Building - Using wood to extend short boom

Print Page | Close Window

Using wood to extend short boom

Printed From: BHPC Forum
Category: Public: Open to anyone
Forum Name: Building
Forum Description: Anything to do with building HPVs
URL: https://forum.bhpc.org.uk/forum_posts.asp?TID=7626
Printed Date: 27 March 2026 at 4:34am
Software Version: Web Wiz Forums 12.07 - https://www.webwizforums.com

Topic: Using wood to extend short boom

Posted By: NigelSlee
Subject: Using wood to extend short boom
Date Posted: 28 January 2024 at 11:35am

When I bought my HPV Grasshopper I didn't realise the boom had been cut back. After a few years of discomfort I had to do something. I only needed an extra half inch of leg length so figured that I could get away with a less than perfect extension as the load wouldn't be too great.

So last winter I came up with a solution. For the extension I used some hang glider keel tube of similar diameter. I had a piece of square profile wood which I whittled down to fit. I fixed the wood into the extension with some epoxy glue and the boom fitted tight with no glue. Last month I gave it an inspection and found that he wood in the boom was now a bit loose so now glued into place like the extension. 

This mod has made a big difference to my enjoyment using the bike and saved me from having to buy an expensive new boom. It also marked a big step up in my bike-modding skills. Wood whittling now added to my use of cable ties and gaffa tape to fix things!

Replies:

Posted By: Yanto
Date Posted: 09 February 2024 at 6:52am

only problem I can foresee is contraction and expansion due to change in humidity, e.g. on a hot dry summer day, that rare occurrence LOL

it will come loose despite the epoxy, time will tell.

Posted By: NigelSlee
Date Posted: 11 March 2024 at 12:32pm

Yanto wrote:

only problem I can foresee is contraction and expansion due to change in humidity, e.g. on a hot dry summer day, that rare occurrence LOL

it will come loose despite the epoxy, time will tell.

Yes that happened! First fit lasted 12 months before coming loose with contraction of wood. Hoping I can get another year out of it since recent refit. Meanwhile on the look out for a suitable boom if one comes up for sale.

Posted By: Kim
Date Posted: 11 March 2024 at 1:35pm

What's the diameter and how much length do you need? I have the boom from my dead Streetmachine GT (the older version) going free to a good home, which has been cut down by about 1".

External diameter of about 49mm, something like 400mm to the back of the derailleur post.

Posted By: NigelSlee
Date Posted: 11 March 2024 at 2:22pm

Kim wrote:

What's the diameter and how much length do you need? I have the boom from my dead Streetmachine GT (the older version) going free to a good home, which has been cut down by about 1".

External diameter of about 49mm, something like 400mm to the back of the derailleur post.

Amazing! I'll check dimensions later today and get back to you ASAP.

Posted By: zoxed
Date Posted: 12 March 2024 at 12:04pm

Or use shorter cranks: if you go from 170 to 140 you can move the pedals in 3cm closer!

Posted By: NigelSlee
Date Posted: 15 March 2024 at 5:42pm

NigelSlee wrote:

Kim wrote:

What's the diameter and how much length do you need? I have the boom from my dead Streetmachine GT (the older version) going free to a good home, which has been cut down by about 1".

External diameter of about 49mm, something like 400mm to the back of the derailleur post.

Amazing! I'll check dimensions later today and get back to you ASAP.

Hi Kim - yes please, 400mm should just do it. Can't find my callipers so perhaps both bikes have same diameter. If you could bring to a race that would be amazing. Thanks!

Posted By: NigelSlee
Date Posted: 15 March 2024 at 5:47pm

zoxed wrote:

Or use shorter cranks: if you go from 170 to 140 you can move the pedals in 3cm closer!

Yes I've thought about that. I'd need a new crankset and rings, not sure where to find that or if I want to invest any more money into this machine right now as it does the job. I'd certainly investigate if someone pointed me in the right direction for buying shorter cranks crankset.

Posted By: Kim
Date Posted: 15 March 2024 at 8:59pm

NigelSlee wrote:

Hi Kim - yes please, 400mm should just do it. Can't find my callipers so perhaps both bikes have same diameter. If you could bring to a race that would be amazing. Thanks!

Okay, I'll bring it to Hull.

My Shiny! New! SMGTe has a considerably larger boom diameter, presumably this changed with the SMGTe switching to aluminium for the main frame. (I assume the new boom is shared with other current models.)

Posted By: zoxed
Date Posted: 16 March 2024 at 5:42am

NigelSlee wrote:

zoxed wrote:

Or use shorter cranks: if you go from 170 to 140 you can move the pedals in 3cm closer!

Single chainring is easy as plenty of BMX cranks, you *may* find some that take 2 chainrings. For triple SJSC used to stock 140+ but they are not listed now. 155 are more common (TA, Miche, and some recumbent makers offer them).