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Author: Subject: J15 Roll bar height?
davetheboat

posted on 13/10/13 at 08:48 PM Reply With Quote
J15 Roll bar height?

Hi fellow J15 builders

After much delay's mainly due to work I am hoping to back into working on my J15
The first thing to sort are the roll bars once they are done I can look at getting the chassis re powder coated.
My J15 chassis was built without a roll bar it did come with a loose full width bar.
But i prefer the look of the seperate hoops. Last year I bought a pair of hoops from Jeremy.

My first question is what height have people got from the top chassis rail to top of the bar? I was thinking of about 15.5in?
I intend the car to be mainly for road use. So its a balance between looks & protection.
The other variable seat's. I am intending to use Jeremys standard seats at least to start with.

Second question position of roll bars across the main chassis rail.
All of the ones I have seen so far are approx 1in in from the side of the chassis. If I put them flush with the edge of the chassis will it cock up the body work? Looks a bit close in the photos I can find.
I am asking this as the hoops I have seem to be a bit wide at the bottom!
They will fit but will virtually touch in the middle & be about 6mm in from the edge of the chassis.

Thanks
David

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theprisioner

posted on 13/10/13 at 10:52 PM Reply With Quote
I originally intended to track my J15. I had a word with the MSA scrutiniser and he advised me to do something about the roll over bar if nothing else. It is about 100mm too low. I spoke to Jeremy who promised to send me a hoop that can be made into a helmet loop but the parts never arrived. The J15 , for me, needs a full roll cage but that should have been thought about much earlier in the build. I have currently given up trying to track the J15 and bought a Megabusa/Westfield to restore over the winter. It will be MSA compliant when I am finished including full roll cage. The J15 just had too many issues for my liking! I am not unhappy with the J15 but it will not be my track car.





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motorcycle_mayhem

posted on 14/10/13 at 07:54 AM Reply With Quote
I have no idea (period), but what I would do (and have done) with all my cars is to simply sit in the thing, helmet on head, and simply ensure that the top of any bar is (at least) 5cm above the helmet.
The bar(s) were then 'created' using pipe insulation, polypipe, wood or anything to hand to ensure that I had something to show the pipe benders.

Caged were my choice for benders, simply due to a T45 fetish, but not locost.

You may not intend to track the car, but you might.... driving on the road is very, very, very boring. If you're not tracking the car, it's next owner might, and he'll be far happier if the bar complies with minimum MSA regs.

I wouldn't use hoops, I'd have a bar. I'd have that bar diagonalised, with backstays (i.e more than theprisoner has on the car on his avatar). If the J15 has the same harness bolts in tension, afixed to a piece of angle, welded onto the top rail (ala Riot), I'd do something about that arrangement as well, but that's me though. I've recently planted the thing into a barrier at Combe, fracturing my sternum (f_me it hurts), tend to look at things a bit harder than most.

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OZDan

posted on 14/10/13 at 01:19 PM Reply With Quote
Hi David,
I am also tinkering with a J15. Mine has a single roll bar with cross brace. There is also removable bracing from the roll bar to the rear chassis. Dimension from top of roll bar to top of "chassis bar" (cross bar at top of seat) is 370 mm (~15" ). The outside edge of the roll bar sits 20 mm in from the outside edge of the frame at the mounting point. Not individual hoops, but hope it helps.
I am fitting seats on runners and expect the top of the runners to be 65 mm above the floor. I have chosen seats from Ultra Shield because there just wasn't much choice for seats less than 390 mm wide (in Western Australia). After some rough measurements, I figured I need a 40deg layback to keep my shoulders below the harness mounts and head below the roll bar, and the Ultra Shield mini cup has this. Unfortunately I have not finished the installation, so can't definitely say this will work, but so far so good.
Apologies, but I can't work out how to upload a picture.
Dan.

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dave

posted on 14/10/13 at 03:18 PM Reply With Quote
I was told the easiest way to check if it is at all suitable is to tie a piece of string to the roll bar and on to the top most chassis cross member at the front, then sit in the car if your head is above the string then the roll bar is not high enough.
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theprisioner

posted on 14/10/13 at 03:42 PM Reply With Quote
That is quite correct, that is what the MSA man did!





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Jenko

posted on 14/10/13 at 03:55 PM Reply With Quote
It is a real issue....I think the twin hoops look good, but if I'm being honest with myself, I will never feel safe doing a track day with them......The main issue of course is the rear clam opening with support struts in place....I had a long hard think of a way around this, and it almost stopped me buying the car, but I keep telling myself whatever I do it will be safer than my old motorbike!.





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nickm

posted on 14/10/13 at 07:55 PM Reply With Quote
Hi
I have twin hoops but never intended it to be anything more than a road car. They do look good but you end up cutting so much GRP out of the central tub in the middle, i would love the rear to be hinged behind the seats but the GRP is especially weak with the cut outs.
The J15 was 2nd at Cadwell this weekend behind a very sorted 6.600 Cobra but im not sure what rollbar Paul has but im pretty sure its not a full cage. Think the Prisoner has a video of it on a track day at Cadwell ?
The J15 in the RGB class has a full cage and the fella Tim Hoverd use to have a blog for another type to look at.
I have Sylvas seats very comfortable and they do "lock you in" with the full pad depth under the thighs it creates a real padded bucket for your a**e and may lower you in relation to the straight line from hoop to front chassis rail but im 5ft7.


Nick M

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davetheboat

posted on 14/10/13 at 08:09 PM Reply With Quote
Right guys some usefull info thanks.

Having had a good think about things and looked at loads of pictures I think I have way forward.
Current thought is to go for bolt on hoops with back stays.

I have seen deecee's car & the back stays are a pain but not impossible deal with.
Since i allready have a loose full width bar I will make this up to suit the mounting holes on each side & also the rear stays.
This should allow for most options in the future.

So I guess what I need to know now is what thickness are the standard seat squabs?

Motorcyclemayhem thanks by coincidence I came up with a figure of 15.5in by sitting in the car with my helmet on whilst sitting on a peice of 2in rigid foam. This gave about 2in clearance between the top of the roll bar & my helmet clad head. So it sounds like its not far out assuming that the seat sqabs are not more than 2in of course.
What is your concern with the harness mounts? I have been told they are a little close together so maybe I should think about them whilst I have the welder out. Sounds painfull hope you heal soon.

Ozdan thanks for the dimension's it helps to know that I am at least in the right area. Not thought about the angle of seat back just leant against the bulkhead! I think the harness mounts should be ok. There are some advantages to not being tall.

Theprisoner I agree its quite easy to make a westfield fully compliant there are lots out there if I had actually got round to building the megabusa I would have fitted a full cage I think the car is more suited to track use.

Dave/theprisoner which part of the chassis is taken as the forward string attachment is the top of the pedal box ok?

Jenko much my thoughts about the hoops but with the addition of rear stays they should be a little more secure. Out of interest what height are your roll bars when you have a minute?

Due to my hoops being wider I suspect I will end up jointing them and welding them together in the middle, in order to keep them about 20mm inside the chassis rails. I dont want to compromise the corner of the body tub by getting them too close to the outside.

My wife also agrees that it would be safer than my motorbike even if there were no roll bars at all!

Many thanks guys
David

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davetheboat

posted on 14/10/13 at 08:23 PM Reply With Quote
Hi Nick M

Good to hear of somebody who is using the sylva seats.
As it happens I am also 5ft7in do you think you have not enough or too much roll bar above your head?
Do your hoops sit slightly forward of the chassis rail? I am intending/hoping to cut mine round the chassis tube by about 1/2 to 3/4 inch in the hope of reducing the amount that needs to be cut away. Not sure it will make enough difference to make it feasable to hinge the tub though?

Thanks
David

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Jenko

posted on 15/10/13 at 07:31 AM Reply With Quote
Dave - I grabbed some pics on my way to work this morning and uploaded to blog:


Hope they are of some use....

Clickster





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loggyboy

posted on 15/10/13 at 07:51 AM Reply With Quote
Use this a good starting point if you dont have/want a full cage.








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davetheboat

posted on 15/10/13 at 09:03 AM Reply With Quote
Thanks Jenko

Exactly what I needed. Thanks for taking the time on your way out this morning.
But I have more question's now.
When you are sat in the car is your head below the roll bar? What's the dimension from the floor of the car to the seat?

Loggyboy thanks looks like I can use the pedal box as the front reference point then.

David

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loggyboy

posted on 15/10/13 at 09:41 AM Reply With Quote
Is a pedal box substantial? Not isure if you need/want to conform to MSA for your planned use of the car, but I would expect a pedal box to squash quite easily? Front of chassis would be the point to give the most sensible 'line of sight'?






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nickm

posted on 15/10/13 at 07:22 PM Reply With Quote
Hi
The roll hoops are im sure vertical to the ground (90 deg) but the chassis rail is square to the angled seat back if you can understand that ? ie the harness mounting points are at 90 deg to the seat back which is sloped.
The hoops sit as forward on that chassis rail as they can with the extra thickness being taken up by a central plate and the triangulated gussets ? my car is elsewhere and im trying to picture it !
The seats are upstairs though, the a**e padding is 60mm before compression with the front being 200mm hence the bucket type effect, my fitting is to the nearest mm as i have trial fitted/checked them.
If you are going to road and track what about leave as is for IVA then add the bracing afterwards as the cockpit could then come into play rather than just the engine area for bracing ?
Remember Jeremy is a good contact to have, and more of his customers probably do their own thing than most other firms !

I went for the original seats as every article said how good they were

Nick M

ps I havent got a motorbike cos i know what im like (so does my missus)

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davetheboat

posted on 16/10/13 at 02:09 PM Reply With Quote
Hi

Loggyboy I don't plan on needing to comply with MSA rules but trying to make sensible decisions for the long term. Pedal box is quite substantial on the J15 but I will certainly bear it in mind.

Nickm Thanks for all the detail. Yes I understand exactly what you mean regarding the position of the roll bar/chassis.

Since I am now intending to go down the route of removable roll bars I can very easily leave any extra bracing until after IVA
I will however weld appropriate brackets onto the chassis prior to having it re powder coated.
I assume you are thinking of a forward facing strut attached to the chassis within the cockpit?

I don't have a motorbike anymore. A, needed the money to complete the car & B My missus did not really approve guess she knows what i'm like!

Thanks Again
David

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theprisioner

posted on 16/10/13 at 02:49 PM Reply With Quote
Now tell me you don't want/need a full roll cage!

http://www.youtube.com/watch?v=xE07N5xrv_8

http://www.youtube.com/watch?v=zYGvL02WA8g

These videos tell me all I need to know





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