Double battery problem

[Hightechpete]

I wasn't sure in which section to post this, but following on from an earlier post I thought this as good a place as any.
I use my Ancheer MTB for day to day running around, for rides with friends on the weekend, I use my old Raleigh fitted with a Cyclotricity front wheel kit.

The bike :-

A single battery (11 Ah rear rack mounted) is sufficient for 90% of Sunday ride outs with my buddies.
I've covered 50 miles with 2000ft of climbing on a full charge. I've also run out of power at 44 miles with 3000ft of climbing, so for the longer rides a second battery would give me peace of mind.
I'm just not fit enough (too old ?) to haul 25+ Kg up the Welsh hills!

Last Sunday, a longer ride was planned, so the second battery (7 Ah bottle) was fitted to the downtube, wired up in parallel with the other, following the excellent advice from this board.
A total of 58 miles with 3200ft of climbing through the Brecon Beacons.

Photo below taken by me at the Crai reservoir.

The batteries performed faultlessly, when I got home there were 3 (out of 4) green lights showing on the smaller battery, and 1 (out of 3) on the larger battery, both battery voltages were the same (of course) at 37v.
I'm guessing enough for another 10+ miles?.

You must be wondering where this is going:-

When cruising downhill at 20mph the front end shimmies like crazy with little weight on the bars, it doesn't feel dangerous, but it is a little disconcerting to say the least.

25Kg bike + my 75Kg = 100Kg all up weight.
Maximum speed achieved 41mph - stable as a rock.
28mm tyres adjusted by the book to 100psi rear 60psi front = shimmy, 100psi rear 80psi front = shimmy.
Steering head bearing is adjusted correctly, the front wheel is true and balanced.
Remove downtube battery, no shimmy
Remove rack battery, no shimmy.
Go figure.

This is more of an observation than a question, according to Sheldon Shacket, bike guru, heavy front wheels can make an existing problem worse. What better reason do I need for a new project?.

Pete.

 

[Nealh]

Last Sunday, a longer ride was planned, so the second battery (7 Ah bottle) was fitted to the downtube, wired up in series with the other, following the excellent advice from this board.
Pete.

Hopefully you wired these two in Parallel other wise you would have had 72v and magic smoke from the controller.

 

[Richardab]

Any bike can have a speed wobble, its a natural resonance frequency, more often than not the speed it occurs is faster than normal speeds so rarely felt, but can be thown off by different weight or weight balance. I have a very light bike that does it at high speed, but its rare as i would not often be riding no handed at those speeds.

On a heavier bike my bar bag can also induce it.

If it does happen try holding your leg/thigh/knee against the top tube, it dampens the ‘harmonic motion’. I’m not sure there is a fix unless you redistribute the weight.

 

[daveboy]

My motorbike did the same thing when I fitted a full top box....
It stopped when I changed the tyres to a more road biased pair.

 

[Hightechpete]

Hopefully you wired these two in Parallel other wise you would have had 72v and magic smoke from the controller.

Thanks for pointing that out, post edited.

If it does happen try holding your leg/thigh/knee against the top tube, it dampens the ‘harmonic motion’

I'll try that later today, I'll be amazed if it works.

My motorbike did the same thing when I fitted a full top box....
It stopped when I changed the tyres to a more road biased pair.

I agree, it's not good to have the weight high and so far from the C of G, but I was surprised that adding weight low down and close to centre had the effect it did.

 

[Benjahmin]

My bike does that. At first I thought it was the front hub, but I understand that the rake angle of the steering head (on hybrids) makes hem prone to this. Strangely mine does it less with a Carradice handlebar box clipped on. Also it only happens with no hands on the bars (trying to be a kid again !).

 

[anotherkiwi]

28 mm tyres! I'm too scared to do those speeds with skinny tyres at my age, give me a 50-55 mm tyre any day.

My first bike had very twitchy steering at speed. Number Two has cheap Suntour forks which wash out at speed in corners so I don't go as fast any more. I can't see your photos so can't form an opinion on how to fix things.

 

[wheeliepete]

I can't see your photos so can't form an opinion on how to fix things.

No, I can't either

 

[Hightechpete]

I can't see your photos

Using 'google pics' for the first time, always relied on photobucket previously. Any good now?.

 

[anotherkiwi]

Nope...

 

[Benjahmin]

Just a grey no entry sign.
I normally just upload from photo's stored on the computer or direct from the camera itself. Google do play silly buggers in all sorts of ways !

 

[Hightechpete]

I normally just upload from photo's stored on the computer or direct from the camera itself.

I didn't know this was possible, how do I do it?. photos are normally hosted somewhere else and linked to the post.

 

[chris_n]

Click on upload a file.

 

[Benjahmin]

The computer sees a camera as an external drive. Plug the camera in (normaly USB) and csroll through the pics. When you find the one you want, make a note of its address. Then go to your post.
Press upload file.
Press browse.
Select external drive.
Type in address.
Press open.
Pictures on my computer are stored in a large folder called, 'Pictures'. Select the sub folder you want, then double click the picture you want.
Works for me

 

[Hightechpete]

Thanks for that Benjahmin, still a bit unclear as to whether some or all can't view the photos.
Anyway, back to my original problem, you'll recall that fitting the second battery induced the steering wobble.

If it does happen try holding your leg/thigh/knee against the top tube, it dampens the ‘harmonic motion’. I’m not sure there is a fix unless you redistribute the weight.

I refitted the downtube battery to test the above theory, up and down the road several times at various speeds and guess what?, I couldn't induce a wobble.
Nothing, zero, nada, zilch, WTF? did I imagine it or what?.

A couple of minutes head scratching later, the only thing that had changed was the positioning of the battery. It's a new battery, and the witness marks of the screw fittings on the holder were about half an inch lower.
Time constraints prohibited any further experimentation, but I think that by moving the battery half an inch up the frame has cured the problem, hard to believe !.

I'll report back once I've confirmed my theory.

Pete.

 

[Richardab]

I refitted the downtube battery to test the above theory, up and down the road several times at various speeds and guess what?, I couldn't induce a wobble.
Nothing, zero, nada, zilch, WTF? did I imagine it or what?.

A couple of minutes head scratching later, the only thing that had changed was the positioning of the battery. It's a new battery, and the witness marks of the screw fittings on the holder were about half an inch lower.
Time constraints prohibited any further experimentation, but I think that by moving the battery half an inch up the frame has cured the problem, hard to believe !.

I'll report back once I've confirmed my theory.

Pete.

Interesting! I know I said redistribute the weight but I am amazed that 1/2" is enough. Who knows?, its a frequency thing, its maybe that you have moved the resonance just enough to a speed that you never/rarely reach.

 

[anotherkiwi]

Move 3 kg half an inch? Sure it can have a huge impact on balance.