Crank height relative to axles? [Frame Design]

[speedmd]

I think the link has it backwards on the effects at least going by what I have read / experienced in the past. Wiki has some on the topic that may shed some . http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry

 

[Punx0r]

I can see why frame builders seem to work to design guidelines established by experience

The frame I'm making will be of mild(ish) steel. It's far from an ideal material in respect of strength:weight, but it does allow me to easily make modifications once complete. I'll get some photos up as I make (slow) progress

 

[speedmd]

Very few frame builders I ever met varied designs much or across many different cycling disciplines. It is just too vast a topic for most. True understanding is rarely found in most complex subjects. I am just scratching at the surface on this one.

For extreme load, or lack of load sensitivity on the front ends feel, the artistic bike seems to have nailed it IMO.

 

[Punx0r]

I normally like to work with paper sketches for home projects, but decided that CAD would be easier for this because it accurately lets me rotate parts and measure resulting angles and distances. So I gave SketchUp a go for the first time.

My first attempt is as below, wheels are 26" (26.5" tyre O.D.) and I just set the crank level with the axles (sagged position [25%]). The front is at normal sagged position (44mm offset, 505mm crown-to-axle). The rear is at full compression (57mm shock stroke).



The trail measurement left me double-checking my measurements. The 68° head angle was selected from the 67-70° range given for typical trail/all mountain bikes. I looks like I'll need to make this a fair bit steeper.

Onto the next iteration...

 

[Punx0r]

Second iteration reveals a head of angle of ~73° gives my desired trail of 58mm:



I found I'd incorrectly worked out the shock travel before, so wheel travel has reduced a little (assuming shock aligned to suspension arc of travel to give a linear response). I've added the current seat tube and kept the standard top tube length to check for wheel collision with seat tube.

The extra length of the forks shows, with the distance from suspension pivot to seat tube top increased from 438mm to 610. The standover height of 938mm is considerably more than my inside length measurement, so this needs reducing to save my under-carriage. I need to angle the top tube.

I'm going to keep fiddling with it until I get the basic proportions right, then see where the other shock mount needs to go, then see if I end up with a frame of sufficient proportions to accommodate the electricals. If not, I need to start making compromises. I expect a few more iterations yet

 

[speedmd]

Remember to figure a reasonable amount of fork dive into your steering. Down hill trails with challenging turns will become a hand full if your caster angles get much steeper than say 84 degrees when braking into the turns. You may see situations where the bars want to be twisted from your hands with ruts and the like. It is a trade off worth making depending on what type of riding you want to optimize for. Also consider lengthening the wheel base if you plan a higher speed / higher horse power build. Like the dirty pictures.

 

[Punx0r]

I've come to the conclusion that many all-mountain-downhill bikes must run a lot more trail than the <60mm I've found so far in research. That seems clear in the cool photo you posted on the first page of this thread. I can't say for sure how tail that bike has, but measuring it off the screen it looks like a lot.

I can't see any way a bike with a ~65° head angle can have 60mm trail with a 26" wheel consider almost all forks have around 44mm offset (it seems to vary no more than +/-2mm).

I'll be honest and say I'm a long way from a downhill rider. What I seem to do a lot of is single-track, with a lot of very low speed manoeuvring around obstacles, so low-speed agility will be important, which is holding me back from increasing the trail from ~60mm. Road bikes seem stable at considerable speed with this sort of trail/head angle/offset. I'm surprised by how "upright" some of the cycle speed record bikes have been. I guess a lot of the advantage of a slack head angle on a downhill bike is it makes the front suspension more effective when hitting bumps. However...

Thanks for spelling out the situation regarding braking. I hadn't realised what a potential issue this is and will definitely check it.

Wheelbase, I will probably grow 10-15mm (currently 110cm). Looking at downhill bikes wheelbase varies from about 110cm to 120cm going from XS to XL frame sizes. One stupid restriction is that I have only one place I can keep the bike and where it must stand is only so long...

Having measured my stepover clearance I'm making some drastic changes to the shape of the top tube. It's starting to looking down a downhill frame outline

 

[Punx0r]

It turns out I was wrong before with my target trail figure of <60mm. It seems this is OK for a roadbike because they don't suffer the geometry changes that come with suspension, and also, apparently, to allow the rider to throw the bike side-to-side when pedalling hard without causing it to weave.

In terms of how much trail a bike "should" have:

An article on PinkBike says: "somewhere between 80 and 90 millimeters seems to be the sweet spot that produces a light feeling at the handlebar, with a stable feel at speed, and with predictable cornering"

Waltworks Bicycles says: "Typical trail figures for mountain bikes are in the 65-80mm range. Downhill bikes can get upwards of 100mm, twitchy crit bikes can be in the 40s."

A PVD Wiki article gives an example of a "typical" mountain bike as having a 71 degree head angle and 70mm of trail.

So it looks like I should be looking at around 80-90mm.

 

[speedmd]

Hi Punx

Now that your drawing in cad, you may want to do the caster angle range calculations for the various suspension setups you have. It will be much clearer which setups get you way out from the old target of 81 degree caster angle. Part of the problem when talking just trail is there is a huge range in wheel diameters now, causing it to be less consistent across many different builds. Fat tire bikes make things even more varied when mixing tire sizes. Some 29ers were labeled as not steering well when in fact their steering geometry was significantly slower than the 26" bike they were compared to. Throw in big head angle changes and you can get way off in a hurry. Caster angle is the base line you need to be looking at IMO.

 

[Thud]

Ive built a few frames...some really good & some really bad.

“Head angle,” “caster angle” and “rake angle” are all the same measurement.

Source of quote:
http://motocrossactionmag.com/home-page/mx-education-why-bikes-turn-why-they-dont-what-you-can-do-about-it

Actualy, a nice little description of the front end geometry...most current down hill/ freeride bicycles use close to motorcycle #s last i checked.

Read these articles when you get time.
http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm
http://www.tonyfoale.com/Articles/Balance/BALANCE.htm

There is a lot of back & forth miscommunication on this subject, mostly nomenclature imo.
The mathmatics of geometry are the same,regardless of where you start the measurments from.
Remember to enjoy the learning as it happens.

 

[speedmd]

“Head angle,” “caster angle” and “rake angle” are all the same measurement.

Source of quote:
http://motocrossactionmag.com/home-page ... o-about-it

Clearly a wrong statement. Typical of a mag writer. All are very different, and it is not just nomenclature.

 

[Thud]

Clearly a wrong statement. Typical of a mag writer. All are very different, and it is not just nomenclature.

did you read the article? or any of the Foale documents...to find the crossed terminology?

From the bike cad site the above picture is from In the motorcycle industry, caster angle is synonymous with what would be called the head angle in the bicycle industry. Sometimes in the bicycle industry, the term caster angle is used to describe the arctangent of the trail divided by the wheel radius. To avoid confusion, BikeCAD does not use the term caster angle. However, for those that are interested in this value, BikeCAD does display a dimension called the arctan(trail/wheel radius), (known by some as caster angle). This dimension is shown above.

this document is a simple trail calculator for bicycle's..& the math will gel for differing wheel diameters....I can find many examples of it used as "offset" or "rake"

We are deep into semantics on terminology.
Call me wrong if you must.

 

[speedmd]

Yes Thud I have read them all. It is Easy to understand trail numbers. Unfortunately, the trail targets vary depending on wheel size. What you do for trail on a small wheel scooter is very different than a fat bike even when your looking at designing in the same steering characteristics. Rake - offset give various degrees of shock transmission, front end flex and feel and many times are not just for hitting trail numbers. Within one product group, I agree, much is used interchangeably and will work, but the math when taken to its basic/simplest form, IMO, it would be the caster angle as shown in the bike cad drawing as the most direct indicator of how the front ends steering will act.

 

[Punx0r]

The terminology certainly is all jumbled up in the various sources out there on the internets. Even the MAM article Thud linked to appears to contain mix-up by the author

Offset is used primarily to determine the correct amount of trail for a given head angle. Motocross front ends typically use between 18mm and 25mm of trail.

I'm sure he means 19-25mm of offset?

Anyways, the caster angle to express trail is better engineering term because it's not derived from wheel radius. However, I can easily imagine the linear measurement for trail on the ground became the popular standard because it's easier to measure on an actual bike.

 

[Thud]

I am not arguing the point above....& the last 2 post clearly show why the terminology is so hard for anyone attempting to learn frame building is easily confused.

Yes Thud I have read them all..

Including this passage? "To avoid confusion, BikeCAD does not use the term caster angle. However, for those that are interested in this value, BikeCAD does display a dimension called the arctan(trail/wheel radius), (known by some as caster angle). This dimension is shown above."

It is Easy to understand trail numbers. Unfortunately, the trail targets vary depending on wheel size. What you do for trail on a small wheel scooter is very different than a fat bike even when your looking at designing in the same steering characteristics. Rake - offset give various degrees of shock transmission, front end flex and feel and many times are not just for hitting trail numbers. Within one product group, I agree, much is used interchangeably and will work, but the math when taken to its basic/simplest form, IMO, it would be the caster angle as shown in the bike cad drawing as the most direct indicator of how the front ends steering will act.

My point is in the body of this reply & you are correct....except the term: "caster angle" & the picture showing the Arc tangent equation to get you on track for a trail measurement.

There is no reference to any measurable frame angle that the words "caster angle" can be attributed too in the Arc-tan eq....thus semantics & nomenclature.

There is the head tube angle & the trail measurement.....the rest is lost in translation & variable for a particular application...off road or street.

 

[speedmd]

Understood. The term caster angle seems headed to be lost to antiquity. It is a simple measurement/target and no confusion once you understand it and see it's value/ importance. Its value to a designer getting into designs that there are no trail guide lines for is huge. Just wanted to share my experience with guys doing new and unusual builds from some of my research in early texts that date back to the late 1800 -early 1900s to current engineering works on the topic as well as a bunch of mixed wheel size design/builds of my own.

 

[Punx0r]

Splitting the difference on the 80-90mm of trail figure I had, I tried 85mm, resulting in a head angle of ~68°, which tallies with the 67-70° range I had down for all-mountain bikes from research. I also lengthened the wheelbased by 10mm. It may still a bit short.

Anyways, taking a worst-case scenario, I checked what would happen to the front end geometry with the rear suspension fully extended and the front fully compressed:



This increases the head angle to 77°and reduces the trail to 53mm while shortening the wheelbase from 1100mm to 1057mm.

These numbers sound OK to me (forks don't tuck under...), but I really have no idea. The main thing I want to avoid is creating something that will be unpredictable to ride because it suffers sudden changes in handling characteristics. That's why I appreciate the input of you guys who have done this sort of thing before

 

[Thud]

punx,
your #s are right in the zone most will fall into..

Regarding the reduced #s under braking: How long will your bike be in "that" situation under normal riding? even if normal means bombing down a run. I guess you will hit those extreams for less than a 1/2 a second maybe 4-5 times per run?

for a full design consideration, you'll calculate your typical sag height(or ride height) where the goemetry is in its sweet spot of door stop stability & snake like agility through the corners.(thats the goal anyway LOL)

keep it good for 96% of the riding situations & depend on your riding skill & arm strength the other 4% of the time.

 

[speedmd]

Agree, you should plan for a fair amount of fork dive but not as if you will need perfect steering when bottoming it out. Half to two thirds compressed max depending on how you like your forks set. Personally I would not go as shallow as 68 if you plan to do much slow / tight trail riding. Certainly better on fast downhills, but not as good in the tight trials type of stuff. Tons of stock bikes out there with numbers like these you can test ride to feel out the difference before you decide. You will know right off after a few minutes on each.

 

[Punx0r]

Thanks for the feedback - this is the sort of stuff I need

In reality I might never see that much brake dive, I'm just trying to avoid inadvertently making a death-trap.

For sure I'll do the design based on the normal sagged (25%) riding position, I just want to make sure nothing nasty happens at the extremes.

So, what sort of HA would you guys choose for a good compromise between stability and agility? 26" wheel, 44mm offset.

 

[speedmd]

It will be only under braking when the front end only is compressing. In most turns / berms and the like, both ends will be compressing to various degrees. A few degrees of dive should cover you reasonably well unless you are trying to dial in a particular track situation. With 26" wheels est. 560mm diameter, at a 72 degree head angle with 44mm offset, you are getting a bit over 80.9 degree caster angle. Dam near perfect steering and very close to the target 81. If you want to shallow it out a degree or two to compensate for dive and higher speeds, that would be reasonable. I would not personally go less than 70 as I don't care for slow speed ride / feel of the slower steering on the tight stuff which is much more important to me in a bike.

 

[Punx0r]

Tyre radius is 13.25" ~337mm (ignoring flattening at the contact patch).

If I'm clear of any dangerzones in the geometry I'm tempted to steepen the HA a bit. There's a lot of clay in my area and when the ground is wet speed is really limited by available traction, making manoeuvring around slowly looking for grip a big thing.

 

[speedmd]

Corrected for your real wheel+tire diameter of 674 mm, a 72 head angle yields a 79.35 degree caster going by my layout/ calcs. I would stay close to the 71 -72 degree head if it were my bike.

 

[Punx0r]

I initially went for 72° and later chickened out and went to 70°. This results in 76mm of trail (12.7° caster angle) in the sagged position.

To maintain my existing reach and seatpost height I end up with a 75° seat post angle, which is a little upright, but should be OK (guideline is 72°).

It shapes up to approximately a "medium" size frame. My short legs are a real nuisance as I had to seriously kink the top tube to give me only 2" (50mm) of standover clearance at full suspension extension.

Next I put the suspension at full compression, added in the rear shock and started filling out a frame around it. I've tried to maintain at least 1" (25mm) clearance to everything to allow for measurement/build error and for future changes. My first attempt is below:



I'm looking to keep the frame on 3" (76mm) wide so I can pedal is effectively and avoid smashing my knees/shins on it when I mess up. I'm not looking to fit a mass of stuff inside, a 72V/10Ah battery, a 12 FET controller and a wattmeter, few switchs/charge connector but it looks like it might be a squeeze.

Any suggestions welcome, I've just finished the above sketch so haven't scrutinised it properly.

 

[speedmd]

Its a few degrees slacker than the old target for neutral steering, but it should absorb road shock better that way and be better at high speed stability. Possibly a good trade off. I have ridden many bikes at both extremes. You may want to try out a production one like what your planning as I found, I don't care much for them in tight stuff or on greasy ground at lower speeds. You will most likely get a good feel right away.