Seeking to cure dangerous instability

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   Hello all
(Mods – I hope "Building" includes adjusting, tweaking and replacing parts.  If not, please do move this to somewhere more appropriate.)

Total newb here.  Never even sat on (in?) a recumbent until I bought an Optima Lynx a few days ago.

Though it felt unstable from the outset, I thought I just needed to relax, loosen my grip and look up, so have clocked up an enjoyable 40 km on it.  Yesterday, however, I found myself heading for the verge at about 25 km/h, unable to brake or steer away quickly enough.  I don't really know what happened, but suspect a moment's inattention or tenseness scuppered me.  No harm done to either myself or the machine, thankfully, but I'm keen to avoid a repeat.  

I know this bent has non-standard parts fitted, e.g. both wheels are the same size, so I thought I'd start measuring.  I know that equalizing the wheel sizes (lowering the rear and raising the front) should give more relaxed geometry (increased rake and trail), though probably not by much.  I've just measured the trail at about 45 mm, but the front axle is about 40 mm forward of the headset axis.  Most photos I see online appear to show a font axle which is in line – or nearly in line – with the headset axis.  This would mean that someone has effectively halved the trail, which might explain some of my difficulty.

Can anyone advise on a typical trail measurement for a stable bent?  Alternatively, does anyone have a standard (or nearly standard) Lynx they could measure, or a link to a catalogue or specification where I might find these numbers?

As a short-term solution, I'm thinking of reversing the forks, which should give me somewhere around 120 mm of trail.  Any advice for or against this would be greatly appreciated, especially if anyone thinks it's a shortcut to another verge encounter!

Thanks in advance
Warren

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weevil Members Profile Send Private Message Find Members Posts Add to Buddy List BHPC Member Joined: 02 June 2021 Location: Cambs Status: Offline Points: 4	Post Options Post Reply Quote weevil Report Post Thanks(0) Quote Reply Topic: Seeking to cure dangerous instability Posted: 03 June 2021 at 6:16pm
	Hello all (Mods – I hope "Building" includes adjusting, tweaking and replacing parts. If not, please do move this to somewhere more appropriate.) Total newb here. Never even sat on (in?) a recumbent until I bought an Optima Lynx a few days ago. Though it felt unstable from the outset, I thought I just needed to relax, loosen my grip and look up, so have clocked up an enjoyable 40 km on it. Yesterday, however, I found myself heading for the verge at about 25 km/h, unable to brake or steer away quickly enough. I don't really know what happened, but suspect a moment's inattention or tenseness scuppered me. No harm done to either myself or the machine, thankfully, but I'm keen to avoid a repeat. I know this bent has non-standard parts fitted, e.g. both wheels are the same size, so I thought I'd start measuring. I know that equalizing the wheel sizes (lowering the rear and raising the front) should give more relaxed geometry (increased rake and trail), though probably not by much. I've just measured the trail at about 45 mm, but the front axle is about 40 mm forward of the headset axis. Most photos I see online appear to show a font axle which is in line – or nearly in line – with the headset axis. This would mean that someone has effectively halved the trail, which might explain some of my difficulty. Can anyone advise on a typical trail measurement for a stable bent? Alternatively, does anyone have a standard (or nearly standard) Lynx they could measure, or a link to a catalogue or specification where I might find these numbers? As a short-term solution, I'm thinking of reversing the forks, which should give me somewhere around 120 mm of trail. Any advice for or against this would be greatly appreciated, especially if anyone thinks it's a shortcut to another verge encounter! Thanks in advance Warren
	A happy (for now!) Optima Lynx rider

Yanto Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 11 July 2005 Status: Offline Points: 1521	Post Options Post Reply Quote Yanto Report Post Thanks(0) Quote Reply Posted: 04 June 2021 at 8:29am
	one for Geoff Bird or Jonathon Woolrich I would have thought, maybe ping them a PM?

AlanGoodman Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Club Chairman Joined: 04 March 2005 Location: United Kingdom Status: Offline Points: 8036	Post Options Post Reply Quote AlanGoodman Report Post Thanks(0) Quote Reply Posted: 04 June 2021 at 8:35am
	I’d suggest posting on the Facebook Group as well.


GeoffBird Members Profile Send Private Message Find Members Posts Add to Buddy List BHPC Member Joined: 20 September 2005 Status: Offline Points: 2396	Post Options Post Reply Quote GeoffBird Report Post Thanks(0) Quote Reply Posted: 04 June 2021 at 4:13pm
	Hi Warren, for my own short wheelbase designs, I have between 60 and 75 mm of trail because I prefer them that way, but many of the Dutch manufacturers use a bit less. You do seem to have an excessive amount of offset on your bike. Reversing the forks would be worth a try, but be careful - there is such a thing as 'trail-induced instability' if you have too much trail. But, you are right, a fork with a small amount of offset or none would be more normal. These people have a wide range of rigid forks for funny wheel sizes, or try one of the recumbent manufacturers that sells spares: https://hollandbikeshop.com/en-gb/bicycle-parts/fork-and-headset/rigid-fork/?page=1 The other option is that it is 'rider induced instability', due to you gripping the bars too tight or applying the wrong inputs. You can only really tell by riding a recumbent that is known to handle well, for comparison. Or you could get an experienced recumbent rider to try it - is there a BHPC race event near you? Or you could revert to factory spec I suppose? What changes have been made and what are the wheel sizes now? Edited by GeoffBird - 04 June 2021 at 4:26pm
	Right Time - Right Place - Wrong Speed

Balor Members Profile Send Private Message Find Members Posts Add to Buddy List Visitor (new) Joined: 25 January 2017 Status: Offline Points: 154	Post Options Post Reply Quote Balor Report Post Thanks(0) Quote Reply Posted: 12 June 2021 at 11:27pm
	There is absolutely nothing wrong with the geometry, and it mere 25 kmh trail forces don't play that much of a role (reversing the fork will introduce so much flop into the system that it would likely make things MORE unstable even at speed - you can only add tons of trail with impunity if your steering angle is much steeper, preferably very close to 90 deg). What is your entire bike arrangement look like? Bike-rider interface is far more important - unless your steering geometry is out of whack (excessive flop or negative trail). You may simply be unused to tiller steering for instance: while comfortable and aerodynamic, takes a lot of getting used to because it totally switches off any 'body english' input to bike steering. I bet it is tiller steering, right? With right steering arrangement a bike with exactly zero trail is absolutely and effortlessly rideable at much higher speeds, too - but NOT with a tiller, that's for sure. I'd say if you are having trouble with a tiller (I can fully sympathise with that, it took me a lot of time and still crashed once purely due to rider error - steering overshoot), I recommend going for 'tweener' bars, much wider. You aero will take a small hit, but your steering will be much improved. A USS will be even better, but I'm not sure this bike is compatible with one. Edited by Balor - 12 June 2021 at 11:43pm