Author Topic: Lowering my 74' CB 750 K4 ? (Read 6899 times)

rddcw · « **on:** July 18, 2015, 08:14:30 PM »

Have purchased a real nice and clean 74' Honda CB 750 K4. Everything is Stock. Problem is being only 5' 3" in Height, very difficult to touch the ground. Had an additional stock seat and had it cut down 2" and the sides rolled. Did help a lot, but still pretty high for me.

Needs tires so was thinking about going with a 120/80 series 18" tire in rear ( slightly shorter in diameter ).

Any other suggestions you guys may be able to provide?

The rear shocks that were on this bike measure 12 1/2" in length, so don't if these are stock to the bike? They appear to be but not sure?

Also, would lowering the fork tubes thru the upper fork tree help any?

Stev-o · « **Reply #1 on:** July 18, 2015, 09:43:42 PM »

My buddy Rick set up his K5 for his wife, who as "vertically challenged". He had the seat cut down, installed a 16" rear wheel which lowered the seat about 3".
There are also lowering blocks, but I won't recommend them as they can affect handling.

Don't feel the need to have to touch the ground with both feet, it just is not necessary during normal riding conditions.

seanbarney41 · « **Reply #2 on:** July 18, 2015, 11:36:55 PM »

+1...just get used to it. Anything that lowers it besides the seat, #$%*s up the handling.

fmctm1sw · « **Reply #3 on:** July 19, 2015, 06:05:04 AM »

Quote from: rddcw on July 18, 2015, 08:14:30 PM

The rear shocks that were on this bike measure 12 1/2" in length, so don't if these are stock to the bike? They appear to be but not sure?

Also, would lowering the fork tubes thru the upper fork tree help any?

The guy I bought my 750 off of did just that. He had short shocks on the back and dropped the clamp an inch or so. I can't really give you any idea of how it rode because I put it right back to stock. About the only thing I can add is that you will create kickstand issues. The bike sat awfully upright like that...

754 · « **Reply #4 on:** July 19, 2015, 08:30:22 AM »

16inch Tire only drops around 1/2..measure you will see.. Lowering blocks and sliding forks up...is easily reversible... You may need to shorten sidestand.

rddcw · « **Reply #5 on:** July 19, 2015, 10:48:07 PM »

I think if I use a 1" lower shock ( 12 " ) in the rear and run the fork tubes up an 1" as well, it shouldn't affect the handling at all. If anything, I would think it would have a better lower center of gravity.

Grabcon · « **Reply #6 on:** July 20, 2015, 06:00:14 AM »

As you lower the bike the handling will be impacted. In any hard cornering you will scrape pegs, side stands, etc. Don't forget that after you lower the bike when you sit on it the suspension will compress making it even lower. Not a fan of doing this. CB750's easily scrap things with stock suspension.

Many folks are on toes when stopped. learn to ride this way.

754 · « **Reply #7 on:** July 20, 2015, 08:30:21 AM »

He asked to lower it so he can actually. Ride it...
Not everybody pushes their bikes to the limits, some never even get closer to it..

Grabcon · « **Reply #8 on:** July 20, 2015, 10:21:33 AM »

754 Just making a statement based on my learnings over the years.

SOHC4 Cafe Racer Fan · « **Reply #9 on:** July 20, 2015, 11:52:18 AM »

Lowering the bike not only affects cornering clearance and the kickstand, but it also affects the handling of the bike.

I would try to get the bike to where you can steady it on the balls of your feet -- that is sufficient.

rddcw · « **Reply #10 on:** July 20, 2015, 05:04:42 PM »

754, Not everyone is gifted with height so I usually have to lower my bikes somehow, whether it's cutting down the seat, using a shorter length shock in the rear, and sometimes even a using a lower profile tire can help a little too.

I don't ride the bike aggressively and the biggest concern that I have for myself is getting comfortable again riding since I haven't been on a motorcycle for years.

seanbarney41 · « **Reply #11 on:** July 20, 2015, 05:15:44 PM »

a cut down seat is a really good way to go...

DSCN0505 by Sean Barney, on Flickr
as you can see, this custom seat drops the height a good 2"...I am 5'8" with particularly stubby legs and I can flatfoot with this seat. I recently restored the stock seat and put it on...back to the tippy toes but I am used to it. The cutdown seat is more comfortable too.

seanbarney41 · « **Reply #12 on:** July 20, 2015, 05:20:10 PM »

a big part of the problem with cb750 is the width, so anything you can do in that department will help too...

754 · « **Reply #13 on:** July 20, 2015, 05:26:17 PM »

Grabcon, thank you for the level headed response.
Over the years many folks have asked me about lowering bikes.. Not many if any regretted it...
Its done a a lotta on this continent.. And like most things there can be drawbacks or compromises.
But what he is doing needs to be done, and it's reversible..

Djfob · « **Reply #14 on:** July 20, 2015, 07:44:34 PM »

I'd be willing to give up a bit on performance to feel confident on a bike.

Grabcon · « **Reply #15 on:** July 20, 2015, 08:29:38 PM »

So I am finding this a civil and interesting thread. It seems that the only manufactures that have addressed the seat height issue are those that make cruiser style bikes. Here we are dealing with 30 and 40 year old bikes and even when these were built Honda had a high seat height.

I am going to add some thoughts and feel free to add and discuss.

First the best way to gain better foot position on the ground is by modifying the seat. Cutting it down and narrowing it. As your leg moves in you get better foot position on the ground. Pros it works well and does not impact any of the bike geometry. Con, it definitely does not look stock unless done by someone very skillful.

From this point there are options that have been discussed. But before any thing is done do your home work. I would suggest looking at overall performance impact of the bike and comfort. Which is a higher priority. Also like 754 had mentioned is what you are doing reversible or does it not really matter. Spend your efforts and money wisely.

Suspension modifications are always an option some issues already have been discussed. Tire size is an other option. As an example on my last build I went with an 18" front and a 17" rear. This does a couple of things one it provides a much broader range of tires and tire sizes over stock tires. Modern tire designs and compounds also provide better handling capabilities. Use a tire height calculator to determine what size tire will give you the best height.

Formula: Diameter in Inches = Rim Diam Inches + [ (2 x Aspect Ratio x Section Width) / 25.4] Use a decimal value for aspect ratio. Again shorter tire height will impact clearances to ground.

Rim diameter and aspect ratio will have the most significant impact on height.

rddcw · « **Reply #16 on:** July 21, 2015, 08:24:16 AM »

Grabcon, I was also entertaining the idea of going with a Front 18" wheel and 17" Rear wheel, but didn't know how that would look like on the bike? Using the current 18" rear wheel that I have and going with a 120/80 series tire, it would be a little less than 1" in overall diameter than the 90 series tire giving me about a 1/2" foot placement advantage. Doesn't sound like much but adds up.

The 17" rear wheel if staying with and 120/80 series tire is another 1" less than overall diameter than the above 120/80 tire and I think that would be an additional 1/2" of foot placement if I'm doing my calculations right?

I had the seat cut down 2" and rolled a little on the sides. It did help but I would like a little better foot placement than the seat gave me.

Getting back to the 18" front wheel and 17" rear. Like to stay with the same style of spoke wheels that I have. Does anyone have any pictures how it would look on the bike? What about the front fender? Is there an 18" front fender that is like the stock front fender that is currently on the bike and would bolt right on?

The rear shocks as I said earlier are 12 1/2" in length. The bike should of came with 13" shocks as stock, so not sure what these are? I was told they are from a Honda 500/550 ?

Has anyone used a 12" rear shock and ran the fork tubes up a little on the K models?

Again, Thank You everyone who has participated in this Post. Any other pictures would be greatly appreciated !

Grabcon · « **Reply #17 on:** July 21, 2015, 08:43:28 AM »

rddcw - Keep in mind that if you use shocks that are shorter from a different weight class of bike you now need to deal with the springs not supporting the weight of the bike and rider.

If you use the option of the bottom mount on the shock being set back to do the lowering, now you need to deal with a new shock angle. Although it might appear slight the angle will influence how the shock operates.

If you are set on shorter shocks you may want to work with one of the shock companies to get some made that do exactly what you want.

Dropping your front end is easiest of all. Put the bike on the center stand loosen the bolts on the triple clamps and move the fork tubes up a 1/2" at a time and try it. Not just sitting on the bike but riding it. Raising the fork tubes will increase the speed of you steering. A little bit does a lot of things.

If you go with different size rims most likely your speedo will change and your RPMs at a noted speed will change. Since the circumference of the rear wheel is smaller it will turn faster at the same speed increasing your RPMs.

rddcw · « **Reply #18 on:** July 21, 2015, 09:11:32 AM »

Grabcon, I think I'll will go with the 80 series 18" tire in the back and keep the rim at 19" in the front. The cost of different wheels and spokes doesn't warrant the change.

The rear shock that I have on the bike as you said probably is not designed for the bike's weight, so will look int that. Then I could go with the 12" shock but with the right spring rates.

I'll drop the fork tubes a 1/2" to start with and see what happens.

I think the seat could of been cut down a little more and may address that issue too. Didn't want to get it too thin where there is now no support.

Grabcon · « **Reply #19 on:** July 21, 2015, 11:18:47 AM »

Your are on the right path. Baby steps. If everything is done at once you will not know which had the most significant impact either on comfort or performance.

Don R · « **Reply #20 on:** July 21, 2015, 11:21:15 AM »

Watch the clearance between the header and the fender, my brother's K4 750 runs lowering blocks and he slides the forks up 1". After a header change he dented the fender when he hit a bump while turning sharply at low speed. He's been set up this way since he bought it new in 74. His riding style isn't really affected by the changes.

evanphi · « **Reply #21 on:** July 21, 2015, 11:23:35 AM »

I had lowering blocks on my bike when I bought it. It makes the shocks VERY stiff. The forks were also raised about 1".

I went back to stock height after my first year of owning the bike. I have a 32" inseam and I can get the balls of my feet planted no problem on stock height.

You could also try boots with taller soles. It was the 70's after all.........................

rddcw · « **Reply #22 on:** July 21, 2015, 05:02:12 PM »

Has anyone used a 12" length rear shock on their CB 750 K' models before? In my case, I believe the 12" shock would be an inch lower than stock.

Like to find a nice cost effective shock that's better than stock without breaking the bank.

mystic_1 · « **Reply #23 on:** July 21, 2015, 10:08:35 PM »

As a guy with about a 29 inch inseam, I can empathize. In addition to the excellent discussion so far, I'll add a few thoughts.

Lowering blocks reposition the shocks, so if your chain guard is stock, it's clearance cutout will no longer line up with the shock. Some lowering blocks also cover up the chain alignment marks on the swingarm, which is very minor but somewhat annoying. Some lowering blocks will move around when the shocks rebound because they are not braced against upwards motion. Do not use those. The better ones have provisions to keep this from happening, either via a hold-down bolt in front of the swingarm shock mount, or by attaching to the swingarm end cap bolts.

Shorter shock (with proper spring rates) are much much (much) better than lowering blocks in basically every way, imho. I've run 12 inch shocks but recently switched back to 13 inchers, because I was tired of dragging the pipes or centerstand lever on tight cloverleafs. I should mention that my bike is a heavily modified chopper so my experiences may not mirror yours.

Start by lowering the rear; do not lower the front unless you have lowered the rear, since this reduces trail in the steering system, and going too far in that direction can lead to fatal instabilities at speed.

If you plan to do any two-up riding, check all of your clearances with two people on the bike. Adjust your shocks to their stiffest setting for two-up riding, and to their softest setting for solo riding.

Grabon's suggestion of narrowing the nose of the seat where your thighs touch it when straddling the bike, is an excellent one; it helps a lot. This of course requires disassembling the seat and modifying the seat pan. Your limiting factor here is the placement of the front seat hinge tab that's bolted to the seat pan, other than that you can cut and reposition the sides of the seat pan then re-weld. Watch out for clearance with the frame rails. This obviously takes a bit of work but even fractions of an inch here can make a big difference on where you can comfortable place your feet.

Finally, consider cowboy boots. No, seriously, they often have a higher heel than your standard work boot or combat boot.

mystic_1

rddcw · « **Reply #24 on:** July 22, 2015, 08:06:11 AM »

I appreciate the input Mystic_1, I'm definitely not going to use the lowering blocks.

I thought when I had my seat cut 2" in front, it would of giving me a lot more legroom. Had the upholsterer round the nose a little but maybe should take the seat back in and have him trim a little more.

I'll look into some "thicker" sole boots since only the balls of my feet touch.

May stay with these 12 1/2" rear shocks ( what ever application they are from ? ) if the spring rates seem to be okay.

Going to run the fork tubes up 1/2" in front.

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Author Topic: Lowering my 74' CB 750 K4 ? (Read 6899 times)