It’s an interesting question.  On the caeser rojo podcast he talked a lot about this issue.  It seemed that on moto gp the consistency of track surface and amount of data mean they have a value for lateral stiffness that they know is fast and can design the frames to.  Cotic are making frames for riders between 7 and 20 stone, for railing world cup tracks and braking down blues.  The perfect amount of lateral stiffness to balance, precision, grip and pump will vary on different corners on the same track.  There isn’t data available to pick a number, all cotic can do is base it on feedback from riders on old frames and protos and guess a compromise.  So for what it’s worth “breathing with the trail” is both rubbish and the truth and we’ll never know which, we can’t even pick a wheelsize how can we get this right?  I’ve got a flaremax because the wheelbase seemed decent for an xc bike and the kit on it was solid enough, although I like the idea of cornering grip through lateral compliance I’ve no idea how I’d decide which bike does it best.  I also like that cotic clearly aren’t big enough for super slick press releases, some of the hyperbole does make me cringe but I’m happy my cash goes into bikes rather than words.

gravity does not exist

People have been lying about gravity for millennia.

Ah… Ok… so as i thought. none of you have even ridden a Session.
Makes sense. As if you had you might have have known the detrimental suspension traits detailed by cheif were present on even the first superlight (and def not all that stiff) Mk1 Aluminium Session frames.
It’s not because of the stiffness.

The Session definitely doesn’t suit every rider but what Paul Aston said about them only suiting a heavier stronger rider is also nothing more than journalist bollocks. Reviews are full of that sort of shit. It’s cyclical and whatever minutae to detail happens to fashionable at the time.

Sorry guys, much as I’d love to show you that bike in full, the simple fact is that it’s toooooo sexy for youze and would likely blow you mind.

It’s not because of the stiffness.

Damn you, spoiling a perfectly good thread with pesky empirical facts.

TO FLEX OR NOT TO FLEX

https://motochassis.com/book/LATSUS.PDF

“https://motochassis.com/book/LATSUS.PDF”

This is worth reading.

Regarding old Trek Sessions vs current ones, there’s nothing empirically accurate about assuming that the kinematics of the new 29er are identical to much earlier bikes with the same name.

You only have to move a pivot point a few mm to have a signicant change in behaviour – and if you have a 26” and 29” DH bike with the same BB height and chain stay lengths then you’ve inevitably moved the rear axle pivot by over 30mm (due to the much larger wheel radius) which will change everything.

I didn’t say they had the same kinematics. I intentionally didn’t mention kinematics at all. I said they all have the trait you were getting your knickers in a twist over.
Try actually riding the bikes you like to disect and fret over to  “princess and the pea” levels of worry instead of reading motocross suspension physics manuals in an attemp to make yourself sound clever on the internet.

If you were paying attention I also didn’t say it was necessarily a negative trait

Im not sold

i see knackered bearings and frame rub! I do not believe that he has designed that amount of stiffness in!

Starling owners do you constantly have to change your bearings, shock bushings and suffer terrible wheel rub?

No issues here. I know (I think) what Andy is saying but the  bike in question was one of the first that Joe built and it was run ragged by testers.

Mine has been fine though – had one experience where the bushes for the shock were a few mm out and there was a bit of flex but once it got sorted it’s been as solid as I need. It’s not as stiff as my Turner was but no heel rub / noticeable flex  and it feels good

Btw – offset bushings were able to knock up what I needed as the Fox plastic bushes weren’t strong enough. It needed the strength of the metal bushings to support it

“I didn’t say they had the same kinematics. I intentionally didn’t mention kinematics at all.”

“you might have have known the detrimental suspension traits detailed by cheif were present on even the first superlight (and def not all that stiff) Mk1 Aluminium Session frames.
It’s not because of the stiffness.”

“If you were paying attention I also didn’t say it was necessarily a negative trait”

Well that’s a lot of contradiction in a short space of time!

A detrimental suspension trait is by definition a negative trait. The two words are synonymous.

And if it’s not because of the stiffness, then this can only be caused by the kinematics – that’s what describes the suspension behaviour. And if they’re suspension traits then that has to be talking about the kinematics. QED.

I don’t need to prove I’m clever on the internet but I can use my ability to understand and explain complex things and shoot down some of the misinformation that abouts across the web. I’m not worrying about anything, I just don’t like people shouting “bollocks!” when actually they’re the ones talking bollocks.

Stiffness is a multi-factorial kind of thing.  flex in the shock linkage could cause side-loading and stiction, resulting in poor perceived performance, whilst in other respects the frame could be relatively stiff.  Or vice-versa.

What would be interesting would be to see some actual measurements for stiffness.  I am sure reviewers do their best, but I would be wary of reading too much into a reviewer’s bald statement that a frame is stiff, or not.  Did they measure it, or are they simply reacting to ride characteristics which they believe to be associated with stiffness, or what the manufacturer said?

Calm down wee man. There’s no contradiction in anything I said. I didn’t say they I thought they were detremental. I said “detrimental suspension traits detailed by cheif” :rolleyes:
I intentionally didn’t mention kinematics because you tend to become a complete tool over the mere mention of the word. As witnessed above.
Of course it’s a suspension trait but the actual kinematics of each new Session design from OG to curent model are not identical.

Did you ever stop to think why the fastest TREK riders in the world don’t complain about the same things you and Paul Aston do?

Ride your bike moar. (Any bike)

“Did you ever stop to think why the fastest TREK riders in the world don’t complain about the same things you and Paul Aston do?”

I know why that is and that’s why they’re professional athletes. I know I wouldn’t make a great professional athlete because I haven’t got the right brain for it but many of the aspects that are detrimental to my athletic performance help make me a very good engineer, product designer, teacher and coach. I earn a living designing products that (within my small sector) are internationally acclaimed, ground-breaking and award-winning. I don’t earn a living from riding my bike, it’s just one fun aspect of my life.

“Ride your bike moar. (Any bike)”

I’m fine thanks, I’m riding my bike just the amount I want to at the moment. I’m playing my bass a lot more. I’m moving heavy barbells in the gym. There’s only so much one pair of legs can handle!

Sorry, I had to do some stuff at work for a few days, absolutely no time for anything else.

Paton’s links are interesting. Basically, Moto-GP bikes are leaned over at extreme angles, up to 60 degrees from the track surface. They are also extremely fast, so even small undulations become high-frequency bumps. If a bike is laid over that far and hits a bumb, the lateral load absorbed by the tyre greatly exceeds the vertical load absorbed by the suspension. Thus, the main compliance (i.e. spring) is provided by the tyre, not the suspension. The frame of the bike cannot provide compliance at the same order of magnitude or else the bike would be too flexy. The suspension damper cannot damp oscillation at 90 degrees to it’s axis, so the lateral tyre oscillations need to be damped. Part of this will come from the tyre itself, but designing a tyre to be the primary damper would compromise its performance in other ways, so some sort of lateral damping is needed. Until recently, there was enough flex in the frame to damp this lateral oscillation, but as materials and design improved, this flex was reduced to the point where lateral damping was insufficient. Therefore, some sort of damping needs to be incorporated into the design of the chassis.

Some key points:

This seems to mostly be an issue with the from suspension, not the rear, as on the Trek. The Trek seems to have a problem with the leverage curve and shock tune. There’s no evidence that it’s related to chassis stiffness.

Having flex in the forks, swingarm, and suspension mounts is detrimental. The damping needs to be provided elsewhere. Stiffer forks, swingarm, and suspension mounts are better (that’s what Paton’s link says, anyway).

This lateral damping is only applicable when a bike is cornering at extreme angles. The MotoGP bikes are apparently hitting about 60 degrees. In that case, the lateral bump force will be twice the vertical (the inverse of the cosine of the lean angle). MTBs cannot lean that far because they don’t generate sufficient cornering G force. Railing around a berm doesn’t count for this, the relevant angle is the angle that the tyre leans in relation to the track surface. At a lean angle of 30 degrees, the lateral bump forces will be small enough that normal suspension travel can take care of them and the regular damper should be enough.

So, to summarize. Very high speed motorbikes racing on very smooth tracks seem to need some lateral damping to stop front wheel oscillations. This is not relevant to MTBs because we can’t achieve the lean angles or speeds where this is a problem. Having flexy suspension linkages and mounts is undesirable. Breathing with the trail is marketing bollocks.

Having flex in the forks, swingarm, and suspension mounts is detrimental.

Everyone has said this. Haven’t they?

It was in the long Cotic piece you selectively quoted from yourself.

You want the flex elsewhere in the frame.

Well, not ‘you’, obviously.

You want the flex elsewhere in the frame.

Not flex. The MotoGP bikes need some lateral damping to control tyre oscillations. Damping is not the same thing as flex.

Also, nobody has shown that what is relevant for a MotoGP bike cornering at 60 degrees is relevant to a MTB. If you want to claim that a flexy MTB full suspension frame is actually desirable, you need to actually produce some evidence. Haven’t seen it yet beyond marketing bollocks.

How is this damping achieved @hols2?

This is getting a bit worrying hols.

Are you OK?

I’m not sure hols2 has even ridden a MTB down a rough rocky like the techier reds and blacks at BPW. Because if he had he’d know that this statement is, appropriately, bollocks:

”At a lean angle of 30 degrees, the lateral bump forces will be small enough that normal suspension travel can take care of them and the regular damper should be enough.”

And the rest of his arguments are equally full of holes. Like how a DH bike will often hit 40mph during a race and the surface is far rougher with much shorter wavelength high amplitude bumps than any track racing motorbike has to deal with. Yes the motorbike might be going four times as fast but the bump wavelength will be far more than four times as long or the track would be unrideably rough.

Anyone who’s ridden a full-sus bike off-road will know that the suspension can’t deal with high frequency buzz, however soft or long travel it is.

If you want to claim that a flexy MTB full suspension frame is actually desirable, you need to actually produce some evidence. Haven’t seen it yet beyond marketing bollocks.

A “flexy” full suspension frame is not desirable.

Some people think that means having no flex at all anywhere in the frame is therefore desirable.

When riders claim otherwise, and if manufacturers embrace that, perhaps the result is bikes that you don’t like to ride. Oh well. If the manner of describing the ride characteristics comes across as bollocks to you. Oh well. That fun little one liner is framed by a long essay on the subject… it’s a catchy and not entirely serious headline/tagline only. Read the rest.

http://www.cotic.co.uk/geek/page/SteelFullSuspension

IIRC SantaCruz did a nice long series of blog posts claiming absolute stiffness is all that matters … hunt it out @Hols2, I think you’d love it. Chimed well wth your view IIRC. Not the only view though.

Only time my bike gets owned over at 30 Deg is when it’s leaned up against the wall I have to admit the suspension doesn’t work at all

I haven’t read the thread after page 2, and never rode a steel fs bike but my question would be, if they are genuinely better at providing grip, then why does not one pro on the wc down hill circuit use one? I’m sure if they were better manufactures would make them.. But for some reason they don’t

Let’s be brutally honest here, what’s more likely …

A small UK manufacturer has worked out that steel frames are a better material for a full suss bike than those produced by all the major manufacturers and everyone but them is doing it wrong

Or

Small UK manufacturer has always made steel frames, has the tools and expertise to make a full suss one, and reckons that by doing so they provide a certain niche that will appeal to some folks.

I’m sure they are great bikes, they look lovely. And I’m sure they may ride slightly differently, which some may prefer . But if you think they provide better grip then I suspect you are kidding yourself.

Are you OK?

Well, last week at work turned out a bit hellish. Mostly we don’t have to do very much, but when we get busy, the deadlines are brutal. When I took the job, a guy who was on the way out told me that he trained himself to only take a dump twice a week so he wouldn’t get distracted when things got busy. I thought he was a bit strange to be honest, he took a job in Viet Nam so he might be a communist, but I’m not sure. He was nice enough to leave a card on my desk saying “Enjoy the view, it’s the only good thing about working here.” Anyway, last week got really hectic so I didn’t have time to take a dump for three days and the smell of my farts got really disgusting, but I got out for a ride on Saturday and the miraculous pre-ride dump materialized and cleared everything out. So now it all seems to be back to normal.

Small UK manufacturer has always made steel frames, has the tools and expertise to make a full suss one, and reckons that by doing so they provide a certain niche that will appeal to some folks.

Well, maybe. For some people being able to do the work ‘in house’ is going to be a big thing. Cotic made their first FS out of aluminium though, and all bar the newest rocket max are still made in the far east, where you could get it made in aluminium too.

Did we decide that frames can flex sideways? Because I can visibly flex my (steel) commuter just by holding onto the saddle and pushing on the bb with my foot. So, frames can and do flex from off axis loading. Given the relatively unconstrained position of the rider above the bike and the nature of the surface being ridden, almost all impacts bar riding straight at a curb seem likely to have an off axis component. Peak impacts from a rider+bike combined weight, moving at speed, similarly seem likely to be much higher than I can manage with my foot. The head tube is a pretty long way from the seat tube, so to say there isn’t going to be a noticeable amount of flex or twist in-between seems pretty unlikely to me.

Whether that it is good or bad is a different thing.