Custom Frame Geometry Help

[1a2bcio8]

So I'm wanting to order a new frame today from Marino and experiment with some geometry. The trouble is I'm very unsure about how to measure everything up appropriately regarding how changes in one area effect another area. My template is the Inspired 4play from which I am making some amendments.

Chainstay: 380mm (same)

Bottom Bracket: +15mm (-5mm)

Head Tube: 74.5

I'm fairly confident with the above but I don't know how to relate it to the wheelbase in order to increase the reach by 30mm so that if I change from a 90mm stem to a 70mm stem (maybe even 50mm) I don't end up with a cramped front end.

Furthermore, I'm also planning to increase the seat tube length by 150mm. I measured the 4play as aprox. 250mm from the centre of the BB to the very top of the tube beyond the centre with which the top tube connects to it. Is this the correct method of measurement?

Finally I want to increase the head tube height from 120mm to 180mm. Does this and the seat tube difference alter the way in which the rest of the geometry needs to be calculated?

I'm extremely ignorant about these matters so any help would be really appreciated.

Thanks.

[Ali C]

[1a2bcio8]

Don't like the geometry, Ali?

[Ali C]

not really

What do you want the bike to be better at? I can't see many advantages from a lower bb? And are you serious about a 180mm headtube length? I can see why you would want to do it, but man that will be one ugly duckling!

[1a2bcio8]

Hmmm, thinking about it, that's a fair point about the head tube. I've not been hugely fussed about aesthetics but it is nice to have a bike that looks okay. However, I thought the increased seat tube height might compensate the appearance. The relative increases would be proportional. Perhaps I'll think about limiting it to 30mm but, either way, I'm definitely increasing it so I'm not using so many stem stackers.

Regarding the BB drop I'm just experimenting. I understand it helps with manuals and stability so I'd like to try it out. Also it should put me a bit more upright which I'm after because I've always been at risk with lower back trouble because of my height.

I've also just noticed on the Marino geometry form that I need to include a seat tube angle...

Does anyone know what the 4play seat tube angle is? It's not on the tech specifications on the site.

[Ali C]

I saw a marino with a 130mm headtube and even that looked a little odd, it had a higher seat tube length than an inspired too. If you just want it to fit and are not worried about looks then go ahead, but it will look very odd and perhaps won't be as strong (spacing the tubes so far apart effectively is turning the front triangle into a square).

The low bb will make is more stable for manuals, but is that a good thing? It will make spins harder, that includes turning while manualing.

If I was you I would find out what rearch you wanted and give all the angles you want and don't worry about the wheelbase, that is the least important measurment you can give. the wheelbase will be whatever it turns out to be once the important stuff is done.

[1a2bcio8]

The wheel base is required on the form and, seemingly, this is how the reach is decided in relation to the other geometrical specifications. Unless I'm misunderstanding which is very possible.

I'm wary about being sucked into your concept of what's important on a bike but when you said that it does make me concerned for 180s out of manuals which I love doing.

[Ali C]

I'm offended that you are wary about my concept of geo To be fair, I mostly take Mark's advice anyway haha.

Send Marino an email rather than fill out the form, that way you can explain every detail you want rather than be restricted by the form. Trials rider seem to be obsessed with wheelbase! As I said, wheelbase is not important, that doesn't tell you how long or short a frame is going to be. you could make a frame with an 1100 wheelbase but make it feel shorter than a bmx if you put a 100mm bb on it and a 60 degree head angle.

Take a look on Tartybikes and find out what the reach is on your current frame, that's the best way to find out how long or short a frame is going to feel. If you just put a steeper head angle on your bike (pivoting at the lower headset cup) then that would keep the same wheelbase but increase the reach by roughly 10-15mm. If you want more reach then it's a case of lengthening the front triangle.

[1a2bcio8]

I'll do that, thanks.

The thing with geo is that it also has a subjective dimension. Being that I'm significantly taller, with longer limbs than most people, it obviously changes the nature of my riding experience and needs. I appreciate the sense of the theory you put forward but the reality doesn't always work out that way according the variables of the person. But still, there's a good chance you're correct about my needs and I have to admit a position of ignorance relative to your own

Thanks for taking the time help.

[Revolver]

Remember bar height changes the feel of BB height, but moving the BB would affect how the frame rides - so you could always add more to the BB for easier spins and compensate for the comfort by raising your bars more. That is if there's any room for more stackers on your forks...

[1a2bcio8]

There's no room. I've used all the space on the steerer... thanks for the advice anyhow.

I'm still struggling here. I've checked out the reach on Tarty but it's to the top of the head tube. I think I may just ignore the changes to the head tube and BB because they were the least important but the ones that mean I have to think about things I don't understand. The head angle and seat tube length were most important so I'm going to just stick with that to make life easy.

[Mark W]

There's a Flash programme online you can use to sort of test geo and work out other measurements, but I can't remember what it is/what it was called. (EDIT: BikeCAD! http://www.bikecad.ca)

I have a feeling the Fourplay ST angle is around 75 or 76° or something like that - they're relatively steep compared to most other frames (mainly comparing them to BMX frames where it's a bit slacker). Speaking of, the main reason for the angle of the ST on the Inspired is to try and get a slammed seat to clear the tyre. Bearing in mind you're raising your seat tube up quite a bit, the seat's going to be further away from your tyre. I'd have a feeling you could probably get your seat tube angle a fair bit slacker, which would in turn make the bike feel a little roomier when doing certain moves. For example with a barspin, when you try and pinch your seat with your legs it'd be further back, so if you're taller it'll allow you to possibly straighten your legs more and be able to get them away from your bars a bit more? Not sure if that's really a problem you've had. Should you happen to want to get involved with moves like hang 5s or hang nothings it'll also probably be a bit more suitable for when you sit down on the seat as it'll effectively make your seat further away, even though the bike could still be the same overall wheelbase. Also, if you're sitting down and pedalling it'll be more comfortable for you - again, not sure what you requirements are from a bike but that may help.

As something else to consider, you could always tweak the back end length slightly to make it more suitable for the gear ratio you run (so you can get away without needing a tensioner, resulting in less stuff to hit/break). I can't remember which ratio you run, but it may be worth checking out this website to see what sort of CS length you need to have your setup without needing a tensioner. Having to use one isn't the end of the world or anything, but it's pretty nice to avoid using them. If you're using horizontal dropouts on it it'd also be good to check out the length you'd need primarily to avoid having to use a half-link too.

Only other thing is that I'd echo Ali's thoughts on the BB height. I found certain spins harder to do on my 2011 Fourplay when it was in standard form just because it always felt like when I was either popping a bunnyhop or doing a manual that my feet were that tiny bit too low. I guess you're taller, but it still seems like a +15mm BB might be a bit too low. Unless you can feel when you're riding that it'd be better to have your BB dropped a bit, it seems like a change that might adversely effect some moves for not much real benefit for other moves (if that makes sense). You're increasing the reach of the frame by quite a large amount so it may be that dropping the BB height as well might make it quite cumbersome in some ways. I suppose I'm sort of looking at it with how it'd feel for me mainly, and I know that you're a fair bit taller than me so your needs are different, but it sounds like it might be a bit much even for you.

Having said that, if you're steepening the head angle that'll probably help, and if you decide to tweak your CS length to resolve any chain tension issues and make the rear triangle a bit shorter that might help make it a bit more nimble/lively. For example, if you're running 22:16 then 374mm is where your wheel would want to sit naturally (without a half-link), so reducing the rear end length to that amount might be a bonus. When I went from the '09 Fourplay to the '11 I instantly noticed that the 5mm difference in CS made it feel nicer for hops and spins, and having had a play on a 2012 Fourplay (which now have 375mm stays) they feel that bit better too. Possibly something to consider...

[Luke Rainbird]

For reference, here's what you've initially outlined, Ben;

I can see what you're aiming to achieve with the longer Seat/Headtube for sure, but I think there's a limit with how far you can go before the aesthetics fall apart a bit. I know you've tried a lot of different high-rise bars though. Perhaps it's time to BMX-bar that shit up. Either way I think you're going to have to compromise with the looks somewhere to achieve the feel you're after.

[RobinJI]

I think/hope this answers your original question? The grey drawing in the background's standard foreplay geo guessing at a 46deg seat angle.

[FamilyBiker]

how tall are you?for 170 to 180 cm go for something like 1020 wheelbase/73 headangle/+35bb,for 180+ go for something like 1040 wb/73headangle/+45 bb-thats awesome for street riding.

get a seattube as short as possible for maximum clearance,i.e. 210 to 240 mm.mine has 1000 wb/73hta/+25 bb and it feels a little bit too short for me,i am 181 cm tall .

chainstays as short as possible,like 365mm

just my 2 cents

edit;you wouldnt even have dreamed of what massive difference a 140 headtube makes to a 120 one...

get a 120 one and fill 20 mm with stackers

Edited April 29, 2012 by FamilyBiker

[1a2bcio8]

Ah man, this is a real lesson in learning... and much more difficult and slower to resolve than I was hoping

Essentially I want a frame in order to get bar spins consistent more safely and learn nose manuals on. I'm intending to go back to inspired frames afterwards.

The seat tube angle, I found out from Dave, is 73. Do you think that's a decent sort of slackness? I do hang 10s and would definitely like to consider geometry which would help them.

Regarding bar spins I actually stood on the pedals and used a ruler to project the line of seat tube and saddle and it seemed pretty spot on regarding fitting between my knees when straight. I'm now considering dropping the seat tube by 70mm and relying on the seat post a bit more just so it doesn't interfere with tailwhips so much when I want to practice them.

I'm going to accept what yourself and Ali says about the bb height and I'll keep it the same. I'm not keen on going in the other direction though because I don't like the idea of a more twitchy bike. I'd rather emphasise feeling in control over increased ability to spin. Perhaps I'm misjudging again though as I'm thinking of my experience of czar which had a crazy bb height and was difficult to land without tilting to the side. Then again the idea of easier 180s from manuals is very appealing... What are you running at the moment and is it working out? You're pretty tall yourself as I remember. Thinking about it some more, every time i increased my bar height I love the increased loopiness. You're saying I'd get more of this, in a sense, if I increased the bb height?

I'm actually running 22:15 at the moment but I was thinking about trying 24:16 or even 15. I don't care for big gaps or sidehops and I think I can adapt with that to still do them even if smaller. I don't think I want to change the chainstay length though. It took me two months to get used to the shorter length on the 2011 4play.

I've decided to keep the head tube at 120mm after all and extend the seat tube by only 130mm. That does look pretty ugly and, in fact, not very easy to ride with regards to some moves

It turns out it's a 73 degree seat tube angle...

That's exactly the sort of thing I'm after though; some way of overlaying my current geometry over the differences I want to employ.

Thank you everyone for your input. It's very helpful and illustrative of just how little I actually know. It's helping me narrow down my thinking.

Edited April 29, 2012 by Ben Rowlands

[Mark W]

+45mm BB sounds like it'd suck The '09 Fourplay felt pretty high at +35, let alone adding another 10mm to it. Having tried a frame out with a steeper head angle, I don't get why you'd want to stick with a 73° head angle either. Just feels pretty sluggish in comparison.

I think Ben's specifically going for a longer seat tube as he's a pretty tall guy, and therefore wants a taller frame in general.

[FamilyBiker]

+45mm BB sounds like it'd suck The '09 Fourplay felt pretty high at +35, let alone adding another 10mm to it. Having tried a frame out with a steeper head angle, I don't get why you'd want to stick with a 73° head angle either. Just feels pretty sluggish in comparison.

I think Ben's specifically going for a longer seat tube as he's a pretty tall guy, and therefore wants a taller frame in general.

thats a point

[1a2bcio8]

Would one of you kind people please create me an image of this geo? Additionally, what wheel base would this give me?

Head angle: 74.5

Head tube: 120mm

Chainstay length: 380mm

BB height: +20mm

Seat Tube: 370mm (assuming the current 4play is the 250mm I measured)

Reach from between the bb and top of the headtube I want to extend by 20mm from 660mm to 680mm (so I can drop down to a 70mm stem)

Seat Tube angle: 73

Any further critique/advice on this geometry?

Edited April 29, 2012 by Ben Rowlands

[FamilyBiker]

What,360reach???

mines got 640

Edited April 29, 2012 by FamilyBiker

[1a2bcio8]

oops...haha

Sorry I'm after 680mm reach when my 4play is 660mm. So an extra 20mm from that.

I can only blame how exhausting I'm finding this whole process...

[LEON]

How tall are you out of interest? I've seen your mentally high front ends and wondered how/why.

But like people have said I wouldn't go bigger than 120mm on the headtube, both mine are, I think the 26GHZ has a 130mm (with a -10 bb I found I actually needed less headtube) but apart from that I've never seen a longer one.

I'd try as many different bb heights as you can, I have +10mm on my 24"...the old fourplay at 35mm was just way too high in my opinion, I haven't tried any other 24's but those are the two extremes, I think I could probably get along with +15, as always with Marino buyers, do your homework! Because once it's welded it's too late.

Edited April 29, 2012 by LEON

[FamilyBiker]

How tall are you out of interest? I've seen your mentally high front ends and wondered how/why.

But like people have said I wouldn't go bigger than 120mm on the headtube, both mine are, I think the 26GHZ has a 130mm (with a -10 bb I found I actually needed less headtube) but apart from that I've never seen a longer one.

I'd try as many different bb heights as you can, I have +10mm on my 24"...the old fourplay at 35mm was just way too high in my opinion, I haven't tried any other 24's but those are the two extremes, I think I could probably get along with +15, as always with Marino buyers, do your homework! Because once it's welded it's too late.

this

[1a2bcio8]

I'm 6'5"

I think all of this is a bit too much for me to think about. I might just get another 4play and wait for things to hopefully change in the direction that I'm after...

Thanks for all the advice though! And sorry if it's gone to waste.

[Blake]

Get some super duper high rise bars maybe? Ali C knows about high rise bars. . . .ask him