in wheel rate each side to get the same "frequency" Fr and Rr. If the front springs are laid over to 60° from vertical would yeild a 1.33 motion ratio So a 94 lb/in wheel rate would require a 166 lb/in front coil 94 x [1.33 x 1.33] = 166.27 This ^^ is for suspension stiffness and not roll stiffness which is another factor [usually requiring ARB's] Incidendally the 5:1 load would require 5" of lift before the wheels lift off the ground [slightly more allowing for the unsprung weight "stretching" the spring] On coil springs you can limit lift by shock travel [which is why spring compressors are used], with a crossleaf lift can be limited by shackle length [totally horizontal at full droop] @AltBantam With a hairpin front suspension [or split bones] side steer is OK but do not use Cowl Steering. You need to be really careful with bump steer Page 12 on this pdf will help https://www.infrastructure.gov.au/s...tion/bulletin/files/Street_COP_Rev2_Sec_6.pdf cheers Kerry
The static deflection of the front and rear springs, assuming linear spring rate should be the same. If not any bounce would result in a pitching couple which gives undesirable ride characteristics. Cant put it any simpler than that. Its basic fundimental physics.
Ok ! As my first question was "Maybe your wording is wrong" You are referring to the "holy grail" of suspension frequency [cycles per second] which is like comparing a toddler to an adult on the same pogo stick. This is the relationship of Suspension stiffness to Sprung weight to Average Speed is all part of the equation. With suspension fequency you want the front suspension frequency to be slightly less than the rear suspension frequency to allow for the split second time difference for the front and rear to go over the same bump. [this is to avoid pitching] Engineers try to design this for an average cruising speed which is slightly under the open road speed limit. [this is deemed to be the speed that happens most often] With accelleration the speed is constantly changing so this would be impossible to factor. If you drive a stock road car around a road racing circuit at city speeds you would question whether suspension is even needed. Then drive that same road car at full racing speeds [trail braking etc] and the car would pitch all over the place. The front and rear will Yo-yo all over the place. Also push down on the fender of a road race car and you would think it has almost zero suspension [this harshness dissappears when you drive them in anger] The safe bet with pitch control [at multiple speeds] is to have the suspension slightly soft [too absorb bumps] and use good quality shocks [which dampen oscillations] [Off topic] I've played around a lot with suspension frequency and in particular controlling pitching. But this was mainly for trailers [car haulers] I'm probably the only fool to ever test a trailer design on a skid-pad. Usually when a trailer is loaded , the weight on the hitch lowers the frequency of the tow vehicle, and the "patch up remedy" is an equalizer hitch [which is effective at load sharing] The end result , I could tow my race car to the track safely using a FWD Honda accord tow vehicle. The biggest mistake a car builder can make is the wrong choice of springs [and their reluctance to change them]
Kerry, no cowling steering. Most likely a reversed Corsair box mounted on the frame with the pitman arm down. I also checked out your pdf. I have seen those drawings, 4 bar and wishbone with steering link, before. Thanks
Funny you should mention that. Here is a car I did in the late '60s (Car Craft magazine project car) in a couple of iterations - first with a homemade tin body and later with a glass T body. Although I used Corvair coil springs and steering it ran on both street and strip successfully. A couple more observations: Your weight bias will most likely have more than 50% of the car's weight on the rear. Do not rule out having wheelie bars on it for the strip. With today's sticky track prep its easy to stand 'er up on end BTDT. A good way to get a rough idea on the correct spring rate (K factor) is to remove all the shocks off the car. Start pushing down on the rear of the car in a cyclical manor, i.e., every time the car bounces up push down again with another push, much like pushing a child on a swing. The correct spring rate will give a natural frequency of about 60 times a minute - or slightly more for a sporty car like yours. Good luck with your project. P.S. I hate what they did to "my car". Chain wheel: yuk
Ignore all that "spring frequency" bullshit [I've previously written], It is just 2 techno boffins trying to "out product knowledge" each other. Just get the stiffness close enough "for comfort" and you'll be ahead of the game Look at a VW super beetle steering box. They are already "reversed" from the factory. Any Beetle box will do , but the early ones clamp to a tube [which could be welded off the side of the frame rail] here is a LHD model You flip it over so the column is on the underside [and sector is horizontal above] With your 4-link try and get the bars parallel and horizontal to prevent rear roll steer. This is an unpleasant sway you feel in the seat on uneven roads Having them pointing down at the front is roll "understeer" which is better than roll oversteer [roll oversteer needs driver steering correction] The other side effects of having them pointing down at the front is the wheels try to drive upwards instead of "hitting" the tires.[the tyres will break loose] But....In cornering the car will get more "side bite" [which is more desirable for road racing] For a street "boulevard bandit" you should aim for having the Instant Centre [IC] 1" higher than the axle centreline. Then the first 2" of suspension compression cancel out any roll steer. You then need to "Tune" the Squat/Anti squat with IC length, not height This ^^^ height also applies to panhard bars [mount the frame 1" higher than the rear-end so it arcs through a 2" range] On droop there is generally no load on the tyres so the effects of the Arc are a moot point.
"Also push down on the fender of a road race car and you would think it has almost zero suspension [this harshness dissappears when you drive them in anger]" I've experienced this at first hand. My wife curses the non-stock coils on my little OT roller skate every time we need to transport anything even half-way delicate, but it smooths right out at 125mph. Having said that, I'm not sure if damping doesn't throw out Maurice Olley's observations regarding road speed, wheelbase, and respective resonant frequencies entirely. Olley's process began during his years at Rolls-Royce, when damping was generally no more than nominal; and continued while he was at GM, when a practical expectation of even 50% critical damping was unlikely. He would certainly have understood that resonant frequencies become mathematically undefined as damping criticality approaches 100%, meaning that speed and wheelbase become moot — which much is reflected in Colin Chapman's later preference for "inappropriately" soft spring rates, strongly damped. Indeed, as you say, "The safe bet with pitch control [at multiple speeds] is to have the suspension slightly soft [too absorb bumps] and use good quality shocks [which dampen oscillations]" Even the conventional "shock condition test" of pushing down on a corner and counting oscillations presupposes 50% critical damping, though far more is today practically available: even over 100% in some modes. Olley's principle is still useful, though, as the location of a vehicle's pitch centre is defined entirely by the front and rear suspension rates, and the ±20% rearward stiffness bias he calls for will tend to place the pitch centre at or near the driver's body, given a conventional layout, so that the driver will tend to experience pitch as tiny rotations rather than possibly large vertical movements.
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