removing an ISP topper and putting cradle on top of mast

Discuss light weight issues concerning road bikes & parts.
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Posts: 354
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Location: Hull

by brearley

Hi Guys,

I currently have a Dengfu FM029 ISP frame.
I'm aware the ISP topper that comes with it is very heavy.
Currently my Seatpost is probably 5mm too high as I never got round to sorting it and never really had a problem.
My latest thinking is to cut a semi-circular groove into the top of the seatmast, drill through the mast itself then put a bit of steel rod through with two holes and attach two lightweight yokes.
Current weight for ISP topper is somewhere around 180g, I imagine this will weigh somewhere around 20-30g so a saving of 150g which isn't to be sniffed at.
Any problems with doing this? how much saddle height would I lose if any doing this?
The setback isn't an issue as i'm in the middle of the rails at 25mm setback so with zero setback i can get the same saddle position.
I have some spare carbon post to create the lower cradle so would only need a few bolts, I'd use stainless steel first.
Alternatively how easy is it to chop the ISP completely and fit a standard seatpost? I know I'd need a dremel probably.
Any thoughts or ideas would be greatly appreciated.

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Joined: Sun Jun 12, 2005 10:56 am
Location: New Zealand

by theremery

Here is how I did mine. Many years later it is still going strong and I hate to think how many K's I have done on it since building it this way (20,000 km?? more??)

Updated: Racing again! Thought this was unlikely! Eventually, I may even have a decent race!
Edit: 2015: darn near won the best South Island series (got second in age
-group)..woo hoo Racy Theremery is back!!

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Location: Hull

by brearley

that is the idea I was having, however the seat mast is 37mm od so I wouldn't be able to put a spacer on it.
for first go my process would be as follows.
1, file a U shape cut out on top of seat mast making sure it's at 90 degrees to the top tube
2, drill horizontal hole through seat mast (not sure what diameter hole as of yet)
3, insert steel tube into drilled hole and epoxy in place
4, insert steel rod and all parts of yoke

for first build I will use a Thomson lower cradle as I know of its integrity.
Later on I will make a lower cradle from cut section of steerer tube as this will be much stronger than cut section of seat mast and lighter too.
This way would cut the need for epoxy and the spacer so around 10g lighter than your setup.
If I'm missing anything please let me know.
If it all goes wrong I can cut it again and put spacers back in and the topper back on.

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by connedale

I would epoxy a section of cut seat post into it. Your saddle is going to be cantilevering your butt. A significant amount of stress is going to be placed on the front hole. The hole is going to want to split toward the top. Another tip is to chamfer the inside downward edge of the reinforcement to reduce a sudden change in cross section.

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Location: Hull

by brearley

OK, I've got an old carbon seatpost spare anyway which will fit.
It's a gloss carbon so will need buffing up and the lacquer removing.
Any ideas how far from the top the through rod should be? Obviously the further away from the top the larger the carbon reinforcement will need to be and the greater the weight.
I have costed this mod as £8 for a pair of ti M6 bolts (40mm + 45mm) and £16 for KCNC yokes so overall £24. I have a small amount of araldite for epoxy. will this work ok as epoxy for this mod?
With everything tightened into place there shouldnt be any area to move if everything is to tolerance anyway.
for stack height issues I could make different height lower cradles to compensate for any height issues.
This mod won't happen until I sort out my training bike as I'm commuting on it at the moment.
I'll then need to strip the bike completely to be able to use all necessary tools (pillar drill etc...)
this is the bike viewtopic.php?f=10&t=116192

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Location: Barcelona

by monchito

Hi brearley

I had the same problem with the ISP of my addict. And this is how i solved:

Sillin por monchito3000, en Flickr

I know that you would need special machines to do the same, but i showed this, ´cos as you, my first idea was to cut a semi-circular groove and drill through the mast. But i realice that all the stress will go to the hole and you will need to put an external ring as backing, and i think that don´t look nice.

So the solution for me was to do an inner cone system in order to have more support surface. I don´t know what machinery you have acces to, but there are some ways to do this system in a less complicate way that i did. (It could be with two semicircular groove, a pair of seatclambs, the cone of an old stem and a tube with the inner diameter of your seatmast and a welded nut inside)

I´m not teaching, just brainstorming.

Good luck with this Project. :up: I really like all what is not just to buy fancy parts. :thumbup:

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by brearley

so this weekend I'm going to try two things.
Firstly I'll take the isp topper off and file a small groove in the top of the seatmast, roughly only so I can place the lower cradle of the seatpost on it then loosely put a saddle on top to measure seat height.
If this doesn't provide enough saddle height I will chop down an old carbon seatpost.
With an angled cut much like a quill stem I can make an expander type seat clamp. I should be able to drop some weight this way and have the ability for height adjustment, this will also reduce requirement to get the process bang on first time.
Obviously weight wise the first option is my favoured approach. as I've said I can afford some height difference through different lower cradle stack heights.
I've not weighed it myself but current ISP topper weighs 165-175g based on weights I've seen online.
For a pair of Ti bolts, carbon spacer and carbon cradles the weight will be <30g
For the expander type seatpost I'll be looking more towards the 100g mark.
I'll let you guys know how I get on.

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by mgrennie

My Woodman IT was 82g, and I saved a gram by going with a Smud cradle. Smud-Carbon now makes carbon toppers as well. I tried making something similar, but my craftsmanship left a lot to be desired.

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by brearley

im not a big fan of buying boutique parts. As much as I'd like to it's still a hobby and I'm saving for other things.
If I can get my internal expander type seatpost to around the woodman range I'll be happy as it'll cost me about £20 if I buy all ti bolts for it.

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