[Closed] Cotic Rocket?

Interesting reading on his geek page, not sure I agree with this, tho he doesn't really explain it:

By moving the caliper as far forward and above the rotor as possible, the brake force on the Hemlock pointed at the instant centre so the braking stabilised.

I can't see a geo page, but it appears to be all about the tech - geo doesn't matter!

Al: you're right - that doesn't make any sense.

(pivot location determines braking response, not the location of the caliper on the seatstay (hemlock) or swingarm (rocket))

Ivan: the frames are months away at least - i guess Cy doesn't want to get people too excited too early. he's given us a few sneak peaks cause he knows we like him.

Ah, I thought I'd read it was feb/march but could well be wrong.

He does invite people to reserve one - why not email and ask for geo, he must have it.

Shame he's come out with that error, tarnishes his other techie stuff.

Sent him an email last night but after being ****ed about by Enigma I'm getting impatient!

very impatient...

I don't know but would guess the enigma is light...that Rocket isn't going to be?

I'm booked in to ride a Rocket at the April demo day ...v much looking fwd to that!

Surely the braking response depends upon the forces acting on the pivot, and the location and direction of these will depend upon the caliper position. Geometry is alluded to in the 'how to Rocket your Hemlock' newletter ~1.5 deg slacker, slightly lower BB, steeper seat angle.

Yeah, after waiting ages for the Enigma and finally cancelling it yesterday I'm in a "I want it now" mood. The Enigma would have been light but also weighted down with disapointment and frustration. So I'm having a change of direction buildwise I think.

[url= http://www.cotic.co.uk/product/rocket ]Rocket Product Page[/url]

As Far as i'm aware the geo for the rocket is very similar/the same as the rest of [url= http://www.cotic.co.uk/geek/ ]Cotic's range[/url].

Frames will be available on or around the 1st Demo day, the 14th of April. Which i will also be attending 🙂
If you are impatient give them a ring on 07970 853 531.

Good things come to those that wait.
Enigma's support has been top notch for me.
An enigma hard tail & a rocket are wide apart - why the change in direction?

The Enigma was going to be a custom design, similar to a brodie holeshot ti crossed with a Soul but slacker and the intention was to build it up into a fairly AM style hardtail. It's not that big a jump to a trail full susser from there. Besides which there's no other ht frame I want now.

Impatience is the last thing you should take heed of when spaffing £1K+ on a frame.

Isn't this droplink the same as used by Ventana at least?

chiefgrooveguru - Member
Surely the braking response depends upon the forces acting on the pivot, and the location and direction of these will depend upon the caliper position

Braking response depends primarily on chainstay growth doesn't it?

In any event doesn't a brake just create a torque on the bike via the frame member it's attached to? (albeit the forces on by which it does so vary according to where it is attached)

Al, it's a bit too complicated to explain here, but i'll try...

imagine a caliper bolted to a swingarm (standard single pivot), when the brake is applied, wheel rotation becomes swingarm rotation.

imagine a caliper bolted to the seatstay on something like an ellsworth dare, the seatstay doesn't rotate as is moves, it just moves up and down, this means that braking forces can't move the suspension.

back to the single-pivot, it doesn't matter where the caliper is mounted - on top of the swingarm, underneath the swingarm, wheel rotation becomes swingarm rotation.

In any event doesn't a brake just create a torque on the bike via the frame member it's attached to?

yes.

albeit the forces on by which it does so vary according to where it is attached

err... this one probably needs a diagram, but in the case of a single pivot it's mostly down to the height of the pivot above the ground.

anyway, if Cy ever makes a steel, 29er, 'Rocket', i'll have his babies.

I'll also be at the demo day, riding a Rocket first then a Solaris. I can email you the techy newsletter update if you want? It's too complicated to copy and paste on here.

I know all of that awhiles, and my latter point was on an oblique reference to moving the caliper mount to direct bracing forces along the seatstay rather than across it.

The sizing/geom details are on the product page now:

http://cotic.co.uk/product/rocket

Geometry is very similar to my Banshee Spitfire. Given that it's sackful of puppies fun to ride, I would expect the Rocket to be similarly entertaining.

Oh, and location of the brake calliper makes ALL the difference. I rode an early Hemlock prototype, before the calliper mount was moved to where it was on later prototypes and the production frames, and you had to stay off the back brake completely on steeper trails. My prod Hemlock doesn't suffer from the same braking issues at all - the behaviour completely fixed by the changed calliper mount/position.

Can you explain that K? No other factors whatsoever?

Even the rear tire could make a difference.

Can I explain why the calliper position makes a difference? No chance! You'll have to ask someone like Cy.

Having just scribbled this out with pencil and paper, I believe that the reaction force from the calliper (looking from the drive side) placing the calliper:

at 3 o'clock (chain stay) would cause suspension jack
at 9 o'clock would cause suspension squat
at 6 o'clock would cause chain growth
at 12 o'clock would cause chain shortening

The reaction from the tyre would cause chain growth and suspension squat. Obviously there are numerous other force to consider, the most problematic being from the organic mass way above the bike's CoG... 😉

Hmmm the geo looks good to me but I think I'm going to pull the trigger on a Covert this week as I'm not in a waiting mood after my Enigma experience.

Thanks for all the info guys.

CGG can you explain your reasoning?

I don't see how the brake produces anything other than a (net) torque and backwards force at the tyre contact patch, no matter where it is.

if this actually made a difference, we'd know about it, and all teh mfrs would have their own special patented location for it, which was way better than everyone elses.

Frame Size Small Medium Large
Seat Tube (centre-top) 16" 400mm 17.5" 445mm 19.3" 490mm
Top Tube Length 22.6" 576mm 23.4" 593mm 24.1" 610mm
Head Angle 66.5° 66.5° 66.5°
Seat Angle 73° 73° 73°
Chainstay Length 428mm 428mm 428mm
BB Drop +8mm +8mm +8mm
Head Tube Length 100mm 105mm 120mm

Usual Height Range 5'5" - 5'8" 5'9" - 6'0" 6'0" - 6'3"
Stem Length 40-70mm 50-80mm 50-80mm
All measurements are static based on 150mm travel fork. 160mm suspension forks will slacken angles by 0.8 degrees. 140mm forks will steepen angles by 0.5 degrees.

Would have liked reach, stack and wheelbase but it's a start.

Would certainly be interested in riding one.

The same reason that when you use a spanner to do up a nut, you create a torque on the nut through the application of a tangential force, and you receive a reaction force back through your hand. This continues through your body with reactions back from the ground etc. I highly doubt that even the idiotic US IP system would allow you to patent disc brake calliper locations with over a century of prior art...

Not buying that.

Forgot the forces through the axle! So you're looking for the net vector force when calliper-disc, disc-axle, wheel-axle and wheel-ground forces are summed. Move the calliper and there is no doubt that the vector forces have to change. Change the vector force and you change the suspension action under braking. QEcan'tbebotheredtoD.

Exactly. It's about force vectors of the caliper and tyre contact patch forces and their influence at the axle - and how this relates to the pivot point. It's more influential on a long travel / softer sprung, heavy-braking ability bike but moving the caliper can affect where the components of the brake reaction are directed. This means a variation in the directional forces at the axle that will have an effect on the suspension under braking. It's been experimented with on DH bikes in the past and it's a known effect on cars and motorbikes. You could get more variation by changing the pivot point some mm / degrees than the caliper position, but that'd affect more than just braking.

Ta...I'll think about that after a coffe in teh morning!

Most of the original questions seem to have been answered (including him buggering off to buy a Transition ;-)), but to summerise:

- Geometry is on the website. I'll pop some reach/stack up soon if people want it. Never understand why wheelbase is such a big deal.
- Delivery is mid-April for production frames. We'll have a couple airfreighted earlier for photos and product launch at Bespoked Bristol show, plus our first demo at Calver on 14th April.
- The braking thing is evident from experiment, as Kelvin said. The Hemlock suffered horrendous brake jack in prototype form until I moved the caliper around. The Rocket is even better under braking because I've got the caliper even further forward, and the suspension works slightly differently. I'll be honest that I don't fully understand every last bit of physics involved, but just as with the whole front wheel ejection thing from a few years ago, the 'ejection force' vector is altered by the position of the caliper. It seems the further away from perpendicular to the suspension movement you can place that vector, the more neutral the braking response.

I'll be honest that I don't fully understand every last bit of physics involved

But "caliper forward" = "ejection force" pointing close to vertical?

I still don't get/buy it...I don't expect anyone to care of course!

But "caliper forward" = "ejection force" pointing close to vertical?

I still don't get/buy it...

Such is the life of a cynic...

All I ask for is an explanation, I'm not stupid, but vectors do confuzzle me.

Why doesn't the same effect occur at the fork, which would mean road rat type forks would handle differently under braking?

Same for say the Chameleon aftermarket disc mounts?

I thought we were looking at braking forces effect on suspension - those two examples don't have any.

I would imagine that the same could be said for a sus fork though - i.e. the ideal position for the calliper would be in-line with the fork leg.

Alex in your earlier post you say "forces would be vertical. More like in line with the pivot point."

What do you mean?

Sorry - meant "wouldn't" be vertical. Too late for an edit 🙁

But you say "in line with" a point (not a vector/line)

Hang on - I'm confused now.
I'm probably not using correct terminology.
I should just let the experts comment 🙂

I'll go ahead and explain what I thought I understood. Then at least when someone comes and shoots it down, I'll be learning 🙂

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In my head (it's a special place) at the point of breaking the caliper becomes the centre of the rotating motion and the axel becomes the point of the wheel which is trying to move away from that centre - a tangent of that circle would be in line with the pivot point.

Uh OK so the caliper -> axle line is perpendicular to the axle -> pivot line.

I still don't get why it could matter!

CY - wheelbase figures allow me to know the front triangle figures.

I like as much info as possible so that I can try and work out the nature of the bike.... of course riding one is the best way to do this and I will be looking to demo when the time comes.

As for the difference caliper position has.... it is fasinating and a little confusing,,, enjoying it.

Just to prove I don't understand.... does disc size make a difference then?? Should I be looking to run a smaller disc to counteract brake jack....

no.

(because..)

the position of a brake caliper has the same affect on the torque generated as the shape of a spanner has on the torque it applies.

it might be bent, it might be straight, it might be thick, it might be thin, it might be hollow, it might be solid, but the torque generated around the point of rotation is equal to the (input force) x (the perpendicular distance).

input force = the deceleration force on the contact patch of the tyre.

the perpendicular distance = the vertical height of the pivot above the ground.

everything inbetween the contact patch, and the pivot, is just a funny looking bent-spanner.

caliper postion has no effect.

i've come to appreciate brake-squatty single-pivot bikes, a big handful of rear brake and the back end sits down a touch = they're less nose divey than true brake-neutral bikes*.

(*which i also like - you can brake through really heavy braking bumps and simply not feel them, everything is good, but for different reasons: i might suggest that slow steep and techy trails might suit a single pivot, and rumble-strip alpine Dh tracks might suit a parallelogram design, i might suggest this, but that would start another massive argument 🙂 )

Just to prove I don't understand.... does disc size make a difference then?? Should I be looking to run a smaller disc to counteract brake jack....

Hang on... a brake caliper set behind the axle will have the effect of "pushing" the rear tyre onto the ground, increasing the amount of braking which can be carried out without skidding? One in front of the axle will have the opposite effect?

no.

caliper postion has no effect.

It will if the wheel is trying to rotate around the caliper?

it's trying, but it can't move, it's a solid structure.

Hang on... a brake caliper set behind the axle will have the effect of "pushing" the rear tyre onto the ground, increasing the amount of braking which can be carried out without skidding? One in front of the axle will have the opposite effect?

But Cy is saying that moving the calliper made a big difference on the Hemlock, so I was just trying to get my head around it.

When you put the bike in a stand and spin the wheel and brake, you can see what goes on, but that is totally ignoring the tyre on ground force. How large each of these forces is, is one for the physics/maths guys.

But Cy is saying that moving the calliper made a big difference on the Hemlock...

i wonder how he quantified that...

the hemlock is one of the 'more' brake-neutral designs out there (especially with the long rocker fitted: it's basically a parallelogram), but this means that heavy braking causes a lot of nose-dive, because there's very little compressive force applied to the rear system.

you can counteract the nose-dive of a hemlock by leaning back a bit, it's not difficult, and quickly becomes second nature.

I suspect that Cy 'fixed' the nose-dive of his hemlock by unconciously leaning back a bit, then concluding that his modified Seatstay was a success.

just a guess of course...

(and happy to be persuaded that i'm completely wrong about all of the waffle above with some lovely force vector diagrams - ooh, i do like a nice diagram)

All I ask for is an explanation...

Did you? Or did you say "I don't buy it" which suggests you don't believe what someone is saying, whether or not you don't get it. Maybe you should take your cynic hat off and put on a confuzzled one instead, thus eliciting a more positive response. 😉

ahwiles, you've missed off half the force vectors! If you want to explain why something doesn't happen, ignoring the parts that don't fit into your incorrect hypothesis isn't the way to go about it. If the calliper is mounted to something that moves relative to the wheel (as with any calliper mounted to a suspension arm but not with any calliper mounted to a telescopic fork lower) then the forces will be non-internal and reactions will be exibited.

I said "ask" not "asked" and you said you CBA explaining 😀

Pretty please I can haz a vector diagram?

ok - mentioning the difference between the fork and the swingarm, might have made a lightbulb come on (although it's only dim).
...
...
no - it's gone.
🙂

I'm thoroughly baffled by this, though strangely interested (its making me feel a bit unclean though).

Ahwiles can you explain this from the Commencal Supreme DH V2 Tech' book?

Disc brake mount adjustment
Your Supreme DH comes with an adjustment that allows you to modify the handling balance of your bike while under braking.
During this phase, the rear braking forces act to compress the rear suspension, resulting in a loss of traction, and therefore its
efficiency. This is sometimes referred to as “brake jack”.
Your disc brake hanger has three positions; position number 3 being the one that least interferes with the rear suspension during
the braking phase. Positions number 2 and 1 are intermediate positions. Don’t hesitate to go out and try each position to determine
which one works best for you.

(source: http://www.commencal.com/news/supreme_dh_uk.pdf)

Unless I've totally misunderstood, isn't this a similar thing to what Cy is suggesting, that caliper position can have an effect? Its certainly the 1st time I've seen this type of adjustment on a frame.

Yes. When you think about it more it also suggests that the larger the disc, the more it matters. I imagine it causes some fun on high performance cars with semi-trailing arm rear suspension - one of the reasons old 911s throw you backwards off the road if you brake when cornering?