How many wheels for a rolling roof?

[Astrokev]

Hi All

Obsy walls are finished and I'm getting close to being able to make my rolling roof. I've bought FH Brundle V track and 4 wheels of 250kg load capacity. The total load will not be a problem for 4 wheels, but I'm wondering whether this is enough to prevent the roof beams from sagging over time between the wheels underneath the roof apex.

The timber I plan to use for the rolling roof frame beams will be 45x70mm. Will this be strong enough, or should I add 2 more wheels as shown in the SketchUp drawing, which is to scale.

Many thanks

Kev

 

 

[Ibbo!]

I am by no means qualified but I would go for the 3 wheel/side option, for no reason except it does look the way to it.

Ps thats the way I did it on my last roll off roof obs and it it lasted about 8 years before we moved and was still good to go.

 

[paul mc c]

I think 4 would be ok,but I would set them centre point between the middle and edge of roof at approx 597mm to lessen any saging.

[mikeDnight]

I'd go for the four wheel option, as I have done for my own ROR,  but I'd bring them slightly closer together. My roof has been operational since 2011 and there's no sag of the timbers. Bringing the wheels closer together allows you to have a supporting H frame closer to the observatory, where a 6 wheel needs a longer track to stop the outer wheels from coming off the frame. With 4 wheels the roof can extend beyond the H frame while the wheels remain on the shorter track. My roof is 8 feet in length and glides with a finger tip push.

[pete_l]

Wheels are cheap and easy to fix during the build. They would be difficult to add at a later time. For the sake of a few £££, I'd add another one under the central upright.

[saac]

7 hours ago, pete_l said:

Wheels are cheap and easy to fix during the build. They would be difficult to add at a later time. For the sake of a few £££, I'd add another one under the central upright.

I'd agree with pete above - from a load and function perspective 4 woudl suffice but if you are using softwood for the roof timbers then sagging at that centre span may later come back to niggle you. For the sacke of two more wheels I would add them now.  It will also give you good support if you intend to use the trailer type lock down levers to secure the roof  For what it is worth I have six wheels on mine  but I have an octagonal roof - the extra two are on the centre beam which is then removed when the roof is retracted.  Brundle is an excellent choice of supplier - really reliable and very good customer service.  

 

Jim

[Astrokev]

1 hour ago, saac said:

I'd agree with pete above - from a load and function perspective 4 woudl suffice but if you are using softwood for the roof timbers then sagging at that centre span may later come back to niggle you. For the sacke of two more wheels I would add them now.  It will also give you good support if you intend to use the trailer type lock down levers to secure the roof  For what it is worth I have six wheels on mine  but I have an octagonal roof - the extra two are on the centre beam which is then removed when the roof is retracted.  Brundle is an excellent choice of supplier - really reliable and very good customer service.  

 

Jim

Thanks Jim. Yes, really impressed with Brundle. They deleivered within 24 hours of ordering!

Thanks to all folks who've replied. being a bit risk averse, as said, for the sake of a few more pounds I think I'll go for 6 wheels for peace of mind.

[Astrokev]

15 hours ago, mikeDnight said:

I'd go for the four wheel option, as I have done for my own ROR,  but I'd bring them slightly closer together. My roof has been operational since 2011 and there's no sag of the timbers. Bringing the wheels closer together allows you to have a supporting H frame closer to the observatory, where a 6 wheel needs a longer track to stop the outer wheels from coming off the frame. With 4 wheels the roof can extend beyond the H frame while the wheels remain on the shorter track. My roof is 8 feet in length and glides with a finger tip push.

I've decided on 6 wheels, but you make a good point Mike about bringing the wheels in a bit. This will allow be o keep the rails as short as possible :-)

[Rusted]

You do get a "jettying" effect from bringing the wheels inboard.

The overhang beyond the wheels helps to counterbalance the mass between the wheels.

I'd still favour six wheels. Larger wheel diameter helps to reduce drag, regardless of load capacity.

 

[Bizibilder]

I would also suggest the six wheel layout.  That said, whether using four or six, if the structure is rigid only three of them will ever be taking the load!

[Astrokev]

58 minutes ago, Bizibilder said:

I would also suggest the six wheel layout.  That said, whether using four or six, if the structure is rigid only three of them will ever be taking the load!

Thanks Roger, a valid point.

The main consideration behind my original question was not cost but how easy it will be to align 3 wheels on a side (and I guess the extra effort of installing 2 extra wheels).

Since I'm using V groove wheels and track, the alignment will have to be pretty good to ensure there is no binding of one of the wheels, or a tendancy for one to 'ride up' the track. I only have simple hand tools so I'll have to be accurate with the drilling of the wheel mounting holes. For the wheels I've chosen, the wheel housing will be mounted underneath the wooden structure and will bear the roof weight vertically (see pic). I may try and create slight slots rather than circular holes (at 90' to the rails) to give a small amount of lateral adjustment, at least for one of the wheels. This will hopefully do the trick.

Actually, thinking some more on this, maybe I could leave the centre wheel mounting bolts slightly loose, to allow the wheel housing to 'auto-align'.

Also, since we build these things out of wood, there's likely to be some warping and movement of the structure over time. This approach would also help allow for this.

[Bizibilder]

It appears you intend to bolt the wheels in place - this will allow you to align things before tightening as you suggest.  I would also recommend using either a drilled plate or very large washer(s) on top of your timbers so that tightening the bolts does not simply crush the timber rails.  I wouldn't leave things loose as any slop will just encourage wear in the timber and could lead to embarassment in the future!  

[Astrokev]

Yeah, I think using bolts will allow me to adjust the alignment from the top, whereas using screws would mean I couldn't adjust and tighten these with the wheels and rails in-situ on the track.

Sensible suggestion about the plate / washer! Thanks Roger

[Gina]

I had six wheels on my roof at first but found only four were actually taking the load so when I upgraded to FHBrundle wheels and track I went for four - inset from the ends a bit as mentioned above.  My support for the wheels is a pair (one each side of the wheels) of 100mm x 25mm timber with the 100mm the height.  This makes a nice stiff frame.  My roof was probably well over-engineered ?