Beefing to reduce fork flex

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RustyKipper   100 W

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Location: South Yorkshire UK the land of flat caps and wippets

Beefing to reduce fork flex

Post by RustyKipper » Nov 07 2018 11:31am

Hey guys, My daily commuter suffers from pretty bad fork flex ie. if I waggle the handle bars quickly or hit a pothole I can build up a fair bit of death wobble, although it doesn't continue it doesn't inspire confidence and makes the bike feel like a blancmange!

I think this is mainly due to the fact that I have to use stupidly thick and heavy inner tubes (3mm thick rubber) to cope with the broken glass and shrapnel covered roads where I work, this builds up huge gyroscopic forces plus I have quite heavy aluminium hand formed mudguards and a handlebar bag containing my waterproofs so a lot of weight hanging of the steering.

So I figured I might have a go at engineering a solution, the flex appears to be centered around the bridge at the bottom of the steerer tube so I figure a good step would be to effectively carry the stanchion tubes right up the the handlebars, not so easy as there are controls on top of the stanchions so I would have to fabricate some sort of joint. So I ordered up some 30mm thin wall stainless tubing to make the tube extensions, and some alumininium tubing and spent a few evenings in the machine shop.
machined parts small.JPG
Machined parts ready for paint
bridge small.JPG
painted and assembled top bridge
assembled small.JPG
Parts assembled on bike
Most of the time was spent machining weight out of the parts.

The parts were then painted and fitted to the bike with heavy duty stainless hose clamps.

The flex was reduced by a good degree but can still get the front wheel to oscillate.
In an ideal world I would swap the standard MTB type forks (Rockshox silver) to a triple clamp set but they are around $1000 plus when the rear mounted hub motor fails I want to go awd and fit a mini hub motor in the front to help with weight balance and triple clamp forks with drop outs seem to be rarer than hens teeth.

The fork extensions seem to be reasonably successful, the forks rarely bottom out unless I land really heavily from jumping of a kerb or speed bump, steering angle is quite reduced but is only an issue when negotiating cycle lane barriers where I have to turn the bars nearly 90 degrees and have added about 1.4Kg of weight.

I'm currently looking at a mark 2 version with the joint between the lower stanchion tubes and the upper tubes from carbon fiber to reduce weight and increase the steering angle and do away with the hose clamps that have some rather sharp edges that will cause a fair bit of bleeding when I come off in the mud.

It has to be said I'm quite pleased with how this setup looks.
2017 Cotic BFE hardtail MTB
Sram 9 speed twist grip
CST rear drive
5P10S 16Ah home built battery with built in block heater
Con121 12FET controller
Sapim 2.3mm black stainless steel spokes
Sunringle 26 inch rims
26 x 2.1 Swarble Marathon Mondail tires
650gm downhill inner tubes --essensial
Shimano Zee 4 pot calipers front and rear
200mm / 203mm rotors
Thudbuster suspension seat post -- essensial
150 Mile per week muddy commute

Tommm   10 kW

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Re: Beefing to reduce fork flex

Post by Tommm » Nov 07 2018 1:08pm

I'm hooked by seeing just the end of the handlebar. Show the full bike.

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neptronix   100 GW

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Re: Beefing to reduce fork flex

Post by neptronix » Nov 07 2018 1:17pm

Interesting attempt..

I think most of your problems are..
1) the bike is a lot heavier than stock.
2) You have wimpy stanchion tubes on that fork.. they're what.. 28mm?

Maybe you don't really need a triple crown.. just a fork with 32mm or wider tubes will be stiffer..
Any play in the axle itself?
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Jonno   100 W

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Re: Beefing to reduce fork flex

Post by Jonno » Nov 07 2018 1:28pm

Wider handle bars would give you more stability and control.
Modified EEB frame, 888 forks, 280mm DNM pit bike shock, QS273 40h V3 , Nucular Electronics 24F controller 300a, 92V @ 36ah 26650 cells, 300a BMS, Hope V4 + Galfer223mm discs + Variable regen braking

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RustyKipper   100 W

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Re: Beefing to reduce fork flex

Post by RustyKipper » Nov 07 2018 2:20pm

Tommm wrote:
Nov 07 2018 1:08pm
I'm hooked by seeing just the end of the handlebar. Show the full bike.
Here she is at 18,000 miles
muddy small.JPG
bike corn field.png
Summer homeward commute-check out the plywood rear mudguard!
bike corn field.png (388.01 KiB) Viewed 806 times

Yes the bike is way heavier, its something like 30Kg, the stanchion tubes are 30mm but probably quite thin, my winter ice bike has the same diameter tubes (cheapo forks) and heavy rubber inner tube but with spike tyres and has a lot less flex, I think maybe this model of Rockshox is probably built for light weight, you have an excellent point about using forks with 32mm tubes.

No there is no play in the axle, if I put the bike upside down and stand on the hand grips I can twist the wheel by maybe 10 degrees in each direction and its not the wheel flexing, I think its a mixture of the bridge and stanchions flexing and the steerer tube twisting.
2017 Cotic BFE hardtail MTB
Sram 9 speed twist grip
CST rear drive
5P10S 16Ah home built battery with built in block heater
Con121 12FET controller
Sapim 2.3mm black stainless steel spokes
Sunringle 26 inch rims
26 x 2.1 Swarble Marathon Mondail tires
650gm downhill inner tubes --essensial
Shimano Zee 4 pot calipers front and rear
200mm / 203mm rotors
Thudbuster suspension seat post -- essensial
150 Mile per week muddy commute

boxtriker   100 µW

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Re: Beefing to reduce fork flex

Post by boxtriker » Nov 07 2018 2:49pm

Nice job I Had a similar problem bike is used for trip to work and the country Green lanes in the summer winter time see's the Montesa trials come out for that. Any how back to the forks the bike was running is standard forks and they are fine till you ad the battery and motor then my fat ass I looked for some triple clamps and found a set of 2nd hand shock works forks for £60 off the bay of Evil now they don't look like downhill forks and I don't jump my bike 4-5 feet airborne but they did the trick bike is stable at its 42-43 mph max and has hit 61mph down hill and they take the bumps well. Its just the back end of the bike could do with a bit of up and down movement now as well so I'm looking for a full suspension frame that will fit my triangular battery
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Chalo   100 GW

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Re: Beefing to reduce fork flex

Post by Chalo » Nov 07 2018 3:30pm

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StuRat   100 W

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Re: Beefing to reduce fork flex

Post by StuRat » Nov 07 2018 3:43pm

RustyKipper wrote:
Nov 07 2018 2:20pm
Tommm wrote:
Nov 07 2018 1:08pm
I'm hooked by seeing just the end of the handlebar. Show the full bike.
Here she is at 18,000 miles
Nice to see a dirty bike. Well used.

I'd say the sus fork has thrown out the trail and rake geometry. Hard to tell from a photo but it looks like you trail is zero, but that's without you on it of course. Front loading will aggravate handling without some rake, and you've only got about 15mm offset on the dropout, that's it.
Was the frame suspension corrected for that much cush?

And yeah, wider bars helps with fighting it.

Nice bike.

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RustyKipper   100 W

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Location: South Yorkshire UK the land of flat caps and wippets

Re: Beefing to reduce fork flex

Post by RustyKipper » Nov 08 2018 3:32am

The forks do definitely have a shorter rake than my other more solid bike but I think maybe as the forks were cheap and heavy they are just made of thicker steel. My commuter is quite stable on road right up to 46MPH (downhill and screaming like a girl!)

Wider handlebars are unfortunately not an option, the roads here in the UK are just not wide enough, the first 4 miles of my commute is a fast downhill section of freeway, I'm doing a good 30mph with cars and trucks brushing my arm when I'm maybe a foot from the kerb and the sight of flowers tied to every second lamp post kind of hints to how dangerous it is, cycle clips are required to keep stuff in! :shock: I did try a pair of Nukeproof wide handlebars but after hitting my arm on a lamp post and then getting stuck trying to get through a gate and finding it impossible to extract by bike from the company cycle rack I ended up cutting a good 6 inches off. Most people round this way use racing type drop handlebars.

nb. Yes its bad practice riding a foot from the kerb but if you aggravate the divers in rush hour traffic they WILL force you off the road and injure you, my boss is a case in point.

I think I will definitely look for forks with thicker stanchions.
2017 Cotic BFE hardtail MTB
Sram 9 speed twist grip
CST rear drive
5P10S 16Ah home built battery with built in block heater
Con121 12FET controller
Sapim 2.3mm black stainless steel spokes
Sunringle 26 inch rims
26 x 2.1 Swarble Marathon Mondail tires
650gm downhill inner tubes --essensial
Shimano Zee 4 pot calipers front and rear
200mm / 203mm rotors
Thudbuster suspension seat post -- essensial
150 Mile per week muddy commute

2old   100 kW

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Re: Beefing to reduce fork flex

Post by 2old » Nov 08 2018 10:15am

Some of the Fox (possibly Rock Shox) ones are 34, 36 mm and not too pricey if you get them on sale from Chain Reaction Cycles in the UK.

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