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Hi Guys,

I've been lurking for a while now and finally have enough of a design to share.

Basically my goal is max performance per dollar. Im a student so my budget is low. Im treating this as a learning exercise trying to do it cheap but cleaver. I'm currently studying mechanical engineering at canterbury university, hence the solidworks and fea modeling(to come).

Bit about the car
honda d15b twin carb 105hp
Mx5 front uprights
5th element mountain bike shocks actuated by push-rods
these allow a decent motion ratio and full compression and rebound adjustments.
more to come

Think of these model pics as a "virtual mock up" theres still much analysis to come but these shots give you an idea of the direction i'm going.

They make mountain bike shocks that can withstand the forces of a car! Holy cow, I goota see that mountain bike; or better yet, I want to see the rider of said mountain bike.

But welcome anyway. I envy your Solidworks ability.

Tom

Why mountain bike shocks? You can pick up motorcycle shocks for around $30 each, and I think they are much more suited to the job. I'd be worried about a car with mountain bike shocks on it, it seems like it's far above the intended use.

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Start looking for a snowmobile engine ..................

welcome aboard dude!!

i'll be watching your build, the only reason i didn't go honda is wanting to avoid the "ricerocket" preclassification, honda makes some good stuff. funny how social appearances have an influence on design. ..
i'm concerned about the mountain bike shocks though, not that they couldn't handle the load short term but that they would have a lifespan suitable for the application. a car in 4 hours of use gets a lot more suspension cycling than a bicycle i suspect. are they rebuildable? what kind and size are the internal seals? can the seals handle the increase in work being applied? can the valving be ajusted or are they fixed compression and rebound? even with levers in the middle theres still a big difference in mass and volume being applied, while it may not show up in pressure readings it will show up in the suspension cycling rate. i'm not trying to put you off, i just think these are questions that need to be addressed otherwise you'll have issues down the road.(literally ) if it can work then its a great thing for a bec, another few pounds reduction in weight. you'll note my recently updated sig line. ..

I raced downhill mountain bikes for years and believe me these suspension units are built to withstand massive impact loading! Im probably going to run external bump stops to help aswell. They run a 12mm cromo shaft and have 35mm diameter piston. Fully rebuildable with replacable seals etc. The shocks have rebound and compression adjustments. The avalible spring rates range from 275lbs to 550+ lbs. The models im using have 3" stroke with i2i of 9.5". Im going to duct in air over the shock bodies to help with heat build up. I plan on trying to predict my damping ratio using the quarter car dynamics model and then i will build up a custom shim stacks and gettting the shocks dyno'd. Im sceptcal at this stage but well see how it goes.



Im using the honda power plant because parts are cheap and thier motors are pretty strong. I can probably build a b16a turbo(circa 250hp) here in new zealand for less than $2000 nz. Gearbox's are plentiful and dispite the honda haters, as a mechanical engineer i love honda stuff!

oh yea don't worry about the body that was just somthing i wipped up in 2mins to show the general idea.

Thanks
Grant

It's not the impact loading I'm worried about, like oldejack said, mountain bike equipment is designed firstly to be lightweight, and is designed to carry about 200lbs max per shock. Motorcycle shocks are designed for easily double that. While the spring rate is nearly the same, the shaft and piston size on my R1 shocks is about 25% bigger than the figures you quoted, and my mind tells me there must be a reason they do that.

I'm not trying to shoot your idea down, I just don't want to see anyone get hurt.

Where do you get your 200lbs max per shock from?

Remember these bikes have linkages and such that magnify the force seen at the shock. Maybe this will help,
look at it like this. The max force the shock will see is when the spring is fully compressed (assuming the spring doesn't bind due to my external bottom out bumpers, neglecting damping force) now i'm using 350lb/inch springs but these shocks can run as high as 550lbs/inch spring rates.

Im going to switch to metric now
550lbs/inch = 96000 N/m
500lbs/inch = 88000 N/m

My shocks have 3" stroke so total spring compression aprox 0.0762m
spring rate*stroke = max force on spring perch = 7315.2 N
R1 shock has a 500lb/in spring rate, R1 stroke aprox 3"
max spring force 6160 N/m aprox

Make no mistake these damper units are designed for the loads i plan on exerting on them.
Possible reasons why there's a large difference in diameter
- Fatigue life, motorbikes are expected to last for 100000km+ push bikes not so much
- leverage ratios are quite high on motorbikes
- oil volume

Remember im using push rods and rockers i have full control over the leverage ratio the shock sees. The dampers even have
steel bodies(not typical for mountain bike shocks) which with help with reliability.

Thanks for the critique and i can see where your coming from but i'm convinced enough to give it a go!

Thanks
Grant

worst case scenario is a shock fails at an inopportune time. .. not altogether uncontrollable, just riding on the bump stop to the inside as you coast out of the way. good inspections would most likely catch an issue before it happened in a race. i can't speak to the difficulty of modeling it in solidworks but i understand its possible.
(i'll pick on myself here for values) suspension requirements need a 1.5-1 ratio with hard stops so the travel would go down at that rate but the load would multiply by the same rate right? 1500# * 60%(rear weight bias)=900# * 1.5(rocker ratio)=1350# static rear weight so 675# per corner applied to the coil spring. will the spring plates hold up? i wonder more about the seals being capable of handling the pressure, the impact loads would be subject to the same math and that gets beyond my scope of knowledge real fast if i can't find a premade spreadsheet. heck even if it doesn't hold up the conversion to motorbike shocks will be easy enough and you'll know how and why it didn't meet your desired results. "cured the disease, killed the patient"

675# is 306.2kg so we can times by gravity 9.81m/s^2 and get a force of aprox 3000 N (I hope no one's offended by metric units) which is well under the max i've shown above. Now im predicting rear corner weight of 180kg so again all good. like you say if they do fail i can always convert to motorbike shocks. Thanks again for taking the time to think about my design.

Some more specifications

Wheelbase: 2655mm
Front track: 1680mm
Rear track: 1650mm
Using 15" wheels. The car will run full slicks
im using mx5 front uprights and steering rack.

Thanks
Grant

MY mistake, forgotten that pedalbikes had rocker arms as well. All I meant is that the typical rear shock in a pedal bike is expected to carry less than 100kgs, since the weight of the rider and the bike are usually close to that, and the weight is not 100% on the rear wheels. A motorbike is more like 300kgs with a rider, again, rear biased but not completely on the rear.

Either way, you seem to have done the math, and if you're comfortable with it, go for it. Out of curiosity, how much do your shocks weigh, and how much do they cost? Are different spring rates easy to get, and reasonably priced?

i'm shooting in the dark here but wouldn't the gravity quotient get multiplied by the rocker arm ratio as well? like i said earlier this conversation left my skillset some time ago. .. i'm just trying to wrap a new bit of info into my awareness.
the rocker requires "the acceleration rate (divided) and the mass weight (multiplied)"doesn't it? in other words, over the same "time" the distance traveled reduces and the weight applied increases by the same (rocker) ratio doesn't it? i'm only on my 3rd cuppa joe here, my brain isn't on yet.

edit>
for some reason it seems to me that its ((ratio)squared) not (ratio). .. been a long time but i'm functioning a bit now

Most of the formula student use shocks from a MTB. Most formula student cars weight about 200kg without the driver. Obviously the springs are more stiffer than on the MTB.

Most MB shocks are also close to being maxed out on FSAE cars.

Itsu, have you spoken with your damper manufacturer directly? Some make dampers marketed specifically for FSAE cars and will valve them accordingly - you will probably need something similar.

Before deciding on a brand, I would make sure that the damper can handle the damping requirements especially since your low motion ratio is going to create very low shaft speeds - it may be difficult if not impossible to get the numbers you need.

I've drawn up a quick Free body diagram for you. Might help. This is for one instant in time. You really need to use a maths program to iterate for all values of theata as the car cycles through its travel to see how the force on the shock changes



Grant