I’m currently working on a race car roof that in order to get the rigidity I need I’m going to hafta use a balsa core. I can post pics if needed. The roof is like most cars it’s has a 1/2 rib running from front to rear helping gain some rigidity however the customer wanted the middle to be flat the area of the flatness is roughly 3’x4’ I have tried foam strips running front to rear in the middle and also an X out if foam strips but they do not seem to stiffen the laminate as much as needed. Another factor I have is weight. They want to keep them in the 6-7 lb range. My current layup is gel, 1.5 mat 1/4x 3/4 foam core strips in and x and 6oz cloth comes in at 7.6lbs and sitting on a table if I pick any corner up it flexes about 6 inches before lifting the rest of the roof. I have no experience with a balsa core. I have 1/4" 2x4 sheets I was planning to cut to size. Could anyone lend advice to how to use them in a layup. I plan to drop to 3/4 mat under the balsa. And 6oz on the other side. I prefer to not have to bag the part to keep cost down but I don’t want any issues with the balsa not bonding. Thankyou I’m advance
I see a few initial issues for this project to have a satisfactory outcome.
1)-Removing the body lines from the OEM roof which was for strength in a metal skin requires you engineer structure elsewhere. In this case , below the surface from view. Perhaps the customer could be persuaded to keep the body lines?
2)-Using fiberglass, let alone non structural mat/CSM etc. is a poor choice for a roof in my humble opinion. Consider Carbon in various weaves including Uni-directionals. If done well, you may be able to avoid a core altogether.
3)- Balsa or any core for that matter work best strength wise, when encapsulated equally in your lay-up schedule . Example> 3 plies of 2x2 twill cf at opposing weaves------core----3 plies 2x2 twill cf.
4)-Without pix of the part, it makes for a difficult assist on this end. Blind leading the blind.
Another thing that pops my mind: You could opt for a bonded substructure. Just like a normal hood / bonnet of a car.
6-7 lbs is not much. I would crank my vacuum pump for that, but I understand your concern for cost. Now your trick is to skip so much structural stuff, that you get within cost, but stay within structural demands. Can be hard at times.
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I know going to other fabric and resin I can do under the target weight but to keep in the price range they want I’m limited to the materials to use.example i could cut possibly 1 lb not using gelcoat but then theirs the added cost of the painting
Sounds like you didn’t quote high enough to make this project work profitably.
I would say don’t use balsa and just use proper foam core across the whole bonnet.
The foam is the lightest part of the whole job. Foam is also half the weight of balsa.
This will give you the stiffness but you may need to reconsider the laminate if that is too heavy.
I considered foam cores but the drill and bolt thru the roofs to mount them and each car has different locations for mounting I’m conserned it will crush with foam the gelcoat is probly the most added weight to the part it takes about 2 1/2lbs to cover it and I was also using 1.5 mat which was about 2lbs of resin. I’m going to try going to either 6oz against the gel or 3.4oz mat against the gel. It takes 1.5 yds to cover the part. I have 1/4 balsa that will be another 1.5lbs added before primed. I’l think I will be close to what I’m looking for if I spray the gel as thin as I can and cutting the resin weight out of the mat. I would like to find some 1/8 balsa with scrib on it to save mabe 1/2lb more but have not located any yet.
When applying gelcoat thin: get the temperature up, catalyse “hot”. All to prevent elephant skin
You could get away with a bit less gelcoat and you should definitely use the lighter 3/4oz 225gm CSM for the skin.
Where there are bolts you could consider adding a localised and thicker solid fibreglass but I know this is different for each part. You just need to make them to order so the reinforcing is right??
I was able to cut 1 lb off of the gelcoat with a layup of 3/4oz csm,6oz cloth 1/4" balsa. 6oz cloth. The part now has the rigidity I am looking for and weighs 8.2lbs. I would like to see it in the low 7s I’m thinking I will drop to a 4oz cloth after the 3/4oz csm to save a little and keep the 6oz on too the balsa.
Thanks to all that’s replied.
What density balsa are you using? If it is standard 150 kg/m^3 you could switch to a lower density balsa 100 kg/m^3 or a foam like Airex C70.55 and save a few pounds.
I’m using sb.50 density. I’m going to do another with the 4oz cloth. And one more using veil mat I place of the 3/4