- Frame flex to help handling when angled over
I know this comes up in the usual Steel Vs Carbon debates so here’s another story I got from following a link from Scarbs…Posted 3 months agoNorthwindSubscriber
Same thing as happened when they did the RC51- up til then, the answer was always “make it stiffer”, then people ended up scalloping the swingarm, leaving frame bolts out, all sorts of madness to make it track better.
It doesn’t really transfer directly to cycling, the application’s too different but Nico Voileywoiley is obsessed with the same thing- he’s done stuff like machining maxles to introduce Friendly Bend.Posted 3 months agobigjimSubscriber
The Cesar Rojo interview on the vital mtb podcasts covers this and is generally interesting. Also interesting that yeti made a big deal of this on their new bikes but everyone is upset because you can grab the top of the back wheel and push the wheel sided to side a few mm (which you obvs do when you’re riding)Posted 3 months agobrantSubscriber
Stanton seem to be going for flex in the back end
Posted 3 months agomildredMember
This is very much horses for courses & based upon. What exactly you’re trying to achieve (comfort vs performance etc).
Many years ago I was involved in Karting & quite a few school karting teams used wooden chassis. They tended to work very well; stiff chassis would invariably lift wheels when cornering but because wood has so much flex it enabled all four wheels/tyre to remain in contact with the ground, thus maintaining the grip of all tyres, thus enabling the driver to maintain higher cornering speed. Very effective when done right.Posted 3 months agothisisnotaspoonSubscriber
my opinion will alwys be that if it is controled then its worth looking at. If its just because a frame is flexy its a waste of time as it means your bearings, shock etc etc are all side loading which isnt idea.
It’s not quite so so simple when you’re looking at something dynamic like a bike moving over a rock.
If you just applied a side load of say 1000N to the back wheel it would probably crumple. If you hit a 1″ rock mid corner the wheel has to move 1″ one way or another. If you make everything stiff then it take a lot more force to get the wheel over that obstacle than if it can flex. That force loads up all your bearings, saps your speed and causes the tyre to lose grip.
As for controlling it (with damping or hysteresis), you need a damper to stop a system from oscillating. I’d hypothesise that frames generally have very high frequencies and correspondingly low amplitudes compared to the impacts so would tend not to need damping.
Honda are using carbon wrapped aluminium frames in motogp, not sure if that’s to make tuning the stiffness easier or add hysteresis but theres a lot more going on with a motorbike. If you hit a rumble strip you don’t want the frame to have the same natural frequency. Whereas mountain bike impacts are random, if you hit one that happened to make the frame resonate, the odds are it’ll be canceled out a split second later.Posted 3 months agoNorthwindSubscriber
Yup. I got to do a pretty perfect comparison when I swapped from my Ragley Mmmbop to a Ti- exact same frame dimensions, same parts, different construction, the Bop was stiff as all hell, the Ti wasn’t exactly floppy but it was much softer. Leaned over? Couldn’t tell you, too much going on. Stood up straight, it made a surprisingly big different, it was faster/less sappy of speed and generally more composed on rough stuff. I don’t think that was all about vertical, after all these are still pretty stiff geometric shapes- I think a part of it was being able to track the rear wheel round stuff slightly instead of only going over.
Changes don’t have to be massive to be noticable though- the difference between 2 bars can be enormous never mind a whole framePosted 3 months ago
Cars and karts are completely different from bikes in this respect. Karts have no suspension to keep things simple and they race on very smooth circuits. The loading of the tyres is affected by the roll stiffness, which you can adjust in a car by changing spring rate and sway bars. If the back of the car is too stiff, it will oversteer because the inside tyre isn’t loaded. If the front is too stiff, it will understeer because the inside tyre isn’t loaded. In a kart, you can only adjust this through frame flex whereas proper race cars have adjustable roll stiffness.
The MotoGP bike thing was because the bikes are leaned over more than 60 degrees, so the suspension becomes increasingly ineffective because the bumps are so far out of alignment with the suspension movement. The tyres still act as springs, but they need to be damped in a direction that the dampers can’t deal with. The frame flex was concerned with damping the lateral forces (in relation to the suspension travel, although they are vertical in relation to the road surface).
This really only becomes an issue when the bike is leaned past 45 degrees in relation to the track surface, so bumps on a bermed corner don’t count because the suspension will still be operating fairly close to 90 degrees from the track surface. In order to generate lean angles greater than 45 degrees, you need a fairly smooth, high-grip surface, so I doubt that many MTBers get remotely close to that level of lean.Posted 3 months agothisisnotaspoonSubscriber
This really only becomes an issue when the bike is leaned past 45 degrees in relation to the track surface, so bumps on a bermed corner don’t count because the suspension will still be operating fairly close to 90 degrees from the track surface. In order to generate lean angles greater than 45 degrees, you need a fairly smooth, high-grip surface, so I doubt that many MTBers get remotely close to that level of lean
If you were to get trigonometric then it’s 60deg. At that point a vertical deflection would require twice as much suspension travel to absorb as it would when upright. But it’s a bit like aerodymanics, there isn’t a number beyond which it has an effect snd below which it doesn’t. An deviation away from an radial load will cause some whether that’s good or bad depends on the scenario and how much.
As a bit of an extreme example. I once had a bad crash on a hardtail coming round a berm the back wheel caught a tiny stump poking out of the trail, it was barely the size of a fist. But as the bike was loaded up it stopped dead and threw me otb. That wouldn’t have been solved by lateral flex, but would have been by vertical suspension. But it shows the sorts of forces involved when there is zero compliance.
And as northwind observes, having a back wheel which can skitter about a bit isn’t always a bad thing as long as the frames stiff enough that it doesn’t affect the riders inputs (I doubt we’ll see flexible forks for example as your hands can’t grab the bars right enough to completely lock them rigid anyway).Posted 3 months ago
If you were to get trigonometric then it’s 60deg. At that point a vertical deflection would require twice as much suspension travel to absorb as it would when upright.
Yes, but, IIRC, damping effectiveness is a function of the square of the damper speed (happy to be corrected if somebody knows better). Which, as I understand it, means that a damper has about 12.5 percent effectiveness at 60 degrees lean, but 25 percent effectiveness at 45 degrees, and 75 percent effectiveness at 30 degrees. My guess is that very few MTBers lean their bikes more than 30 degrees very often (not saying never, just not very much), so the frame flex helping suspension theory from MotoGP doesn’t seem very relevant.Posted 3 months ago
“This really only becomes an issue when the bike is leaned past 45 degrees in relation to the track surface, so bumps on a bermed corner don’t count because the suspension will still be operating fairly close to 90 degrees from the track surface. In order to generate lean angles greater than 45 degrees, you need a fairly smooth, high-grip surface, so I doubt that many MTBers get remotely close to that level of lean.”
Whenever you post about this, I wonder how you ride a MTB? There are three glaring errors in your assumption:
1. If you’re riding well, you lean a MTB more than your body. That’s why you can have a tyre like a High Roller where the edge knobs are right over to the sides and then a big channel between them and the centre knobs.
2. You can generate far more grip and thus greater lean angles by pumping turns. Any good rider does this.
3. Small improvements in grip are more than valuable than you seem to realise. And good riders (and even riders like me!) can tell the difference between tyre pressure changes of less than 10% so why shouldn’t frame flex be both noticeable and beneficial?Posted 3 months ago
“You’re saying you lean the bike more than 45 degrees relative to the track surface? Chapeau!”
Maybe I do, maybe I don’t. Assuming no binding, a 25 degree lean angle will make your suspension 10% less compliant. I’d hope that a half decent rider notices their suspension being 10% too stiff. But once you put a sideways load on suspension components, especially fork bushings, they don’t always move with as low friction as they should, so the real world situation will be worse.
Then add in off-camber bumps, like when you’re holding a flat turn around a tree, bumping over large roots that are biggest/highest on the inside of the turn, and the forces are even further from being parallel to the suspension movement.Posted 3 months ago
“You’re saying you can feel the difference in suspension movement leant over? Chapeau!”
Of course I can’t, because I don’t have anything to compare to against – going around a corner the forces are totally different to going in a straight line. But I can totally tell the difference by changing the pressure in my forks or shock or tyres by 10%, which was my point.
Bikes are not infinitely stiff. Flex affects handling, grip and more. Being an armchair expert who likes to oversimplify things and deem small differences insignificant whilst saying hat repeatedly doesn’t change that.Posted 3 months ago
Au contraire, I think the subject is massively complex and also not well understood even at MotoGP level – I think mountain biking is more dynamic yet involves far less energy, so I think it’s premature to state with any certainty that what works well for one will work well for the other. YMMV.Posted 3 months ago
“I think it’s premature to state with any certainty that what works well for one will work well for the other.”
You can barely read a review of a DH or enduro bike nowadays without there being comments about the frame flex. And there’s a clear correlation with bikes that are not the stiffest but are the quickest down a track due to more grip in the turns and reduced rider fatigue. And people choosing to run less stiff wheels to give stiffer frames more compliance.Posted 3 months agoDickBartonMember
Watch a full suspension bike rear wheel in slow motion HD – rear wheel is twanging all over the place. It doesn’t track with the frame.Posted 3 months ago
Orange 5 is a great example of this mainly due to a ‘single’ arm from wheel to frame. Frame with a wider gap between seat and chain stays show less but it is present.greyspokeMember
Assuming no binding, a 25 degree lean angle will make your suspension 10% less compliant
That is what the trigonometry says. If the bike is leaned over 25 degrees, the suspension will have to compress around 10 percent more to achieve the same vertical wheel movement as compared to the no lean situation. At 90 degrees the only vertical compliance will be from flex.Posted 3 months ago
Except greyspoke. if a bike is leant over 25 degrees it’s more than likely leant over because the rider is turning. and if the bike is turning in a berm/rut or any sort of turn with support the wheels are in reality going to be far closer to perpendicular to both the supporting ground and many of the bump forces. Even turning on flat ground terrain is unlikely to be completely smooth therefore you cannot say exactly what the forces on the wheels and suspension will be. Nevermind quantify a percentage of loss of compliance.
Put yer trigonometry away and go ride yer bike. looking at that graph paper seems to have made you forget what a mountainbike ride actually feels like.Posted 3 months ago
If you look at the video in the link below (just the first random thing I saw that was relevant), at about 55 seconds you get a nice slow motion bit. Although the bikes are leaned over a lot relative to vertical, this is only on heavily bermed turns, the tyres are still not far from 90 degrees from the track surface. This is a completely different situation to the MotoGP issue that the OP linked to, where the bikes are leaned past 60 degree lean angles and the damping becomes ineffective.Posted 3 months ago
It isn’t completely different, it’s just a less extreme example, just as the dynamics of racing cars and road cars are related, even though the forces are much lower for the latter.
Also, has anyone noticed that a MTB trail tends to be a little bumpier than a Moto GP track?Posted 3 months ago
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