Steer Clear - understanding steering for twinshocks

Need help finding information or parts for that old machine in your shed? Someone in here will know!

Moderator: Moderators

David Lahey
Champion
Champion
Posts: 4062
Joined: Thu Jun 05, 2003 7:01 pm
Bike: Many Twinshocks
Club: CQTC Inc, RTC Inc
Location: Gladstone, Queensland

Re: Steer Clear - understanding steering for twinshocks

Postby David Lahey » Fri Jul 24, 2020 4:53 pm

tat ty wrote:TO THE INTRIGUING TRIALS PART OF THE STORY ..........

Appearing on of all places Australian Ebay approximately 5 years ago was a trials bike hand made as a prototype by an Australian rider who had spent time competing (not trials) in the US.

The bike had been won in a raffle by the South Australian vendor. Perhaps the raffle resulted from dispersal of the deceased estate of the bloke who built the bike.

That bike used the same ATK system and countershaft disk brake.

No regretfully I did not buy the bike but always wondered.

Someone may know it.
Alastair.


That prototype 1980s trials bike by Kelvin Franks now resides in Brisbane and was the subject of one of my columns in VMX magazine.
When the ATK with the rear brake disc beside the sprocket came out, I wondered how any of them were ever approved for road use because of the risk of loss of braking that would result if the chain broke.
Attachments
Avenger13.jpg
Avenger13.jpg (198.45 KiB) Viewed 2334 times


relax, nothing is under control

JC1
Expert participant
Expert participant
Posts: 387
Joined: Tue Apr 26, 2011 4:15 pm
Bike: Bul, KT, TY
Club: wdtc
Location: Toowoomba, Qld

Re: Steer Clear - understanding steering for twinshocks

Postby JC1 » Sat Jul 25, 2020 9:41 am

tat ty wrote:The explanation of the loop factor did effectively loop my brain. I do have a reasonable mathematical ability, but I'm sure I'll need to read it at least 5 more times before there is a chance that I may "unloop"

I dip my helmet to JC1.

So here is the question that I've needed resolved and I suspect JC1 may have the explanation.

In the mid 1980s an American company named ATK produced a revolutionary dirt bike. Either side of the swing arm pivot (above and below) was an idler pulley, which ensured that the chain ran parallel to the swing arm.

My hunch is that the system (Anti-Tension Kettenantreib) had to do with the loop factor. So the question is out.

Alastair.


When I first wrote that post many years ago I thought of that design too Alastair.

ATK called it a parallelogram system that was marketed as isolating the chain torque reaction from the suspension so that they were independant from each other - ie it was supposed to make the suspension more supple and responsive: no effect on the suspension from the chain pull force because, according to ATK, the chain pull was parallel to the swingarm.

I immediately realized that the ATK designer didn't understand how chain-pull torque worked, since the chain-pull torque has nothing to do with whether or not the chain-pull direction is parallel to the swingarm.

Look at the diagram again and compare it to the ATK in David's post.
(Or google-image ATK406 and you can see some pics where the configuration of the system is more apparent.)

Chain Torque JPG.JPG
Chain Torque JPG.JPG (14.08 KiB) Viewed 2299 times


In his system the dimension 'a' is huge (and constant) compared to a normal system, so the squat torque from the chain pull is also constantly huge. To compensate he is forced into having an extreme swingarm angle (and a hi swingarm pivot location) so that the dimension 'b' is even more huge to counteract it.

To some extent it did counteract loop factor (anti-squat) a little because 'a' is so large causing squat, but at the cost of actively softening the rear suspension when under power (which was what my next post was about back then on page 4 of this thread).

It did achieve more or less constant chain tension but at the cost of extremely hi wear of chain and front sprocket since the chain wrap-around of the front sprocket was barely 90deg, so all the power was transmitted thro just a few teeth. And I suspect it would be prone to the chain jumping teeth whenever the system is slightly out of adjustment.


There was also an accessory marketed, called the AMP Link if I recall correctly, which was also based on the parallelism claims, which tried to emulate the ATK system but is long gone.

They both were little more than a novelty which soon wore off as it didn't achieve what was claimed. It's noteworthy that nobody else took it on board.

For good reason.


"Men are never more likely to settle a matter rightly than when they can discuss it freely"

tat ty
Expert participant
Expert participant
Posts: 253
Joined: Sat May 07, 2005 6:12 pm

Re: Steer Clear - understanding steering for twinshocks

Postby tat ty » Sat Jul 25, 2020 11:28 am

Thanks JC1.

I think my brain has unlooped and I pretty much get it.

Reminds me a little of a conversation I had with a cyclist friend of mine a few years back.

He had bought an eyewateringly expensive push bike with a carbon fibre frame. The whole idea was a combination of weight saving and extreme rigidity. It was all about maximising energy to road contact.

I guess it's more vital when the energy is your morning weet bix as opposed a gerry can from the back of a ute.

Alastair




Return to “Twinshock & Classic Trials”

Who is online

Users browsing this forum: No registered users and 35 guests