Crank height relative to axles? [Frame Design]

I'm rebuilding my bike frame as a steel box to contain the ebike parts rather than having them strapped to the outside of the original frame. To complicate matters I'm also increasing both fork and rear shock travel. This was originally a paltry 1" rear 2" front. I upgraded this to 2"/2" (the forks were 4" but only 2" travel in reality). I now plan to fit a 3 or 3.5" rear shock and proper 4" forks.

I don't want to completely screw up the geometry and in building the frame I'm free to move everything about. So far I have moved the rear shock mounting point to keep the original wheel height the same when sat on the bike, there will just be more travel.

Am I right in thinking that 4" forks will be longer and I should move the head tube to suit?

I figure the front and rear axles should be the same height (level)? What height should the cranks be relative to them?

I guess there must be "golden rules" of frame/suspension geometry but I'm sadly ignorant of them...

Thanks

Before you started, you needed to actually measure the angle, with an angle finder. You can get them cheap at harbor freight. Same item surely on ebay. Even a simple plumb bob and a protractor can do it too.

A few degrees slacker headset might even be desirable if you will be running faster. Especially MTB's, they may be needing some more slack angle to run 40 mph or more.

I don't know much about what angle to aim for now, but I think my beach cruiser angle is 22 degrees.( off of 90) Any slacker than that would be too slack I think, but nothing wrong with aiming for beach cruiser geometry now IMO.

I would go to Surley's (or anyone's really). They post the geometry of their frames so you know what your buying.

A 26"wheeled bicycle typically has the bottom bracket dropped aprox. 2" from axel height.

http://surlybikes.com/bikes/steamroller/geometry

I use slack head angles of 24deg. & make rake adjustments to keep the trail measurement in line for the speeds your designing for.
the faster you go...the closer to motorcycle trail #s you will aim for.

you should familiarize with these terms & understand what they do if your going to be chopping bikes.....cause one you start its hard to stop.
have fun & good luck!

Standard bikes are 65 -70mm drop typically. It is just a common measurement used to gauge how it will keep your pedals from hitting the ground in mild turns. Ideal is as low as possible for stability, and as high as possible for clearance (off road). You are totally free to put it where you wish/ need with those issues in mind. Suspension bikes will have much higher BB typically. Small wheels, even higher relative to axles.

You will want to keep your front ends caster angle around 8 - 9 degrees from vertical to have good steering regardless of the head angle. Faster you go, the more wheel base you will want.

Thanks guys, these tips and sources really help.

I already planned to slacken the head angle and increase the wheelbase a little. The starting geometry for the bike is of little use as it's a BSO and I've already messed about with the front and rear suspension, so having specs for proper bikes to helps.

I had a Duh! moment with the crank height when I realised my initial placement would mean the crownwheel would strike the ground at full suspension compression It didn't occur to be that a bike with more travel needs to be taller...

Which made me wonder: On proper bikes at full front & rear suspension travel, does the lower pedal clear the ground, or if the rider expected to maintain level cranks?

I've got everything laid out on a big piece of cardboard at the moment, marking the key points/pivots and motions of suspension.

As you slacken the head angle, you will want to add rake/offset to get back to a suitable caster angle without slowing the steering too much. You need to keep your toes up while landing jumps.

Punx0r said:

I figure the front and rear axles should be the same height (level)? What height should the cranks be relative to them?

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The axles will be at the same height if the wheels are the same diameter. If not, then the axles will be at different heights.

Bottom bracket height is always relative to the ground. Wheels vary in height, but the ground does not. The lowest bottom brackets on mass market bikes for adults were about 260mm to the center. Riding a bike that low with normal length cranks requires a little bit of discipline about not pedaling through turns. The highest BBs on unsuspended mass market bikes were about 330mm high. If you are using dual suspension, it wouldn't be a bad idea to shoot for a value in the 300 to 350mm range, depending on your height, your habits, and how long your suspension travel is.

Remember that your effective BB height will be affected by suspension sag.

Thanks, it's good to have some guideline numbers to work to

I'm looking for forks with around 120-130mm travel, but might go as much as 150mm if such a pair comes up at a good price. Rear wheel travel will be around 7-8" depending on whether I end up with a 3, 3.25 or 3.5" stroke rear shock. I'm leaving room to accommodate a short range of rear shock mounting positions for tweaking. I've pretty much rejected the idea of an adjustable headtube as too heavy and complicated considering the reduction in strength it bring.

I don't expect to end up with the perfect frame as a result of this exercise. I expect I will make some mistakes and I expect it to be a learning exercise

Chalo said:

Bottom bracket height is always relative to the ground. Wheels vary in height, but the ground does not.

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This made me realise I'd been going about this incorrectly. I'd used the frame as my fixed point of reference and was moving the wheels around it. I've scrapped that, taped a big roll of paper to the floor with the edge as my "ground" and drawn the axle centre line on 13.25" above that. Now I have proper reference points to work to

Careful, the reference line will move significantly with tire selection /size changes alone. Throw wheel sizes in there and you have some big angle/ height changes. You will want to use a range of heights depending on tires you may want to ride. Maybe best to make suspension / ride height adjustable if not set on tires/ wheels.

I'm set on 26" wheels and future tyres won't depart much from the ones I have now

Is there any obvious way to make front ride height adjustable I'm perhaps missing? I thought about welding the head tube to a flat rectangular plate and bolting that to a similar plate welded to the (box) frame. This would allow adjustment (height and angle with spacers) but it'd be pretty ugly.

Front ends with triple clamps on most you can slide up/down the fork tubes. You can also get differing amounts of offset on the triple clamps for rake/ trail adjustment. Lower head tube / bearing Spacers are also a option. Internally shock ( dampening rods ) tubes can be modified also to lower front ends a bit if you don't need the travel.

Like the Marzocchi 888? I've seen a couple of sets of those for sale.

Do the regular suspension forks I've seen with adjustable travel achieve a variable crown-to-axle length?

I'll bear the spacers in mind. You've also reminded me of the Crane Creek offset headsets, but they ain't cheap.

Not sure all the zocchi forks will adjust. Most older ones like the 888 are no issue. Some of the Newer ones seem to have specific mount points. Looks like they lightened them up significantly. Nothing on the front end is cheap. If you are set on wheels and tires, design it correctly and you will not need much if any adjustments. You could start with a slightly longer head tube and shorten it when the build nears completion if need be. Head Bearings have different stack heights also for a bit of fine adjustment. Most can be adjusted significantly with shock air pressures. Sounds like a fun project.

After a sobering looking at my bank account I ended up with a 3"/76mm Fox Van RC rear shock and Marzocchi 55 ATA forks (125-165mm travel). With the current rear swingarm (which I'll keep for now) this gives me almost exactly equal front and rear wheel travel at ~5.5".

I've been doing some more research on geometry and it seems I want to be aiming for something like an "All Mountain" style of frame with 67-70° head angle. However, it seems that pretty much universally for bicycles (road to mountain) the trail needs to be at least 55-60mm (or more specifically, 56-58mm) for neutral handling. My fork offset/rake is 44mm, so I figure the head angle should be compromised for proper trail, if required.

Less significantly, I should be looking at around a 72-74° seat tube angle.

Do these numbers sound sensible to you guys? My riding is a mixture of street and trail. I pedal, so want to be comfortable doing that, but don't need to extract maximum effort/efficiency and I'd prefer a more upright posture to avoid backache on long rides than getting optimum weight distribution off road.

Hi Punx..

I did not understand front ends until I learned about caster angles many years ago. It seems to be lost in antiquity and I could not find any of my old texts to reference it, nor find much of use on the web. This is the only photo I could find. Vertical angle from the head tubes axis focal point on the ground to the wheel axle center.


Ideal trail will vary with head angle, wheel -tire size and rake, but caster angle is a relatively steady target. Going by memory, you are looking for 80 -82 degrees. 80 is slower steering and 82 is fast steering. The faster your target speed the shallower you go with this angle. For bicycle speeds you don't want to vary it much from this.

speedmd said:

Ideal trail will vary with head angle, wheel -tire size and rake, but caster angle is a relatively steady target. Going by memory, you are looking for 80 -82 degrees. 80 is slower steering and 82 is fast steering. The faster your target speed the shallower you go with this angle. For bicycle speeds you don't want to vary it much from this.

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Commercial bicycles mostly have head angles from 67 to 75 degrees. The steepest head angles you'll find mostly on track bikes; the slackest angles on cruisers and roadsters. Obviously there are outliers like choppers and recumbents that violate the general established principles of bicycle steering design. Usually these bikes exhibit handling that helps illustrate why normal design values are normal.

If head angle is what you mean by "caster", then I have to note that I've never seen a commercial bicycle of any kind that was even close to 80 degrees. I once made a couple of bike frames with zero-offset forks, and I think I settled on 76 degrees as a head angle that allowed normal trail with a no-offset fork.

Head angle and fork offset ("rake" in bicycle jargon) are a system as you note. The slacker the head angle, the longer the fork offset-- to keep trail within a normal range. Too much trail gives a bike a tendency to "flop" its front wheel into a turn; too little trail makes a bike hard to keep straight and steady, and can make it impossible to ride no-handed.

Motorcycles' head angle ("rake" in motorcycle jargon) varies around 60 degrees, rather than varying around 70 degrees like bicycles.

Speedmd, is this caster angle measurement another way of expressing trail, as an angle rather than a linear measurement?

It took a while to get my head around the effect of offset, trail and head angle and how they interact, but based on your descriptions it sounds like I have the basic gist of it

It's definitely a complex subject when you factor in the rest of the geometry of the frame. I had to consciously step back and just focus on the front end, else I'd end up futilely trying to design the "ideal" frame the uniquely fits me and never getting it done. Like they say "better is the enemy of good", I just want/need something better than the BSO I started with, which frankly, I got on OK with, probably because I'm short

Caster angle is not head angle so that is the first thing you need to understand. Its the red marked angle in the diagram. The head angle is used to locate the caster angles ray from vertical. Trail target numbers vary with wheel diameter.

Much science has been done on the topic and I hope my understanding helps yours. If you were to build a standard type bike with no rake, a 80-81 degree head angle would be good neutral controllable steering. 76 degrees would be slow steering, but still ok for most riders. 82 degrees would be fast steering. The Ride quality over bumps would be hard as hell, but steer relatively well. This is why the steeper angles were used only on track bikes as road shock would numb your hands in short order. Rake and a shallower head angle give much more pedaling clearance which is also badly needed in track balance maneuvers making it a more suitable setup overall.

As far as I understand it, the terms fast and slow, the research I have found most always looks at hands free reaction of the front end. The slower the steering, the harder/ more you need to lean in /mussel the bars. Faster the steering, the easier the front end falls/ turns into the turn. Fast steering requires very little rider balance movement to turn. Turn your head slightly and the bike moves off line. Motor cycle riders understand this phenomena as they are mostly setup with very slow steering which wants to stand up the bike in a turn, forcing you to load it hard to stay in a tight radius. It wants to keep you going straight. If you go too steep of a caster angle, you will have a front end that can easily turn sideways on you or plow/ slip and more easily goo off line or cause a crash.

Beauty of the caster angle measurement is that it works with any wheel size combination. Any head angle, any fork offset /rake. You are free to experiment with different wheel sizes / tires combinations, fork lengths/styles, suspension angles and you can always simply come back to looking at your steering and make the adjustments as needed to know it will steer well. No need to look up at some general tables that are based on conventional builds.

Thanks for clearing that up. It's always good to have another analytical tool.

In my experience, desired trail is not proportional to wheel diameter. Trail is the lever the bike uses to steer back underneath you. You don't want a much smaller trail value just because the wheel might be much smaller, because that gives the bike less authority to stay balanced. The bike is going to be the same height and carry the same weight regardless of wheel size. It needs an appropriate lever length for the job.

The bigger the wheel the more trail is needed. Lets make it a bit more extreme and it may make more sense. Tiny wheel only need a relatively small amount of trail to stay solidly directional.


Picture a huge caster wheel now. A bikes front end acts much the same way. The classic example shines through when you work out trail numbers on mixed wheel size stayer.


One of my all time favorite shots of one.

The semantics confused me a little as first because if you search the internet for "caster angle" all you get are phrases along the lines of "on a bicycle, caster angle is the steering axis angle", which is not what we're discussing here 8)

This was the only mention of this angle I could find (called "rake angle" here):




How about this for plenty of trail, but 90° head angle and zero caster/rake angle?:




Full article here: http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm

speedmd said:

The bigger the wheel the more trail is needed. Lets make it a bit more extreme and it may make more sense. Tiny wheel only need a relatively small amount of trail to stay solidly directional.

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I guess I'll find out soon enough... because I'm in the middle of building a front loader cargo bike that has a 20" front wheel, and I gave it about the same trail as I would have used on a big wheeled utility bike. If I'm wrong, I'll need to transplant both the fork dropouts and the drum brake anchor point.

My guess is that regardless of wheel size, a bike with the same weight, wheelbase, C of G height, handlebar width, etc., needs the same trail dimension (not caster angle) for the same degree of steering self-correction. I mean, that's how long a handle it has to work with, right?

It's true that most small-wheeled bikes and scooters I have seen have less trail than normal bikes. It's also true in my observation that they self-correct a lot less affirmatively than normal bikes.

Punx. You know better now than most all that dabble in it, on what it is truly all about. 8) When you get your suspension close to set and fixed on wheels/tires and the rest, do what DM suggested and measure the angles so you can do the reverse engineering (geometry) to see where you are and adjust if needed. If you plan a higher speed build, IMO it is better to err on the slower steering side. This is advisable when using suspensions also as the head angle increases greatly under breaking.

On the 20 inch cargo bike, hopefully you can try out a few different fork rakes without too much trouble to see if matters much on steering. Most of this is really based on hands free balance which may or may not matter on a cargo bike that may not have even loading or really even want sensitive to balanced steering. Interested to hear what you find. Would think that a steeper head angle with less trail would give you much less sensitivity to load-weight in the steering and not impact stability much at all given the added wheel base.

Jeepers, I'd forgotten about brake dive. Something else to research tomorrow

Chalo, I read something specifically trail and touring bikes. It's about carrying weight in front of the steering axis and so requiring less trail than normal. Might be significant to your front-loader cargo bike?

http://www.spectrum-cycles.com/geometry.php
(Last paragraph)