pushrod/bellcrank question

[Sean in CT]

First of all, this is for my Amod, not a FSAE team.
I am unable to package my front bellcranks/coilovers in a manner to get full range of motion with the correct motion ratio. I have used long control arms with a narrow front chassis (UCAs 10" apart, LCAs 6" apart) and I am hoping to have long suspension travel available. My push rods are probably only 25* from horizontal, requiring a 2.6:1 bell crank to get a 1" wheel travel to .75" shock travel ratio. I am thinking of adding a puchrod where the coilover would go, and placing the coilovers up by my thighs, maybe with a linkage to multiply the ratio there along with a 1:1 bellcrank. My wheelbase is much longer than FSAE (90") so there is plenty of room above my thighs. Have any teams done something like this. It seems that as long as it is rigid there should be no problems...

Thank you,
Sean

[Brian Evans]

Why do you want a higher motion ratio? What is the benefit in your application? Historically, motion ratio's near 1:1 came about because shocks had a hard time damping the very low motion's/velocities with low ratios when springs got very stiff and suspension travel got very small. You want big suspension travel, as I recall, with a quite light car. I still recommend a MR in the .5 range for your application.

Back of the envelope calc says that if your sprung weight on the front corner is 150 lbs, and you want 6" of travel with a 3" ride height, then front wheel rate is going to be about 50 lbs. Spring rate would be 50 lb/inch with 1:1, 88 lb/in with .75 MR, and 200 lb/in with .5 MR. It's just so much easier to buy shocks and springs if your spring rates are in the 200 lb range than in the 50 lb range...

Brian

Brian

[Sean in CT]

Brian, thank you for your input.
I am guessing the front corner weights will be closer to 200. I am thinking of a motion ration closer to 0.75 with a wheel rate of .75...therefore a 200lb corner weight will require a 200 lb spring. I am planning on using bilstein 7" shocks. Can they dampen sufficiently with a .5 motion ratio? I think even with a .5 motion ratio, i will still have packaging issues, due to the angle on the pushrod. I actually am hoping to have 9" travel available, but a little less is OK. I expect to only use 1/2 the available travel 90% of the time.
If my bellcrank has a 3" arm (on the coilover side), 180* motion (which is impossible without binding) will only give 6"...4.5 to 5 is more likely) I can not make the arms much longer than 3" without having the bellcranks outside the chassis or the coilovers touching one another.
If my idea is faulty, i might be able to use an outboard coilover/bellcrank (like the Stohr cars, etc). But, If I use the double pushrod idea, i can get the motion ratio exact and it would be easier to adjust

[Brian Evans]

Don't forget that the corner weight you are working with is the sprung weight. The entire weight of the wheels, tires, brakes, uprights, and a proportion of the suspensions members is UNsprung weight. The other thing not to forget is that usable suspension travel requires that the spring be under load. If the spring rattles on the shock, it's not supporting the car so it's not suspension travel.

I still think you are going to have a really hard time finding springs that are soft enough to do this. The soft springs will be very long - I have some that are 100 lbs/in, and they are 12" long, and they will coil bind at about 8".

I honestly think there is no justification for a high MR.

Here are some numbers: unsprung corner weight will be at least 40 lbs on your car (that's what it is on a FFord, which is about a 930 lb car). 160 lbs sprung weight. 200 lb spring at .75 MR is 112.5 lb/in wheel rate (don't forget it's MR squared). Static bump from sprung weight load will compress the suspension 1.42", so the maximum droop travel you can have before the springs rattle is 1.42". 9" less 1.42" is 7.58". To get 7.58" of bump will require 853 lbs of load, which is 5.33 times your sprung weight, which seems excessive. You'd have to run that much ground clearance to get that much bump travel, obviously. Your spring would have traveled .75 times 9", or 6.75", to get a spring that can do that without coil binding you will probably need a spring that is 20" long open (which does make it easier to get soft springs).

To get more static droop, you need a softer wheel rate. You can obviously do that with MR or spring, but the big advantage of MR is that you don't need such long springs and shocks. If you set up more static droop, you can run a lower ride height, and you don't need to accomodate so much bump travel, so everything can get proportionatly smaller.

I think the failure in your thinking is the need to have such great wheel travel with such a light car. You said you have rough pavement to deal with, but I would run the numbers based on a 3" ride height, 3" of bump travel and 2" of droop. My car is about the same weight as yours, it's a road race car but often hits curbs (FIA style, not street curbs). I run rates that give me 3/4" of droop and 1.75" of bump travel. Works great!

Brian

[Sean in CT]

Now i understand better....basically the advantages of running a .5 MR (opposed to .75) out weigh the disadvantages. Some loss of low speed damping will be compensated for by greater piston travel and proper valving.
I already have the front crossmember fabricated for 4" ride height, so i am not changing that, but I could settle for less overall travel....I roginally picked the 9" because bilstein makes a 9" shock. My control arms set up allows about 11" travel before the ball joints bind, so i figuered the 9" seemed reasonable. I hadnt considered the length of spring needed, but it seems by using teh .5 MR that you suggested, I should be able to come up with "good" suspension travel.
I wish i had pics of some of theses surfaces we run on. Off camber, dips, waves, etc. Usually most of the course is OK with just 1 or 2 areas that require more travel...of course tip-toeing through 1 or 2 turns kills the times...hence the rational for this car.

Years ago I ran a formula 440 based car with a GSXR1100, independant rear, LSD etc, and last year I ran a Legends car. Both these cars had short wheel bases (72,73) and relatively stiff suspension with limited travel. They were both excellent on smooth surfaces, but got very unstable over rough surfaces, with occasional episodes of going airborn (esp. the formula car)
I want my new car to be more foregiving because I dont get out autocrossing too much (family, business, etc) and I like to bring a friend along who ususally doesnt have much autocross or formula car experience.

[Brian Evans]

BTW, Sean, it just occured to me that what you are designing is almost, in many respects, a duplicate of my 1964 Merlyn Mk 6A sports racer...

Tube frame, rear engine, up to about 200 Hp (mine had 145), 1,000 lbs more or less, in it's original 1964 configuration it had 4" ride height, about 7" of total travel, front MR was .5, rear was .65, and it ran 90 lb springs in front, about 80 in the rear. A real old school, soft sprung car in the style of the early 1960's. When I had it, I ran rates about 300% of the original rates. It was a very competitive car in it's day.

http://sports.racer.net/chassis/merlyn/images/brian_eva...allclassic2003-1.jpg (http://sports.racer.net/chassis/merlyn/images/brian_evans/fallclassic2003-1.jpg)

Brian