Custom hardtail - thoughts and questions about geometry. Long!

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Custom hardtail - thoughts and questions about geometry. Long!
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Posted: Jan 20, 2020 at 19:33 Quote
Hi guys.

Looking at getting myself a steel hardtail. I want something that will force me to be a better rider and be more fun on easier tracks.

I really want something super long and slack, but not too low (I'm sick of pedal strikes). The only bike I can find that looks pretty sweet is the Cromag Doctahawk, but it's designed around a 180mm fork and isn't cheap. I was thinking more like a 130 or 140mm fork.

I'm thinking something with less than 65 degree effective headangle (prefer between 63 and 64), greater than 500mm reach (thinking more like 520 or 530), but not too low to the ground, so maybe only a 15mm bottom bracket drop.

However, I've found the Marino site, and they look good and have plenty of positive feedback, however there's a few things I'm not sure about. I know that hardtails will sag at the front so the effective headangle is not the same as the actual headangle, same for the seat angle. I know that lots of hardtails are now specced with fat tyres (2.8 or wider) which I don't really like the idea of. Also, I've never built a bike up before.

So, a bunch of musing and a bunch of questions. FYI I'm 188cm and around 110kg, so not a lightweight. Current bike is an s3 stumpjumper evo. Awesome bike, except not long enough, and too low to the ground!


1) 29 or 27.5? I really like 27.5 but everyone I know is all excited about the bigger wheels. I'm a big guy, so they make sense for me, but they're also maybe not quite as strong, so maybe they don't? I really don't know. I'm no big hucker, but I presume 27.5 would be more playful.

2) Boost or non-boost? Boost obviously is the answer, but non-boost stuff is sometimes really cheap, which makes building the bike up more affordable.

3) How slack is too slack? I was thinking a 63 degree headangle, but a 140mm fork with 35mm sag would leave me with nearly 65 degree effective headangle. Maybe 62 degrees?

4) Can I get away with not being too low? I know that low to the ground is great for handling, but it sucks for pedal strikes.

5) Built in headset? Obviously it can't be changed, but would offer advantages in terms of durability I presume.

6) How long should the headtube be? What type should I select out of 44mm, 1 1/8, or tapered 41.8 / 51.8 - 45x45? I have no idea what that stuff means, but normally 1 1/8 is normal?

7) Chain stay length? Normally I like long and stable, but for a playful hardtail maybe a bit shorter. 425mm?

Cool Seat tube angle? Steep is good, but also, if I get front sag presumably that means that a 76 degree seat angle would effectively be nearly 78 degrees? That sounds right doesn't it? And 78 seems pretty good to me.

9) What the hell is bottom bracket offset?

10) What's the best standard for pressfit? Have a choice of 68mm, 73mm, 85mm, or 100mm, all BSA. Or 73mm press fit (my instinct is that one).

11) IS or post mount? I feel that IS is more standard, but post mount seems increasingly common.

12) What the hell is fork ATC? Is that the travel or the overall length of the fork?

13) What dropouts are best?


So obviously a bunch of stuff in there. The big thing is the geo though.

My evo is super long and slack and low. So I'm used to it. I have no problems with the headangle, in either the low or high position, and feel it would totally work lower. I have no problems with the reach, I just wish it was longer, even though it was crazy long when they came out. I do have an issue with the bottom bracket, on technical stuff, especially climbs, I'm always cracking my pedals on rocks.

Hopefully that all makes sense.

Any feedback would be welcomed, including suggestions for bikes that might exist that fit my requirements, if they can be bought in Australia!

cheers.

Posted: Jan 20, 2020 at 21:08 Quote
It sounds like you are on a good track. I spoke with lynskey before I bought my bootleg. They estimate 1deg steeper STA and HTA with sag. I'd probably go 74 or 75 deg STA.

As for fork travel, I keep finding I like longer and longer. I'd build it around 160 in 27.5 or 150 in 29er. Another option would be to run mullet, which is what I would do. HTA at 63-64 seems to be pretty ideal to me.

Chainstay no longer than 425 IMO. But I guess it depends on how much fun you want to have.

I'd go for a standard tapered headtube. Get a nice king headset - it will last as well as any Integrated job.

Make sure you post photo when you have one. Sounds like a nice ride

Posted: Jan 20, 2020 at 21:33 Quote
No offence, but some of your questions suggest that you may not fully have the grasp of the bike geometry thing and you are mostly influenced by what's en vogue these days. I'm not saying I'm any better at this by any means!

The safest way to go would be to copy a bike you like and change a few things that you think need improvement. A variation of this would be to copy a popular frame that resembles what you have in mind. The trap is to pay attention to the "headline" numbers like head angle, seat tube angle and reach, and ignore other key values.

Front vs rear centre length for example can really define the bike's handling among other things. A really long reach with a 63deg HA would need longer chainstays or a steep SA to allow you to weight the front wheel. But a very steep SA combined with a slack HA would give you a very short effective top tube for a given reach, like many current bikes have and I personally don't like.

The ideal scenario would be to do it like the surfers. Find a builder you trust, tell them what you want out of the bike, where you ride and what bikes you've ridden and liked so far and let them decide on numbers.

Most of us, even those who have ridden plenty of bikes, don't really understand how numbers combined make geometry work a certain way. For every characteristic you want to exaggerate, you compromise another. Just like surfboard shaping.

My two suggestions: For 27.5" (normal or plus) check out Whyte's 900 series hardtail geometry and make something based on these. Adjust bb height if you don't like plus tyres, it's too low. For 29" check out Pipedreams' Moxie. Both these examples get great reviews and their geos have been getting refined for a few years now.

Posted: Jan 21, 2020 at 1:11 Quote
bike geometry goes two ways

one way makes it go straight (stable)

the other way makes it turn (nimble)

what ever you do you are just doing a tug of war at every step. where in the world can you build a bike with such options?

1) 29 or 27.5? I really like 27.5 but everyone I know is all excited about the bigger wheels. I'm a big guy, so they make sense for me, but they're also maybe not quite as strong, so maybe they don't? I really don't know. I'm no big hucker, but I presume 27.5 would be more playful.

my 5 year old used a 20 inch wheel, now he's 8 and on a 24 inch. the 11 year old is most comfortable on a 26 inch. give him a 29 inch and his butt will kiss the tyre when moving back. if you're tall then naturally a 29er. but if you want 27.5 nobody can stop you.

2) Boost or non-boost? Boost obviously is the answer, but non-boost stuff is sometimes really cheap, which makes building the bike up more affordable.

doesn't matter

3) How slack is too slack? I was thinking a 63 degree headangle, but a 140mm fork with 35mm sag would leave me with nearly 65 degree effective headangle. Maybe 62 degrees?

if you want to climb then you want 66 degrees or more. that's why the hardtails don't come in 63. 63 only goes down. that's why they're on downhill bikes.

4) Can I get away with not being too low? I know that low to the ground is great for handling, but it sucks for pedal strikes. use 170mm cranks with low profile pedals.

5) Built in headset? Obviously it can't be changed, but would offer advantages in terms of durability I presume.

if someone makes them?

6) How long should the headtube be? What type should I select out of 44mm, 1 1/8, or tapered 41.8 / 51.8 - 45x45? I have no idea what that stuff means, but normally 1 1/8 is normal?

tapered.

7) Chain stay length? Normally I like long and stable, but for a playful hardtail maybe a bit shorter. 425mm?

if you want a long bike you want a longer chainstay. but it's up to you. the shortest chainstay is limited by your wheel size.

Cool Cool Seat tube angle? Steep is good, but also, if I get front sag presumably that means that a 76 degree seat angle would effectively be nearly 78 degrees? That sounds right doesn't it? And 78 seems pretty good to me.

it doesn't matter. just move your saddle fore-aft.

9) What the hell is bottom bracket offset?

is it BB drop?

10) What's the best standard for pressfit? Have a choice of 68mm, 73mm, 85mm, or 100mm, all BSA. Or 73mm press fit (my instinct is that one).

there is no best standard. the most important thing is frame shell tolerance quality control.

11) IS or post mount? I feel that IS is more standard, but post mount seems increasingly common.

no idea

12) What the hell is fork ATC? Is that the travel or the overall length of the fork?

no idea

13) What dropouts are best?

no idea

Posted: Jan 21, 2020 at 6:48 Quote
29 vs 27.5 is preference. The idea of playfulness is created by the geometry not the wheel size. The benefits of smaller wheels are lower rotational weight, shorter wheelbases and maybe a little easier to spin up (I’m not sure the physics bear this out but you can be the judge of that). The idea that 29ers aren’t nimble is a throwback to the old days of massive compromises in geo. Long chain stays with long wheel bases and steep head angles and short front centers led to really poorly balanced handling. Just remember that smaller wheels bring your bb lower to the ground all other things being equal. 29 wheels will give you the ability to drop your bb more with less risk of penalty.
To boost or not to boost. You could go 145 and change between 142 and 148 depending on the axle you use. I think surly does this.
So bb offset is how far forward the bb is from a neutral position. This lets you move the rear wheel forward to shorten chain stays, but will slacken your effective seat tube angle.
Atc is axle to crown. This is the measurement that determines the fork length your geometry will be based on. Every fork maker will provide this as a measurement.
I would say if you are a home mechanic and don’t have access to a full set of mechanics tools stay away from pressfit bb setups. The most common and plentiful in terms of available cranks and bottom brackets is BSA 73. BSA68 is generally for road cranks but can be spaced out to 73 with spacers without issue. BSA83 will limit you to dh cranksets with their longer spindles. 100mm is for fat bikes.
Go with the 44mm straight head tube. Headsets are plentiful and it allows the widest range of forks. This is just personal preference though. The only benefit of inset cups is stack height. They are not appreciably different from external cup headsets. They use the same bearings and the same methods to seal the bearings.
IS vs PM is just another preference thing. You will have to buy adapters regardless to run anything bigger than a 160mm rotor.
I personally like sliding dropouts but I’m a single speeder so there is that.
I personally rode a moxie on 29 wheels with 170 cranks. It is designed around a 140mm fork but I’m running with a 130mm with a 51mm offset and find it gives me a nice balanced ride. Fork offset coupled with head angle will determine your trail. Trail plays a big part in a nimble front end. Too much trail will be slow and floppy feeling at low speed but will be nice pointing straight downhill. Lower trail will give you a more speedy front end but might not feel as stable at speed. As with everything you’re gonna have to compromise.
Pedal strikes are as much a technique issue as a geometry issue.

Posted: Jan 21, 2020 at 13:12 Quote
Hey I can answer any specific Marino questions that you may have... Just send me a PM. As for the other stuff I think I can answer some questions there too...

1) 29 or 27.5? At your height 29". Sure there are some benefits to smaller wheels (strength is one to consider with your weight) but I'd take the feeling and roll over of 29" over smaller wheels any day. You're big, why shouldn't your wheels be?

2) Boost or non-boost? Boost.

3) How slack is too slack? If you don't want to climb a lot sure 63 is not too bad... But I'd go for something like 65 for an allround bike.

4) Can I get away with not being too low? If you suffer from pedal strikes get shorter cranks. With a 29er HT with a relatively long fork you should be looking at a BB drop of between 40 and 70mm.

5) Built in headset? Get a straight 44mm tube and go for an external headset.

6) How long should the headtube be? Depends how high you want the front end to be. Some like it low (100-120) others like it high (130-150) note that this matters a lot for climbing as a very high front end mayb make that suffer.

7) Chain stay length? Again, I prefer a balanced bike... Depending on your reach I'd go for a bit longer chainstays. With a reach of 520 if look to get a chainstays around 450. However, static. reach =/= the reach when you are sitting at sag, that will be longer (so 520 reach is crazy long).

Cool Seat tube angle? Steep is good. I don't think we have found the limits for seat angle steepness.

9) What the hell is bottom bracket offset? How far offset the seat tube is in relation to the BB. Increases tire clearance for instance.

10) What's the best standard for pressfit? Go for 73mm BSA.

11) IS or post mount? IS is cheaper and works fine.

12) What the hell is fork ATC? Axle to crown length.

13) What dropouts are best? Depends if you want to single speed or not... Sliding if you want to single speed...standard if you don't care about that.

Hope that covers it...

Posted: Jan 21, 2020 at 17:17 Quote
Some really solid answers here, thanks!

In terms of geometry, it's obviously hard to work out when it's all theoretical.

However, when I bought my Evo, everyone was talking it up as being somewhat "extreme" geometry that wouldn't be an all rounder.

Whereas, when I ride it, it seems completely normal. It climbs, it descends, it turns. It's not outrageous. Even in the low setting, with a 63.5 degree HA it's completely fine.

I do find myself wishing it was longer, and slightly higher (I'm getting some 170 cranks for it, but I definitely pedal strike a lot).

My thinking is that the steep seat angle will help with the climbing. And it needs to be a bit longer to compensate for that seat angle. I want that effective headangle of around 64 degrees. I want it low, but not crazy low, and I want it stable. Evo has chainstays of 443mm and it's just not something I notice, so presumably that's pretty good. I think a bit shorter just because this will be a more "playful" bike.

Apart from those basic ideas, the rest is probably not as important (the dropouts, the brake mounts etc).

Definitely very helpful!

Posted: Jan 21, 2020 at 19:56 Quote
1.I won't even try to start this debate, but #27.5gang for life.
2.Boost, this is slowly becoming the standard and if you ever want to move wheels to a new bike you will want boost.
3.All depends on the travel. In my opinion hardtails shouldn't go past 130mm travel simply because you have to slacken and lengthen the bike so much to minimize hta change that soon you will have a dh bike without the capability of one. Go with 130 mm and somewhere between 64 and 66 degrees.
4.This is another reason I say no more than 130 travel. Short cranks should solve most problems with a moderate bb height.
5.In my experience all headset configurations work, but you may want to try angle sets if you are still iffy about ht angle.
6.130-150 seeing that your front end will lower at sag. Either get a longer ht or riser bars.
7.Short ass chain stays are a must on hardtails. This minimizes bb drop and short ends aren't just industry propaganda, they are fun to ride.
8. Moderately Steep
9.Already explained.
10.Get threaded if you can. Press fit belongs on bmx and djs where you won't feel bad hitting it with a hammer.
11.Theoretically post mounts have less flex, but you won't notice it. Go with the cheap option.
12.Axle to crown.
13.If geared go with a through axle ready dropout, if not get sliding which will accommodate single speed and geared.

Posted: Jan 21, 2020 at 20:22 Quote
Nice response!

I have to admit that I really like the feel of 27.5. That being said, I've never ridden an XL 29er so I could get a feel for it on a bike my size. Maybe I need to do some demoing to try out different wheel size

Posted: Jan 21, 2020 at 22:59 Quote
I think our mates here are doing a good job answering your specific questions, I'll just add some comments regarding the specificity of hardtail design.

Hardtails behave very differently from FS when it comes geometry changes. That's something a lot of people seem to forget, including a ton of brands. You shouldn't just copy the geo you know from an FS.

- from the moment you seat on it, hardtail's head angles steepen, while FSs slacken even further. The head angle of an HT with a 140mm fork steepens 1,5° when sagged.

- in a FS the reach shortens when sagged, on a HT the reach gets longer. Using the same 140mm example, the reach grows about 15mm after sag. And it keeps growing, generally when you least need it, like on a steep section where you need to put your weight backwards. Proper HT design must consider this

- the longer the fork on a HT, the worse the negative traits of it are accentuated. When faced with an impact, obstacle or G out, the geometry changes caused by fork compression on a HT are all contrary to what you'd want: head angle gets steeper, wheelbase shortens, reach grows. The longer the fork travel, the worse this geometry change is.

Some brands do a great job by showing a geometry table with sagged values (Cotic, Bird or Nordest I believe).
BTR, for example, takes all these factors in account when designing their frames, go have a look at their reach and head angle values. They also have some great writings on this topic, go read them

Posted: Jan 21, 2020 at 23:22 Quote
gotohe11carolina wrote:
29 vs 27.5 is preference. The idea of playfulness is created by the geometry not the wheel size. .

This answer was fantastic but I disagree with this one part. Anyone who has swapped out a 29 rear wheel on a bike for a 27,5 will tell you the handling is significantly different. I think of wheels like I do downhill skis. More sidecut will make for a ski that turns sharper. Less sidecut is great for flotation but doesn't turn sharp. Bike wheels are the same. When you corner, you lay your bike sideways and the arc of the wheel that contacts the ground is like sidecut of a ski. 27.5 has a sharper contact arc and therefore turns more sharply.

I have a 29er hardtail with a shorter wheelbase and shorter chainstays than my 27.5 hardtail and it doesn't corner nearly so well and is much less playful. When I mulleted the 29er it ended up riding more like the 27.5

I still say mullet the bike.

Posted: Jan 22, 2020 at 2:29 Quote
Arierep wrote:
I think our mates here are doing a good job answering your specific questions, I'll just add some comments regarding the specificity of hardtail design.

Hardtails behave very differently from FS when it comes geometry changes. That's something a lot of people seem to forget, including a ton of brands. You shouldn't just copy the geo you know from an FS.

- from the moment you seat on it, hardtail's head angles steepen, while FSs slacken even further. The head angle of an HT with a 140mm fork steepens 1,5° when sagged.

- in a FS the reach shortens when sagged, on a HT the reach gets longer. Using the same 140mm example, the reach grows about 15mm after sag. And it keeps growing, generally when you least need it, like on a steep section where you need to put your weight backwards. Proper HT design must consider this

- the longer the fork on a HT, the worse the negative traits of it are accentuated. When faced with an impact, obstacle or G out, the geometry changes caused by fork compression on a HT are all contrary to what you'd want: head angle gets steeper, wheelbase shortens, reach grows. The longer the fork travel, the worse this geometry change is.

Some brands do a great job by showing a geometry table with sagged values (Cotic, Bird or Nordest I believe).
BTR, for example, takes all these factors in account when designing their frames, go have a look at their reach and head angle values. They also have some great writings on this topic, go read them

There's no reason to ride a hard tail other than on gravel roads. That's my motto.

Posted: Jan 22, 2020 at 22:25 Quote
chickenrunz wrote:
There's no reason to ride a hard tail other than on gravel roads. That's my motto.

Wrong thread mate.

Posted: Jan 23, 2020 at 0:43 Quote
My suggestion if you are looking for a custom frame in Australia is Mooro cycles, lovely looking things: https://www.moorocycles.com/catalogue?lightbox=dataItem-jtv34grx1

Posted: Jan 23, 2020 at 1:46 Quote
Just checked them out. Their road bikes look cool, but the mountain bike is WAY too short and steep.



Having read all the feedback, I'm thinking the following.

62.5 degree headangle. With a 140mm fork at 25% sag, that's roughly a 64.25 degree effective headangle. If I change to 160mm that would be 63.5 degrees effective. I'm very happy with both those numbers.

76 degree seat tube angle. Again, that gives me roughly 77.75 degrees effective angle, so should climb well.

25mm bottom bracket drop. I do a lot of riding with more technical and rocky kind of stuff, plus I've got big feet, so pedal strike sucks. That's still low, but definitely not crazy low.

520mm reach. 490 feels WAY too short for me, so happy to stretch it out. If I hate it I can always get one of those fancy zero mm stems, but I feel good about 520.

425mm chainstay length. That's short, but not crazy short, but should add a little bit of pop.



Definitely going to go for the 140mm fork option, as I don't want a rugged enduro bike, I want a cool trail bike.

Still torn on the wheel sizing, but strongly leaning towards the 27.5, for no real reason, apart from I like it.

I'll continue to ponder!

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