[Closed] XC race bike head angle

Bit of a tangent, but I just listened to this podcast yesterday, Cesar's got some very interesting thoughts about XC bike geometry...
http://www.vitalmtb.com/features/The-Inside-Line-Podcast-Cesar-Rojo-Founder-of-UNNO-and-Cero-Design,2045

It covers head angles, BTW.

Got hold of some 51mm offset forks. They are better than the Rebas as forks (Fox F32), slightly shorter A2C too. But the bike is completely different in corners. Really solid, stable and planted, I can feel the front hooking up really well. And it's really nippy in the twisty bits.

I could definately tell the difference between my dads 42/44mm offset Lyriks and my 46mm Pikes. They do have less of a tendency to tuck under. I find higher offsets more reactive and more forgiving when weighting up the front too much.

Giant went back to 46mm offset Lyriks on the 2018 Reign, after running 46mm offset Pikes on the 2015 and 42mm offset Lyriks on Lyriks on the 2016 and 2017.

The complete opposite to Mojo and Transition.

But moving from 46mm offset to 51 mm offset should have exactly the opposite effect to what he is claiming it to have .

Maybe, depends how we describe things perhaps. My 29er has a long offset fork (55mm) and lower trail than most MTBs by 25% or more yet it's very stable, corners beautifully. It's just not the slack/floppy kind of stable. Trail, weight distribution and HA-offset combo are a 3-way thing that can be balanced up more ways than I can get my head around.

But as far as effect, 5mm change on offset isn't hard to spot if you're in tune with the bike, whether it matters or not is another Q.

I could definately tell the difference between my dads 42/44mm offset Lyriks and my 46mm Pikes. They do have less of a tendency to tuck under. I find higher offsets more reactive and more forgiving when weighting up the front too much.

But moving from 46mm offset to 51 mm offset should have exactly the opposite effect to what he is claiming it to have .

What makes you say that?

The front wheel is now further away from me, so it's a bit like having a slacker Ha in that respect. Trek seem to have specified a steeper actual HA whilst relying on the increased offset to stop it feeling nervous.

I think the riser bar has definitely helped the overall handling. Previous setup was flat 695mm bar with a 70mm stem flipped. That was probably the wrong choice too. But the big difference is in the feedback when I start to turn and when I am leaning over. There is push back from the front tyre, it feels secure and solid. Whereas before I'd lean over and the front wouldn't push back which is why it felt insecure, vague and made me nervous.

Why think about cutting them down if they help the riding position

They'd still be risers.

Today though I put the saddle forward 5mm or so and the width felt easier to deal with so maybe I'll stick with it a while.

It's good to be reminded that it is worth persevering with setup

This... I think a lot of people get rid of bikes because they aren't 'right' when really buying a bike may be only the start of the process of getting it right.

Same trail figure from a steep HTA and less offset is less carvy, easier to overweight or over-lean,

That's how it felt. HA is something like 71 degrees which is why I think it needed the 51mm offset.

This brings and interesting comparison with my Anthem which feels much more stable at 69.25 degress eather than 71 static.

You fitted an angleset? Or the bushings?

The front wheel is now further away from me, so it's a bit like having a slacker Ha in that respect. Trek seem to have specified a steeper actual HA whilst relying on the increased offset to stop it feeling nervous.

What you are describing is precisely the effect Trek (and Gary Fisher before Trek) meant to achieve with the G2 geometry.

It is nicely illustrated here:

[url= http://http://blog.artscyclery.com/ask-a-mechanic/ask-a-mechanic-29er-forks-46mm-or-51mm-offset/ ]null

Good article Ramsey but it's coming at it from a slightly different point of view. He seems to be talking about choosing offset to get a particular characteristic. However Trek designed the rest of the package to take advantage of the extra offset, so it would seem that reducing it moves the steering forces into something undesirable - for me at least. Without it, it's too steep to be good imo.

It's not a new thing for Trek/Fisher anyway, nor is it a 29er thing - I had a 2004 26" bike with Genesis geometry and apparently it goes back to '99.

You fitted an angleset? Or the bushings

A 120mm for for -1 and a single offset bushing in the rear shock for -.75

Interestingly less trail seems to be the way it's going on modern 29ers , I find it all very confusing . I am 6ft tall and have a 19.5 Superfly FS and use it as a race bike although hip surgery has meant no racing for me this season . I think I have a 90mm Hope stem not slammed with a 700mm flat bar and it seems fine on the ups and downs and the nadgery bits inbetween I am certainly not a riding god though and tend to just jump on a bike and adapt to how it is rather than try to change stuff . Glad it's worked out for you .

So what we've learnt is that if you have a bike designed for a specific fork offset, then it rides better with that fork and not one which it wasn't designed around. Earth shattering 🙄

It's not like Gary Fisher and Trek haven't had years designing these things. The original Superfly was one of the bikes that convinced the masses that 29ers were the future. That bike blew up big time in the USA and was the hottest bike for a season, everyone wanted one and that really kick started things over the pond.

What a pleasant kind post dragon.

Yes, we've learned that it matters. There are a lot of posts on the internet saying 'can I use a G2 frame with a normal fork?' and a lot of peopel say that it won't matter, as 5mm isn't significant. And a lot of people wondering how much it matters.

Well, I think I've answered that, for myself at least. It does matter, quite a bit.

it seems fine on the ups and downs and the nadgery bits inbetween

If you bought the bike complete you'll already have the right offset fork.

I'm not surprised you can feel a 5mm change in fork offset. One of my bikes has adjustable geometry via moving the dropouts, so the slacker setting drops the BB by 6mm and takes half a degree off the angles. Totally changes the feel of the bike.

Some people just don't notice this stuff though - lucky sods! 😉

Interestingly less trail seems to be the way it's going on modern 29ers , I find it all very confusing . I am 6ft tall and have a 19.5 Superfly FS and use it as a race bike although hip surgery has meant no racing for me this season . I think I have a 90mm Hope stem not slammed with a 700mm flat bar and it seems fine on the ups and downs and the nadgery bits inbetween I am certainly not a riding god though and tend to just jump on a bike and adapt to how it is rather than try to change stuff . Glad it's worked out for you .

There's two debating schools of thought, one the one had some companies like Transition and Mojo are going with shorter offsets - others are going with Longer - eg Giant.

I think it has a lot to do with your style of riding, I use countersteering a lot and tend to weight the front of the bike quite heavily in steep switchbacks, that's where longer offsets feel particularly good to me.

I'm not surprised you can feel a 5mm change in fork offset. One of my bikes has adjustable geometry via moving the dropouts, so the slacker setting drops the BB by 6mm and takes half a degree off the angles. Totally changes the feel of the bike.

I've found that out of half decent riders - there are two general types - those who would be suited to racing who will go fast whatever. And those who would be good at development riding/driving etc - who are more analytical. The rare unicorns, are people like Valentino Rossi - who is both.

some companies like Transition and Mojo are going with shorter offsets - others are going with Longer - eg Giant.

What is different about the rest of the bike though?

As for fast riders being fast - of course, but if there was something really not right about a bike they would call it out.

The flip side to my previous post I have a Trek Remedy 29er from 2014 which has had the original Fox 34 fork with a 51mm offset 140mm travel which I swapped for a 150mm travel Yari with a 46mm offset . Other than the Yari being way better than the Fox in terms of suspension performance I have noticed no noticeable difference in the handling but as I said before I don't tend to be able to notice small differences .

But with a longer fork and slacker angles it may have remained within a range of workable parameters. Maybe mine, being step to begin with, the offset had more effect. Plus you fitted a longer fork which would also move the front wheel away from you, so reducing the offset might've been a good move in your case.

As for fast riders being fast - of course, but if there was something really not right about a bike they would call it out

Not suggesting Im fast but:
Yep. When i bought my Race HT it came with a 69 degree HA. I knew immediately I was more comforfotable with it than the Anthem, and since then have replicated every facet of its measurements that I can with positive results.

Before then, I’d have put myself in Neils camp of not really feeling small changes. But now the Anthem setup has changed its noticable on the trail. One conundrum remains though; obviously I cant change the changestay length, but the HA changes and the lower BB resulting from the offset bushing make it [i]feel[/i] as though they are shorter, and I dont understand the physics of that yet.

make it feel as though they are shorter, and I dont understand the physics of that yet.

You've possibly made the bike better-balanced by lengthening the front end.

My new bike has 455mm chainstays and 460mm reach. The rear end doesn't [i]feel [/i]any longer than my HT which is about 445mm/435mm (and longer stem).

I dont understand the physics of that yet.

Well my suggestion would be as follows:

When you turn a corner, you and the bike are pivoting around the contact patch of the rear wheel (in your moving frame of reference). The force to make you pivot comes from the front wheel contact patch. So the closer your centre of mass is to the rear wheel AND/OR the further the front wheel is from the CoM, the more leverage the front tyre has so the more easily it can pivot.

Shorter chainstays move the rear wheel closer to your CoM; raising your bars makes you sit up more which moves your CoM closer to the rear wheel, as does shortening the stem. Slackening the seat angle does the same thing. Longer forks, more trail or a slacker HA increase the leverage the front tyre has on your CoM.

I have found in the past that moving the saddle backwards and shortening the stem (or even rotating riser bars) can move your whole CoM backwards and change the manoeuvrability.

You seem to be talking about rotating the bike there chaka, rather than railing the corner. Surely if there is no weight over the front at all, the front wheel will slide out, as in order to rotate the bike you have to overcome some angular inertia (there is still mass to move, even if no weight over the front).

It's got to be about getting the front/rear balance right, and probably there is a cornering technique to suit different points over a range of weight distributions.

You seem to be talking about rotating the bike there chaka, rather than railing the corner.

The bike has to rotate in order for it to end up pointing in a different direction. The closer your CoM is to the back wheel the less force is required. So even if you have less weight on the front, you need less. In my experience, very twisty trails like say Seagull in Swinley are much easier with your weight over the back - usua\lly. If it's really muddy or slippery, then traction can be an issue.

That applies to twisty tight corners. For longer fast corners you need much more force to change direction, so you need to balance the weight on both wheels which means moving forward a bit. The bike is only pivoting slowly.

I agree up to a point. But in the absence of any data or (very difficult) calculations, we can't make any progress with this as we need to know how significant the forces are when comparing the need to initiate and then cancel some rotation with the need to provide some cornering force. Also, most of us probably use a fair bit of body movement to momentarily rotate the bike under us (I have always thought this was particularly important with a weight-rearward style) so it isn't just the front tyre that provides that.

Or we could just use our experience. Honest question - have you ever ridden Seagull? It's an extreme test case, because it's a smooth narrow surfaced trail that is just a series of narrow radius but smooth bends that almost double back on each other, without much if any interconnecting straights. So you really need to think about technique or you'll be slow.

I thought my XC bike was quick handling and my big bike slow handling, so I assumed the XC bike would be better for that trail. But it was much worse. I tried to get my weight foward but that makes it nearly impossible to go fast. Only way to get any speed is to put your weight way back, turn the bars a lot and almost pull the bars around.

EDIT er.. wait.. is it Seagull? Or one of the others? Ergh.. the dead windy one anyway.

I don't come from that way, but it sounds like good woodland fun. Can't think of anything round here (Cardiff) that has the smooths, and that twisty tends to imply steep as well. Are the turns so close that you are straight(er) lining with your upper body as the bike follows the trail?

I'm also in Cardiff. And you're right there isn't much along those lines here.

Are the turns so close that you are straight(er) lining with your upper body as the bike follows the trail?

No, you have to completely change direction on each bend. Quite artifical, natural trails aren't like that.

Since you are from Cardiff, you'll know the trail that promted this whole problem with the bike and fork - the fence line at Castell Coch. With the old fork and bar I just couldn't go at any decent speed down it. Felt terrifying all the way down. I can hit it properly now.

Handlebars and roots down there. I will experiment next time.

Another nice feature of my Spitfire is that in addition to being able to move the dropouts vertically to change the angles and BB height, you can also swap the dropouts for different lengths (and axle standards). As it had acres of mud clearance with the 27.5 dropouts I swapped them for the 26 ones which shortened the chainstay length by 10mm (still running 27.5 wheels).

With the shorter chainstays It flicks into turns noticeably more easily, especially sequences of berms - responds better to subtle hip movements.

Since then I've lengthened the front centre with a -2 deg Works headset so it's over 20mm longer and it's still nicely flicky.

Adjustable geometry is ace. Sliding shock mount on my Patriot made it awesome.