The Grim Donut finally tested!

the head angle means that flat hits (like the huck to flat) there’s a lot of sideways load on the bushings – so they perform badly – but whilst riding along and hitting a log the force comes from the front – it could easily be that angles like that are more perpendicular to the force hitting the wheel and thus actually work better (same for the force going through the headtube)

but as this has been done without all that (from what I can see from the various videos and articles on it), then I’m more sceptical.

They did the FEA and the rig testing on it according to the vid.

Im sure Cy talked about it in his lectures at Sheffield (10 years ago?). The loading in the CEN test for pulling the fork forward is actually mimicking a landing so hard your legs would buckle.

Podcast discussing it here

https://www.pinkbike.com/news/the-pinkbike-podcast-episode-23-grim-donut-2-is-live.html

Here you go,

The CEN test is 1200N pulling horizontally away from the rear axle, 50,000 times.

At 2012 geometry (yes slacker will be a bit less) that was 50,000 9G landings to flat.

Not just forward – it loads in both directions alternating 1200N fwd and 600N rwd iirc.

The loads are set to get a balance of short (cheap) test without overdoing it (plastic failure). Fatigue cycles add up – if you want to test at smaller loads then crack on but it might have to run for a million cycles. Mixed load cycles would be best but that needs more complicated rigs and control. It is basically a stake in the ground to show you’ve tried to makes a safe bike.

I would like to see someone try to separate length from head angle. That wheelbase looks long enough to have a normal head angle and the bars far enough behind the steering axis to be connected by a backwards stem. Then it would be interesting whether that behaved well or whether a remote steering linkage would be required.

https://www.btr-fabrications.com/gallery/hack-bike-derby/

A backwards stem would cause quite a lot of steering instability.

I know some brands like Nicolai are pushing the limits of whats available but it’s good to see what PB have done.
I think the industry just likes to do incremental changes for their next model bikes so they can do the same again for the next model. rinse repeat.
No one seems to do any empirical data collection and just work out what is actually fastest.

I like the grim donut though.

A backwards stem would cause quite a lot of steering instability.

Aft of steerer mounted bar pivot point with linkage to steerer needed then….

Problem for me is there was no A-B-A type test.

Watching the video,there are several very significant sections where on the second run (on the GD) that YB is hitting much faster / straighter lines. However, without a second run on his normal bike, we don’t know if he was able to hit those lines due to additional familiarity (due to it being his second run) or actually because the long/slack bike ENABLED him to hit those lines without crashing……..

A backwards stem would cause quite a lot of steering instability.

out of interest, why? the steering geometry itself would be the same (for the same frame), just the arc in which your hands move would change (I think?). As we’ve moved from 150mm stems to 40mm stems, that arc has changed quite a bit already, why would it all go wrong if we went backwards (say) 40mm?

I think the article says it’s a track he knows well. Sure, it’s not a double blind randomised trial, it is interesting and it is entertaining, and I don’t doubt the result.

We’ve been hearing for a while that many manufacturers don’t make the enduro bikes that racers would like.

I wonder how commencel feel about it 🙂  To be fair they have probably won more world cup DH and EWS rounds than most other brands so i doubt to bothered .

I can see Yoann asking Max to knock up a 60° head angle prototype.

“out of interest, why? the steering geometry itself would be the same (for the same frame), just the arc in which your hands move would change (I think?). As we’ve moved from 150mm stems to 40mm stems, that arc has changed quite a bit already, why would it all go wrong if we went backwards (say) 40mm?“

I wrote a reply but the internet ate it…

It’s because most destabilising inputs to a MTB cause deceleration. And that causes the rider’s mass to apply a forwards force to the grips. The longer the stem, the more that force has a self-centring effect on the steering. A negative stem will do the reverse unless both hands are equally weighted.

I thought that was amazing. And there is no way I’d be mincing down that track. But the bit that I found most impressive was not the bike. No, it was the calm cool commentary on both runs. Just remarkable. Bike seems like it could compensate for even my skills deficiencies.

Not just forward – it loads in both directions alternating 1200N fwd and 600N rwd iirc.

The loads are set to get a balance of short (cheap) test without overdoing it (plastic failure). Fatigue cycles add up – if you want to test at smaller loads then crack on but it might have to run for a million cycles. Mixed load cycles would be best but that needs more complicated rigs and control. It is basically a stake in the ground to show you’ve tried to makes a safe bike.

If you watch the video he makes that point. 600N seems like a high number, but achievable in the real world.

1200N pulling has no real world application and broke the original Soul in about 1/4 the required cycles (and everyone elses frames it seems).

No, it was the calm cool commentary on both runs. Just remarkable.

Yeah – and he calmly says at the start he’ll be going about 80% of capacity and then proceeds to shoot down it at unreasonable speeds and takes 15 seconds or something off the Strava KOM 😐

Yeah – and he calmly says at the start he’ll be going about 80% of capacity and then proceeds to shoot down it at unreasonable speeds and takes 15 seconds or something off the Strava KOM 😐

Have a look at some of Yoann’s other videos…
Rides tech like he’s riding to the shop for a pint of milk.

There’s an unrelated Finn Isles vid this week where he’s training too, again, the same…flying down, destroying the trail like he’s strolling for a sandwich.

My <b>35kg</b> kid has been playing with a measured static 62.1 deg HA on a correspondingly long frame for his size and 142mm cranks.
Goes downhill VERY fast… uphill … a bit of a pig. Seatpost is very slack though.

It’s because most destabilising inputs to a MTB cause deceleration. And that causes the rider’s mass to apply a forwards force to the grips. The longer the stem, the more that force has a self-centring effect on the steering. A negative stem will do the reverse unless both hands are equally weighted.

that interesting, I hadn’t thought about it.

Based on this – if I flipped my 35mm stem (I’m not actually going to..) – that would have a similar difference from going to that 35mm stem from a 100mm stem – that is to say, it would be gradually less stable, rather than suddenly unstable? on a 35mm stem and a bit of backsweep, the riders mass is applied pretty much in-line with the head tube?

I think you’ve got four main systems providing stability to a bike:

1. Trail
2. Jacking (head tube rising when you turn the bars)
3. Gyroscopic
4. Handlebar weighting

Trail and jacking increase stability with decreasing head angle and offset. Gyroscopic stability increases with rim+tyre mass. Jacking has the same effect at all speeds, the other two increase with increasing speed.

The handlebar weighting (tiller) stability does nothing if the grips are in line with the steerer (typical bars and 30mm stem). As the stem length increases it adds stability. As the stem length decreases it deducts stability. It isn’t proportional to speed but is to input force.

so if you have enough trail and jacking (by using a 58 deg head angle) – the instability caused by reversing the stem could (maybe) be offset (no pun intended)?

I still can’t get my head round how decreasing offset increases trail.

Does anyone have a diagram 😉

Watching all those SIDs waggle around on the huck to flat is, er, educational.

Did anyone else think the Commencal looked really short after seeing the Donut?

I would like to see someone try to separate length from head angle. That wheelbase looks long enough to have a normal head angle and the bars far enough behind the steering axis to be connected by a backwards stem. Then it would be interesting whether that behaved well or whether a remote steering linkage would be required.

I don’t think that would work very well. If you were tall enough to be able to weight the forks straight down through the steering axis it might but for anyone shorter than ideal it wouldn’t be very nice to ride. I have no idea in reality, it’s just a gut feeling that it wouldn’t be quite ‘right’ for some reason.

Are you so sceptical of bridges, aeroplane wings not snapping and the existence of helicopters?

Speaking of… I’m still awaiting the Brighton i360 to blow over….
Now I’m no structural engineer, but I’m not fick or nuffink…and it just looks ‘too thin’ to be safe.
So there. If it blows over, they should have listened to me.

DrP

we don’t know if he was able to hit those lines due to additional familiarity

It was explained that it’s one of his regular test tracks, so not an issue there.

We’ve been hearing for a while that many manufacturers don’t make the enduro bikes that racers would like.

We often hear that racers don’t want the longer, slacker bikes that some of us punters are moving towards.

Watching all those SIDs waggle around on the huck to flat is, er, educational.

I got hooked on the huck to flat videos to see forks flexing then I looked at the rear suspension and now can’t take my eyes off the chains and rear mechs flailing around

That wheelbase looks long enough to have a normal head angle and the bars far enough behind the steering axis to be connected by a backwards stem. Then it would be interesting whether that behaved well or whether a remote steering linkage would be required.

I’m not sure what to make of the “Dangerzone” theory, but surely, someone, somewhere must have tried it.

https://www.pinkbike.com/news/exploring-the-relationship-between-handlebar-vs-stem-length.html

I’m a big fan of slacker head angles and balanced rear stay length but less so of much longer reaches & lower BBs.

I could see myself getting a frame with 60 degree ha in a few years.

Speaking of… I’m still awaiting the Brighton i360 to blow over….
Now I’m no structural engineer, but I’m not fick or nuffink…and it just looks ‘too thin’ to be safe.
So there. If it blows over, they should have listened to me.

have you seen it up close? its all perforated so the wind just goes straight through it. The really crazy thing is how it was built – they built the top section, lifted it up, then built the top-but-one section underneath it, lifted that up, and so on..

I think the article says it’s a track he knows well. Sure, it’s not a double blind randomised trial, it is interesting and it is entertaining, and I don’t doubt the result.

It’s still pretty odd that he smashes the KoM (which presumably includes many of his own runs) on his first go that day

We often hear that racers don’t want the longer, slacker bikes that some of us punters are moving towards.

Yeah, there’s a mix for sure. The PB video speaks to a bike company, which says that racers are conservative. And to Arron Gwin, who I would say is one of the conservative riders too.

Jordi at fox has mentioned before how some enduro riders alter their sag front and rear to effectively reduce HA. And I think one of the main drivers for racers trying 29er front ends on their 27.5 bikes was to slacken the HA a bit.

Really like Paul Astons’s bike. Looks great and is probably perfect for what he does.

RE the i360 being built that way…look..you’re right…

But… I wouldn’t have done it like that. I’d have lay all the pieces down on the beach.. got Norman Cook to have DJ’d a cracking set whilst workers/ravers bolted it all together laid down.. then hired a crane for the afternoon, pulled it up in one go, and then post-creted the base in.
But like I say..no one asked me, so hey ho..

DrP