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I have just finished a front end inboard conversion on a very old Birkin 7. I know it's not a Locost but it may be of interest to the members on this site. Have a look and give me your opinions.

i see only one week point, the pivot bracket welded to the chassis, this point is carrying the whole weight of the car at the heavy end and it looks like there is only about an inch of weld each side, this point is stressed at double the spring rate.

Good on your for having a go at something different.
Along the lines of what John is saying, it does feed a lot of load into the middle of a tube.
Ok, so there's a small tube triangulating it, but it's in a different direction than the suspension load.
It'll be interesting to see how well it stands up.
Cheers - Gavin.

first off,the weld is in tension, not good.

second, there is not enough weld area to hold the brackets, the weld may not fail but the tube wall will.

third,remove the suspension from the bracket and measure the distance from the bottom of the chassis to the hole for the bell crank, then assemble the suspension and add 50 lbs. to the front end, then set the car on the ground and measure again, if there is less than 1/16" deflection thats o.k. by me, if there is more than a 1/16"deflection, it's bending the tubes and they will continue to bend over time.

the concept is good, but i would like to see the tubes captured by the bracket top and bottom, if you do that, then you can add a support tube down to the lower chassis rail, adding to the existing welds, this could be a bolt in piece.

keep at it, looking good.

Thanks for the input so far. Yes, I had not actually considerd the weight load of the car (640kg with driver) on those two brackets, 320kg each. The brackets are DOM and are very strong as well as being welded on the opposite side but I do hear the point that they are in tension and carrying the weight of the car. The small triangulated tubes were added for extra weld surface area for the rocker brackets and yes they arent doing much to assist in the verticle plane. I need to do the deflection test as suggested by John, the results will be interesting. Possibly I could add a strengthening plate that runs from the top of the rack mounting brackets up to the rocker pivots!

This is the 6th car I have done this mod to, the first one I did was two years ago. They are all race cars and compete roughly once a month in the various classes of our local road series. That first car won the championship last year. As yet, there have been no failures but I am thankfull for the constructive input thus far and will analyse my options based on the recommendations above.

Doing some napkin math on the welds, between the 8 welds, I'd say there's a good 6 inches of weld there. If the effective throat was 0.125", that gives 0.75 sq.in. of weld. At 70ksi weld material, there's nominally a 50,000 lb strength to those all together. With a 320kg (700lb) front end, that ends up being a fairly large multiple, even at large spring travels.

I'd say the weld is fine*--even in tension, which half the welds on the chassis are, anyway. I have no idea on the tube structure, though, except that more triangles are better than less, right?

How was the outboard shock/spring arranged before the inboard conversion?


*This is not advice, or anything to be considered worthy of listening to. Please, no one sue me.

hey Matt i'm calling my lawer now,

thanks for the calculations, as i said above it's not the weld that i'm worying about but the tube wall tearing out, if the tube is 16 guage and the stress is in shear of the tube wall, that is far less area than the weld, your calculations would hold up however if the metal of the bracket and the tube had thickness larger than the weld cross section.

i appologies for my concerns but i have seen this type of suspension fail at the point of attachment due to tube wall failure, both in compression where the bracket digs into the tube and tearout were the bracket pulls off the tube taking the tube wall with it, mostly due to too light construction and lines of force which were not along the axis of the mounting bracket, if it gets out of line for any time at all in the travel, it will break eventually.

these, however are minor points, what is important is the original concept is inovative, we can all improve on someone else"s ideas.

A few things,

1. I would run a tube up from the lower frame rails vertically to meet beneath the brackets. This will strengthen them and allow you to weld that tube to the existing tube as well as the brackets themselves.

2. You suspension may be falling rate. It looks like the pushrods are greater than 90* at rest and would thereofre increase their leverage on the bellcrank as the suspension travels. This may be OK because it looks like you have the opposite going on at the shock so you may end up with a near linear or rising rate suspension. I would run the math on it to ensure. You generally want linear or slightly rising rate.

Otherwise looks nice. let us know how it works when you get it on the road.

Even though a perfect world 1" x 1/4" weld by the book might hold 4 or 5 tons and even though some may scoff at it, I still like the old timers rule of thumb, ie: 1" of weld for 1 ton - it covers all the variables, ie: home hobby welder, inconsistent current, moisture in the air, inferior rods/wire/gas, impure sufaces, rust, substandard materials etc, and the operator himself.

Thanks for all the comments. I have inspected the other cars I have done this mod to and there are no signs of fatigue or cracking as yet. I will come up with a plan just to support the rocker brackets further in the vertical plane. Bear in mind that the above pic shows the car without the radiator and the oil cooler fitted which crowds the area further complicating the location of certain proposed tubes. I drew it all in CAD first and I worked the motion ratio out like that but to be honest I did not account for much in the way of take up once the car was on the floor. I will re-analyse the rocker geometry and maybe come up with something better. I appreciate the input from everyone.

Thanks for taking the time to show your work. I had such a hard time thinking about inboard suspension that I think I decided full body work was the answer if I really wanted the inboard stuff...